< draft-ietf-ospf-version2-09.txt		draft-ietf-ospf-version2-10.txt >
	Network Working Group J. Moy		Network Working Group J. Moy
	Internet Draft Cascade Communications Corp.		Internet Draft Cascade Communications Corp.
	Expiration Date: July 1997 January 1997		Expiration Date: August 1997 February 1997
	File name: draft-ietf-ospf-version2-09.txt		File name: draft-ietf-ospf-version2-10.txt
	OSPF Version 2		OSPF Version 2
	Status of this Memo		Status of this Memo
	This document is an Internet-Draft. Internet-Drafts are working		This document is an Internet-Draft. Internet-Drafts are working
	documents of the Internet Engineering Task Force (IETF), its areas,		documents of the Internet Engineering Task Force (IETF), its areas,
	and its working groups. Note that other groups may also distribute		and its working groups. Note that other groups may also distribute
	working documents as Internet-Drafts.		working documents as Internet-Drafts.
	skipping to change at page 3, line 20 ¶		skipping to change at page 3, line 20 ¶
	1.3 Brief history of link-state routing technology ........ 11		1.3 Brief history of link-state routing technology ........ 11
	1.4 Organization of this document ......................... 12		1.4 Organization of this document ......................... 12
	1.5 Acknowledgments ....................................... 13		1.5 Acknowledgments ....................................... 13
	2 The link-state database: organization and calculations 13		2 The link-state database: organization and calculations 13
	2.1 Representation of routers and networks ................ 13		2.1 Representation of routers and networks ................ 13
	2.1.1 Representation of non-broadcast networks .............. 15		2.1.1 Representation of non-broadcast networks .............. 15
	2.1.2 An example link-state database ........................ 16		2.1.2 An example link-state database ........................ 16
	2.2 The shortest-path tree ................................ 20		2.2 The shortest-path tree ................................ 20
	2.3 Use of external routing information ................... 22		2.3 Use of external routing information ................... 22
	2.4 Equal-cost multipath .................................. 24		2.4 Equal-cost multipath .................................. 24
	2.5 TOS-based routing ..................................... 24		2.5 TOS-based routing ..................................... 25
	3 Splitting the AS into Areas ........................... 25		3 Splitting the AS into Areas ........................... 25
	3.1 The backbone of the Autonomous System ................. 26		3.1 The backbone of the Autonomous System ................. 26
	3.2 Inter-area routing .................................... 26		3.2 Inter-area routing .................................... 27
	3.3 Classification of routers ............................. 27		3.3 Classification of routers ............................. 27
	3.4 A sample area configuration ........................... 28		3.4 A sample area configuration ........................... 28
	3.5 IP subnetting support ................................. 34		3.5 IP subnetting support ................................. 34
	3.6 Supporting stub areas ................................. 35		3.6 Supporting stub areas ................................. 35
	3.7 Partitions of areas ................................... 36		3.7 Partitions of areas ................................... 36
	4 Functional Summary .................................... 38		4 Functional Summary .................................... 38
	4.1 Inter-area routing .................................... 38		4.1 Inter-area routing .................................... 38
	4.2 AS external routes .................................... 39		4.2 AS external routes .................................... 39
	4.3 Routing protocol packets .............................. 39		4.3 Routing protocol packets .............................. 39
	4.4 Basic implementation requirements ..................... 41		4.4 Basic implementation requirements ..................... 42
	4.5 Optional OSPF capabilities ............................ 43		4.5 Optional OSPF capabilities ............................ 43
	5 Protocol data structures .............................. 44		5 Protocol data structures .............................. 44
	6 The Area Data Structure ............................... 46		6 The Area Data Structure ............................... 46
	7 Bringing Up Adjacencies ............................... 49		7 Bringing Up Adjacencies ............................... 49
	7.1 The Hello Protocol .................................... 49		7.1 The Hello Protocol .................................... 49
	7.2 The Synchronization of Databases ...................... 50		7.2 The Synchronization of Databases ...................... 50
	7.3 The Designated Router ................................. 51		7.3 The Designated Router ................................. 51
	7.4 The Backup Designated Router .......................... 52		7.4 The Backup Designated Router .......................... 52
	7.5 The graph of adjacencies .............................. 53		7.5 The graph of adjacencies .............................. 53
	8 Protocol Packet Processing ............................ 54		8 Protocol Packet Processing ............................ 53
	8.1 Sending protocol packets .............................. 54		8.1 Sending protocol packets .............................. 54
	8.2 Receiving protocol packets ............................ 56		8.2 Receiving protocol packets ............................ 56
	9 The Interface Data Structure .......................... 59		9 The Interface Data Structure .......................... 59
	9.1 Interface states ...................................... 62		9.1 Interface states ...................................... 62
	9.2 Events causing interface state changes ................ 64		9.2 Events causing interface state changes ................ 64
	9.3 The Interface state machine ........................... 66		9.3 The Interface state machine ........................... 66
	9.4 Electing the Designated Router ........................ 68		9.4 Electing the Designated Router ........................ 68
	9.5 Sending Hello packets ................................. 71		9.5 Sending Hello packets ................................. 71
	9.5.1 Sending Hello packets on NBMA networks ................ 72		9.5.1 Sending Hello packets on NBMA networks ................ 72
	10 The Neighbor Data Structure ........................... 73		10 The Neighbor Data Structure ........................... 73
	skipping to change at page 4, line 23 ¶		skipping to change at page 4, line 23 ¶
	10.9 Sending Link State Request Packets .................... 94		10.9 Sending Link State Request Packets .................... 94
	10.10 An Example ............................................ 95		10.10 An Example ............................................ 95
	11 The Routing Table Structure ........................... 95		11 The Routing Table Structure ........................... 95
	11.1 Routing table lookup ................................. 100		11.1 Routing table lookup ................................. 100
	11.2 Sample routing table, without areas .................. 101		11.2 Sample routing table, without areas .................. 101
	11.3 Sample routing table, with areas ..................... 102		11.3 Sample routing table, with areas ..................... 102
	12 Link State Advertisements (LSAs) ..................... 103		12 Link State Advertisements (LSAs) ..................... 103
	12.1 The LSA Header ....................................... 105		12.1 The LSA Header ....................................... 105
	12.1.1 LS age ............................................... 105		12.1.1 LS age ............................................... 105
	12.1.2 Options .............................................. 106		12.1.2 Options .............................................. 106
	12.1.3 LS type .............................................. 106		12.1.3 LS type .............................................. 107
	12.1.4 Link State ID ........................................ 106		12.1.4 Link State ID ........................................ 108
	12.1.5 Advertising Router ................................... 108		12.1.5 Advertising Router ................................... 109
	12.1.6 LS sequence number ................................... 108		12.1.6 LS sequence number ................................... 109
	12.1.7 LS checksum .......................................... 109		12.1.7 LS checksum .......................................... 110
	12.2 The link state database .............................. 110		12.2 The link state database .............................. 110
	12.3 Representation of TOS ................................ 111		12.3 Representation of TOS ................................ 111
	12.4 Originating LSAs ..................................... 112		12.4 Originating LSAs ..................................... 112
	12.4.1 Router-LSAs .......................................... 115		12.4.1 Router-LSAs .......................................... 115
	12.4.1.1 Describing point-to-point interfaces ................. 117		12.4.1.1 Describing point-to-point interfaces ................. 118
	12.4.1.2 Describing broadcast and NBMA interfaces ............. 118		12.4.1.2 Describing broadcast and NBMA interfaces ............. 119
	12.4.1.3 Describing virtual links ............................. 119		12.4.1.3 Describing virtual links ............................. 119
	12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 119		12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 120
	12.4.1.5 Examples of router-LSAs .............................. 119		12.4.1.5 Examples of router-LSAs .............................. 120
	12.4.2 Network-LSAs ......................................... 122		12.4.2 Network-LSAs ......................................... 122
	12.4.2.1 Examples of network-LSAs ............................. 122		12.4.2.1 Examples of network-LSAs ............................. 123
	12.4.3 Summary-LSAs ......................................... 123		12.4.3 Summary-LSAs ......................................... 123
	12.4.3.1 Originating summary-LSAs into stub areas ............. 126		12.4.3.1 Originating summary-LSAs into stub areas ............. 126
	12.4.3.2 Examples of summary-LSAs ............................. 126		12.4.3.2 Examples of summary-LSAs ............................. 127
	12.4.4 AS-external-LSAs ..................................... 127		12.4.4 AS-external-LSAs ..................................... 128
	12.4.4.1 Examples of AS-external-LSAs ......................... 128		12.4.4.1 Examples of AS-external-LSAs ......................... 128
	13 The Flooding Procedure ............................... 130		13 The Flooding Procedure ............................... 130
	13.1 Determining which LSA is newer ....................... 133		13.1 Determining which LSA is newer ....................... 134
	13.2 Installing LSAs in the database ...................... 134		13.2 Installing LSAs in the database ...................... 134
	13.3 Next step in the flooding procedure .................. 135		13.3 Next step in the flooding procedure .................. 135
	13.4 Receiving self-originated LSAs ....................... 138		13.4 Receiving self-originated LSAs ....................... 138
	13.5 Sending Link State Acknowledgment packets ............ 138		13.5 Sending Link State Acknowledgment packets ............ 139
	13.6 Retransmitting LSAs .................................. 141		13.6 Retransmitting LSAs .................................. 141
	13.7 Receiving link state acknowledgments ................. 141		13.7 Receiving link state acknowledgments ................. 142
	14 Aging The Link State Database ........................ 142		14 Aging The Link State Database ........................ 142
	14.1 Premature aging of LSAs .............................. 142		14.1 Premature aging of LSAs .............................. 143
	15 Virtual Links ........................................ 143		15 Virtual Links ........................................ 144
	16 Calculation of the routing table ..................... 145		16 Calculation of the routing table ..................... 146
	16.1 Calculating the shortest-path tree for an area ....... 146		16.1 Calculating the shortest-path tree for an area ....... 147
	16.1.1 The next hop calculation ............................. 152		16.1.1 The next hop calculation ............................. 152
	16.2 Calculating the inter-area routes .................... 153		16.2 Calculating the inter-area routes .................... 153
	16.3 Examining transit areas' summary-LSAs ................ 154		16.3 Examining transit areas' summary-LSAs ................ 155
	16.4 Calculating AS external routes ....................... 157		16.4 Calculating AS external routes ....................... 157
	16.4.1 External path preferences ............................ 159		16.4.1 External path preferences ............................ 159
	16.5 Incremental updates -- summary-LSAs .................. 159		16.5 Incremental updates -- summary-LSAs .................. 160
	16.6 Incremental updates -- AS-external-LSAs .............. 160		16.6 Incremental updates -- AS-external-LSAs .............. 161
	16.7 Events generated as a result of routing table changes 161		16.7 Events generated as a result of routing table changes 161
	16.8 Equal-cost multipath ................................. 161		16.8 Equal-cost multipath ................................. 162
	16.9 Building the non-zero-TOS portion of the routing table 162		16.9 Building the non-zero-TOS portion of the routing table 162
	Footnotes ............................................ 164		Footnotes ............................................ 165
	References ........................................... 168		References ........................................... 169
	A OSPF data formats .................................... 170		A OSPF data formats .................................... 171
	A.1 Encapsulation of OSPF packets ........................ 170		A.1 Encapsulation of OSPF packets ........................ 171
	A.2 The Options field .................................... 172		A.2 The Options field .................................... 173
	A.3 OSPF Packet Formats .................................. 174		A.3 OSPF Packet Formats .................................. 175
	A.3.1 The OSPF packet header ............................... 175		A.3.1 The OSPF packet header ............................... 176
	A.3.2 The Hello packet ..................................... 177		A.3.2 The Hello packet ..................................... 178
	A.3.3 The Database Description packet ...................... 179		A.3.3 The Database Description packet ...................... 180
	A.3.4 The Link State Request packet ........................ 181		A.3.4 The Link State Request packet ........................ 182
	A.3.5 The Link State Update packet ......................... 183		A.3.5 The Link State Update packet ......................... 184
	A.3.6 The Link State Acknowledgment packet ................. 185		A.3.6 The Link State Acknowledgment packet ................. 186
	A.4 LSA formats .......................................... 187		A.4 LSA formats .......................................... 188
	A.4.1 The LSA header ....................................... 188		A.4.1 The LSA header ....................................... 189
	A.4.2 Router-LSAs .......................................... 190		A.4.2 Router-LSAs .......................................... 191
	A.4.3 Network-LSAs ......................................... 194		A.4.3 Network-LSAs ......................................... 195
	A.4.4 Summary-LSAs ......................................... 195		A.4.4 Summary-LSAs ......................................... 196
	A.4.5 AS-external-LSAs ..................................... 197		A.4.5 AS-external-LSAs ..................................... 198
	B Architectural Constants .............................. 199		B Architectural Constants .............................. 200
	C Configurable Constants ............................... 201		C Configurable Constants ............................... 202
	C.1 Global parameters .................................... 201		C.1 Global parameters .................................... 202
	C.2 Area parameters ...................................... 202		C.2 Area parameters ...................................... 203
	C.3 Router interface parameters .......................... 203		C.3 Router interface parameters .......................... 204
	C.4 Virtual link parameters .............................. 205		C.4 Virtual link parameters .............................. 206
	C.5 NBMA network parameters .............................. 206		C.5 NBMA network parameters .............................. 207
	C.6 Point-to-MultiPoint network parameters ............... 207		C.6 Point-to-MultiPoint network parameters ............... 208
	C.7 Host route parameters ................................ 207		C.7 Host route parameters ................................ 208
	D Authentication ....................................... 208		D Authentication ....................................... 209
	D.1 Null authentication .................................. 208		D.1 Null authentication .................................. 209
	D.2 Simple password authentication ....................... 208		D.2 Simple password authentication ....................... 209
	D.3 Cryptographic authentication ......................... 209		D.3 Cryptographic authentication ......................... 210
	D.4 Message generation ................................... 211		D.4 Message generation ................................... 212
	D.4.1 Generating Null authentication ....................... 212		D.4.1 Generating Null authentication ....................... 213
	D.4.2 Generating Simple password authentication ............ 212		D.4.2 Generating Simple password authentication ............ 213
	D.4.3 Generating Cryptographic authentication .............. 212		D.4.3 Generating Cryptographic authentication .............. 213
	D.5 Message verification ................................. 214		D.5 Message verification ................................. 215
	D.5.1 Verifying Null authentication ........................ 214		D.5.1 Verifying Null authentication ........................ 215
	D.5.2 Verifying Simple password authentication ............. 214		D.5.2 Verifying Simple password authentication ............. 215
	D.5.3 Verifying Cryptographic authentication ............... 214		D.5.3 Verifying Cryptographic authentication ............... 215
	E An algorithm for assigning Link State IDs ............ 216		E An algorithm for assigning Link State IDs ............ 217
	F Multiple interfaces to the same network/subnet ....... 218		F Multiple interfaces to the same network/subnet ....... 219
	G Differences from RFC 1583 ............................ 219		G Differences from RFC 1583 ............................ 220
	G.1 Enhancements to OSPF authentication .................. 219		G.1 Enhancements to OSPF authentication .................. 220
	G.2 Addition of Point-to-MultiPoint interface ............ 219		G.2 Addition of Point-to-MultiPoint interface ............ 220
	G.3 Support for overlapping area ranges .................. 220		G.3 Support for overlapping area ranges .................. 221
	G.4 A modification to the flooding algorithm ............. 221		G.4 A modification to the flooding algorithm ............. 222
	G.5 Introduction of the MinLSArrival constant ............ 221		G.5 Introduction of the MinLSArrival constant ............ 222
	G.6 Optionally advertising point-to-point links as subnets 222		G.6 Optionally advertising point-to-point links as subnets 223
	G.7 Advertising same external route from multiple areas .. 222		G.7 Advertising same external route from multiple areas .. 223
	G.8 Retransmission of initial Database Description packets 224		G.8 Retransmission of initial Database Description packets 225
	G.9 Detecting interface MTU mismatches ................... 224		G.9 Detecting interface MTU mismatches ................... 225
	Security Considerations .............................. 225		Security Considerations .............................. 226
	Author's Address ..................................... 225		Author's Address ..................................... 226
	1. Introduction		1. Introduction
	This document is a specification of the Open Shortest Path First		This document is a specification of the Open Shortest Path First
	(OSPF) TCP/IP internet routing protocol. OSPF is classified as an		(OSPF) TCP/IP internet routing protocol. OSPF is classified as an
	Interior Gateway Protocol (IGP). This means that it distributes		Interior Gateway Protocol (IGP). This means that it distributes
	routing information between routers belonging to a single Autonomous		routing information between routers belonging to a single Autonomous
	System. The OSPF protocol is based on link-state or SPF technology.		System. The OSPF protocol is based on link-state or SPF technology.
	This is a departure from the Bellman-Ford base used by traditional		This is a departure from the Bellman-Ford base used by traditional
	TCP/IP internet routing protocols.		TCP/IP internet routing protocols.
	skipping to change at page 22, line 18 ¶		skipping to change at page 22, line 18 ¶
	N3 RT3 7		N3 RT3 7
	N4 RT3 8		N4 RT3 8
	Ib * 7		Ib * 7
	Ia RT10 12		Ia RT10 12
	N6 RT10 8		N6 RT10 8
	N7 RT10 12		N7 RT10 12
	N8 RT10 10		N8 RT10 10
	N9 RT10 11		N9 RT10 11
	N10 RT10 13		N10 RT10 13
	N11 RT10 14		N11 RT10 14
			H1 RT10 21
	__________________________________		__________________________________
	RT5 RT5 6		RT5 RT5 6
	RT7 RT10 8		RT7 RT10 8
	Table 2: The portion of Router RT6's routing table listing local		Table 2: The portion of Router RT6's routing table listing local
	destinations.		destinations.
	Routes to networks belonging to other AS'es (such as N12) appear		Routes to networks belonging to other AS'es (such as N12) appear
	as dashed lines on the shortest path tree in Figure 5. Use of		as dashed lines on the shortest path tree in Figure 5. Use of
	this externally derived routing information is considered in the		this externally derived routing information is considered in the
	skipping to change at page 28, line 49 ¶		skipping to change at page 29, line 4 ¶
	Figure 7 shows the resulting link-state database for the Area 1.		Figure 7 shows the resulting link-state database for the Area 1.
	The figure completely describes that area's intra-area routing.		The figure completely describes that area's intra-area routing.
	It also shows the complete view of the internet for the two		It also shows the complete view of the internet for the two
	internal routers RT1 and RT2. It is the job of the area border		internal routers RT1 and RT2. It is the job of the area border
	routers, RT3 and RT4, to advertise into Area 1 the distances to		routers, RT3 and RT4, to advertise into Area 1 the distances to
	all destinations external to the area. These are indicated in		all destinations external to the area. These are indicated in
	Figure 7 by the dashed stub routes. Also, RT3 and RT4 must		Figure 7 by the dashed stub routes. Also, RT3 and RT4 must
	advertise into Area 1 the location of the AS boundary routers		advertise into Area 1 the location of the AS boundary routers
	RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are		RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are
	flooded throughout the entire AS, and in particular throughout		flooded throughout the entire AS, and in particular throughout
	Area 1. These LSAs are included in Area 1's database, and yield
	routes to Networks N12-N15.
	...........................		...........................
	. + .		. + .
	. | 3+---+ . N12 N14		. | 3+---+ . N12 N14
	. N1|--|RT1|\ 1 . \ N13 /		. N1|--|RT1|\ 1 . \ N13 /
	. | +---+ \ . 8\ |8/8		. | +---+ \ . 8\ |8/8
	. + \ ____ . \|/		. + \ ____ . \|/
	. / \ 1+---+8 8+---+6		. / \ 1+---+8 8+---+6
	. * N3 *---|RT4|------|RT5|--------+		. * N3 *---|RT4|------|RT5|--------+
	. \____/ +---+ +---+ |		. \____/ +---+ +---+ |
	. + / \ . |7 |		. + / \ . |7 |
	skipping to change at page 30, line 4 ¶		skipping to change at page 30, line 4 ¶
	. +--+SLIP +----+ . . +---+ .		. +--+SLIP +----+ . . +---+ .
	. |2 . . |4 .		. |2 . . |4 .
	. | . . | .		. | . . | .
	. +---------+ . . +--------+ .		. +---------+ . . +--------+ .
	. N10 . . N7 .		. N10 . . N7 .
	. . .Area 2 .		. . .Area 2 .
	.Area 3 . ................................		.Area 3 . ................................
	..........................		..........................
	Figure 6: A sample OSPF area configuration		Figure 6: A sample OSPF area configuration
			Area 1. These LSAs are included in Area 1's database, and yield
			routes to Networks N12-N15.
	Routers RT3 and RT4 must also summarize Area 1's topology for		Routers RT3 and RT4 must also summarize Area 1's topology for
	distribution to the backbone. Their backbone LSAs are shown in		distribution to the backbone. Their backbone LSAs are shown in
	Table 4. These summaries show which networks are contained in		Table 4. These summaries show which networks are contained in
	Area 1 (i.e., Networks N1-N4), and the distance to these		Area 1 (i.e., Networks N1-N4), and the distance to these
	networks from the routers RT3 and RT4 respectively.		networks from the routers RT3 and RT4 respectively.
	The link-state database for the backbone is shown in Figure 8.		The link-state database for the backbone is shown in Figure 8.
	The set of routers pictured are the backbone routers. Router		The set of routers pictured are the backbone routers. Router
	RT11 is a backbone router because it belongs to two areas. In		RT11 is a backbone router because it belongs to two areas. In
	order to make the backbone connected, a virtual link has been		order to make the backbone connected, a virtual link has been
	skipping to change at page 30, line 35 ¶		skipping to change at page 30, line 38 ¶
	The backbone enables the exchange of summary information between		The backbone enables the exchange of summary information between
	area border routers. Every area border router hears the area		area border routers. Every area border router hears the area
	summaries from all other area border routers. It then forms a		summaries from all other area border routers. It then forms a
	picture of the distance to all networks outside of its area by		picture of the distance to all networks outside of its area by
	examining the collected LSAs, and adding in the backbone		examining the collected LSAs, and adding in the backbone
	distance to each advertising router.		distance to each advertising router.
	Again using Routers RT3 and RT4 as an example, the procedure		Again using Routers RT3 and RT4 as an example, the procedure
	goes as follows: They first calculate the SPF tree for the		goes as follows: They first calculate the SPF tree for the
	backbone. This gives the distances to all other area border
	routers. Also noted are the distances to networks (Ia and Ib)
	and AS boundary routers (RT5 and RT7) that belong to the
	backbone. This calculation is shown in Table 5.
	Network RT3 adv. RT4 adv.		Network RT3 adv. RT4 adv.
	_____________________________		_____________________________
	N1 4 4		N1 4 4
	N2 4 4		N2 4 4
	N3 1 1		N3 1 1
	N4 2 3		N4 2 3
	Table 4: Networks advertised to the backbone		Table 4: Networks advertised to the backbone
	by Routers RT3 and RT4.		by Routers RT3 and RT4.
	skipping to change at page 32, line 39 ¶		skipping to change at page 32, line 39 ¶
	N14| | |8 | | | | |		N14| | |8 | | | | |
	N15| | | | |9 | | |		N15| | | | |9 | | |
	Figure 8: The backbone's database.		Figure 8: The backbone's database.
	Networks and routers are represented by vertices.		Networks and routers are represented by vertices.
	An edge of cost X connects Vertex A to Vertex B iff		An edge of cost X connects Vertex A to Vertex B iff
	the intersection of Column A and Row B is marked		the intersection of Column A and Row B is marked
	with an X.		with an X.
			backbone. This gives the distances to all other area border
			routers. Also noted are the distances to networks (Ia and Ib)
			and AS boundary routers (RT5 and RT7) that belong to the
			backbone. This calculation is shown in Table 5.
	Next, by looking at the area summaries from these area border		Next, by looking at the area summaries from these area border
	routers, RT3 and RT4 can determine the distance to all networks		routers, RT3 and RT4 can determine the distance to all networks
	outside their area. These distances are then advertised		outside their area. These distances are then advertised
	internally to the area by RT3 and RT4. The advertisements that		internally to the area by RT3 and RT4. The advertisements that
	Router RT3 and RT4 will make into Area 1 are shown in Table 6.		Router RT3 and RT4 will make into Area 1 are shown in Table 6.
	Note that Table 6 assumes that an area range has been configured		Note that Table 6 assumes that an area range has been configured
	for the backbone which groups Ia and Ib into a single LSA.		for the backbone which groups Ia and Ib into a single LSA.
	The information imported into Area 1 by Routers RT3 and RT4
	enables an internal router, such as RT1, to choose an area
	border router intelligently. Router RT1 would use RT4 for
	traffic to Network N6, RT3 for traffic to Network N10, and would
	dist from dist from		dist from dist from
	RT3 RT4		RT3 RT4
	__________________________________		__________________________________
	to RT3 * 21		to RT3 * 21
	to RT4 22 *		to RT4 22 *
	to RT7 20 14		to RT7 20 14
	to RT10 15 22		to RT10 15 22
			to RT11 18 25
	__________________________________		__________________________________
	to Ia 20 27		to Ia 20 27
	to Ib 15 22		to Ib 15 22
	__________________________________		__________________________________
	to RT5 14 8		to RT5 14 8
	to RT7 20 14		to RT7 20 14
	Table 5: Backbone distances calculated		Table 5: Backbone distances calculated
	by Routers RT3 and RT4.		by Routers RT3 and RT4.
			The information imported into Area 1 by Routers RT3 and RT4
			enables an internal router, such as RT1, to choose an area
			border router intelligently. Router RT1 would use RT4 for
			traffic to Network N6, RT3 for traffic to Network N10, and would
			load share between the two for traffic to Network N8.
			Router RT1 can also determine in this manner the shortest path
			to the AS boundary routers RT5 and RT7. Then, by looking at RT5
			and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or
			RT7 when sending to a destination in another Autonomous System
			(one of the networks N12-N15).
			Destination RT3 adv. RT4 adv.
	_________________________________		_________________________________
	Ia,Ib 20 27		Ia,Ib 20 27
	N6 16 15		N6 16 15
	N7 20 19		N7 20 19
	N8 18 18		N8 18 18
	N9-N11,H1 29 36		N9-N11,H1 29 36
	_________________________________		_________________________________
	RT5 14 8		RT5 14 8
	RT7 20 14		RT7 20 14
	Table 6: Destinations advertised into Area 1		Table 6: Destinations advertised into Area 1
	by Routers RT3 and RT4.		by Routers RT3 and RT4.
	load share between the two for traffic to Network N8.
	Router RT1 can also determine in this manner the shortest path
	to the AS boundary routers RT5 and RT7. Then, by looking at RT5
	and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or
	RT7 when sending to a destination in another Autonomous System
	(one of the networks N12-N15).
	Note that a failure of the line between Routers RT6 and RT10		Note that a failure of the line between Routers RT6 and RT10
	will cause the backbone to become disconnected. Configuring a		will cause the backbone to become disconnected. Configuring a
	virtual link between Routers RT7 and RT10 will give the backbone		virtual link between Routers RT7 and RT10 will give the backbone
	more connectivity and more resistance to such failures.		more connectivity and more resistance to such failures.
	3.5. IP subnetting support		3.5. IP subnetting support
	OSPF attaches an IP address mask to each advertised route. The		OSPF attaches an IP address mask to each advertised route. The
	mask indicates the range of addresses being described by the		mask indicates the range of addresses being described by the
	particular route. For example, a summary-LSA for the		particular route. For example, a summary-LSA for the
	skipping to change at page 41, line 13 ¶		skipping to change at page 41, line 13 ¶
	multicast address.		multicast address.
	LS LSA LSA description		LS LSA LSA description
	type name		type name
	________________________________________________________		________________________________________________________
	1 Router-LSAs Originated by all routers.		1 Router-LSAs Originated by all routers.
	This LSA describes		This LSA describes
	the collected states of the		the collected states of the
	router's interfaces to an		router's interfaces to an
	area. Flooded throughout a		area. Flooded throughout a
			single area only.
	________________________________________________________		________________________________________________________
	2 Network-LSAs Originated for broadcast		2 Network-LSAs Originated for broadcast
	and NBMA networks by		and NBMA networks by
	the Designated Router. This		the Designated Router. This
	LSA contains the		LSA contains the
	list of routers connected		list of routers connected
	to the network. Flooded		to the network. Flooded
	throughout a single area only.		throughout a single area only.
	________________________________________________________		________________________________________________________
	3,4 Summary-LSAs Originated by area border		3,4 Summary-LSAs Originated by area border
	routers, and flooded through-		routers, and flooded through-
	out the LSA's associated		out the LSA's associated
	area. Each summary-LSA		area. Each summary-LSA
	describes a route to a		describes a route to a
	destination outside the area,		destination outside the area,
	yet still inside the AS		yet still inside the AS
	(i.e., an inter-area route).		(i.e., an inter-area route).
	Type 3 summary-LSAs describe		Type 3 summary-LSAs describe
	routes to networks. Type 4		routes to networks. Type 4
	summary-LSAs describe		summary-LSAs describe
			routes to AS boundary routers.
	________________________________________________________		________________________________________________________
	5 AS-external-LSAs Originated by AS boundary		5 AS-external-LSAs Originated by AS boundary
	routers, and flooded through-		routers, and flooded through-
	out the AS. Each		out the AS. Each
	AS-external-LSA describes		AS-external-LSA describes
	a route to a destination in		a route to a destination in
	another Autonomous System.		another Autonomous System.
	Default routes for the AS can		Default routes for the AS can
	also be described by		also be described by
	AS-external-LSAs.		AS-external-LSAs.
	skipping to change at page 46, line 44 ¶		skipping to change at page 47, line 5 ¶
	The OSPF backbone is the special OSPF area responsible for		The OSPF backbone is the special OSPF area responsible for
	disseminating inter-area routing information.		disseminating inter-area routing information.
	The area link-state database consists of the collection of router-		The area link-state database consists of the collection of router-
	LSAs, network-LSAs and summary-LSAs that have originated from the		LSAs, network-LSAs and summary-LSAs that have originated from the
	area's routers. This information is flooded throughout a single		area's routers. This information is flooded throughout a single
	area only. The list of AS-external-LSAs (see Section 5) is also		area only. The list of AS-external-LSAs (see Section 5) is also
	considered to be part of each area's link-state database.		considered to be part of each area's link-state database.
	Area ID
	A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
	reserved for the backbone.
	List of area address ranges
	In order to aggregate routing information at area boundaries,
	+----+		+----+
	|RT10|------+		|RT10|------+
	+----+ \+-------------+		+----+ \+-------------+
	/ \ |Routing Table|		/ \ |Routing Table|
	/ \ +-------------+		/ \ +-------------+
	/ \		/ \
	+------+ / \ +--------+		+------+ / \ +--------+
	|Area 2|---+ +---|Backbone|		|Area 2|---+ +---|Backbone|
	+------+***********+ +--------+		+------+***********+ +--------+
	/ \ * / \		/ \ * / \
	skipping to change at page 47, line 32 ¶		skipping to change at page 47, line 33 ¶
	|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|		|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|
	| RT8 | | RT7 | | +-------------+ +------------+		| RT8 | | RT7 | | +-------------+ +------------+
	+--------+ +--------+ |		+--------+ +--------+ |
	|		|
	+-------------+		+-------------+
	|Neighbor RT11|		|Neighbor RT11|
	+-------------+		+-------------+
	Figure 9: Router RT10's Data structures		Figure 9: Router RT10's Data structures
			Area ID
			A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
			reserved for the backbone.
			List of area address ranges
			In order to aggregate routing information at area boundaries,
	area address ranges can be employed. Each address range is		area address ranges can be employed. Each address range is
	specified by an [address,mask] pair and a status indication of		specified by an [address,mask] pair and a status indication of
	either Advertise or DoNotAdvertise (see Section 12.4.3).		either Advertise or DoNotAdvertise (see Section 12.4.3).
	Associated router interfaces		Associated router interfaces
	This router's interfaces connecting to the area. A router		This router's interfaces connecting to the area. A router
	interface belongs to one and only one area (or the backbone).		interface belongs to one and only one area (or the backbone).
	For the backbone area this list includes all the virtual links.		For the backbone area this list includes all the virtual links.
	A virtual link is identified by the Router ID of its other		A virtual link is identified by the Router ID of its other
	endpoint; its cost is the cost of the shortest intra-area path		endpoint; its cost is the cost of the shortest intra-area path
	skipping to change at page 53, line 27 ¶		skipping to change at page 53, line 32 ¶
	Two graphs are possible, depending on whether a Designated		Two graphs are possible, depending on whether a Designated
	Router is elected for the network. On physical point-to-point		Router is elected for the network. On physical point-to-point
	networks, Point-to-MultiPoint networks and virtual links,		networks, Point-to-MultiPoint networks and virtual links,
	neighboring routers become adjacent whenever they can		neighboring routers become adjacent whenever they can
	communicate directly. In contrast, on broadcast and NBMA		communicate directly. In contrast, on broadcast and NBMA
	networks only the Designated Router and the Backup Designated		networks only the Designated Router and the Backup Designated
	Router become adjacent to all other routers attached to the		Router become adjacent to all other routers attached to the
	network.		network.
			These graphs are shown in Figure 10. It is assumed that Router
			RT7 has become the Designated Router, and Router RT3 the Backup
			Designated Router, for the Network N2. The Backup Designated
			Router performs a lesser function during the flooding procedure
			than the Designated Router (see Section 13.3). This is the
			reason for the dashed lines connecting the Backup Designated
			Router RT3.
			8. Protocol Packet Processing
			This section discusses the general processing of OSPF routing
			protocol packets. It is very important that the router link-state
			databases remain synchronized. For this reason, routing protocol
			packets should get preferential treatment over ordinary data
			packets, both in sending and receiving.
			Routing protocol packets are sent along adjacencies only (with the
			exception of Hello packets, which are used to discover the
			adjacencies). This means that all routing protocol packets travel a
	+---+ +---+		+---+ +---+
	|RT1|------------|RT2| o---------------o		|RT1|------------|RT2| o---------------o
	+---+ N1 +---+ RT1 RT2		+---+ N1 +---+ RT1 RT2
	RT7		RT7
	o---------+		o---------+
	+---+ +---+ +---+ /|\ |		+---+ +---+ +---+ /|\ |
	|RT7| |RT3| |RT4| / | \ |		|RT7| |RT3| |RT4| / | \ |
	+---+ +---+ +---+ / | \ |		+---+ +---+ +---+ / | \ |
	| | | / | \ |		| | | / | \ |
	+-----------------------+ RT5o RT6o oRT4 |		+-----------------------+ RT5o RT6o oRT4 |
	| | N2 * * * |		| | N2 * * * |
	+---+ +---+ * * * |		+---+ +---+ * * * |
	|RT5| |RT6| * * * |		|RT5| |RT6| * * * |
	+---+ +---+ *** |		+---+ +---+ *** |
	o---------+		o---------+
	RT3		RT3
	Figure 10: The graph of adjacencies		Figure 10: The graph of adjacencies
	These graphs are shown in Figure 10. It is assumed that Router
	RT7 has become the Designated Router, and Router RT3 the Backup
	Designated Router, for the Network N2. The Backup Designated
	Router performs a lesser function during the flooding procedure
	than the Designated Router (see Section 13.3). This is the
	reason for the dashed lines connecting the Backup Designated
	Router RT3.
	8. Protocol Packet Processing
	This section discusses the general processing of OSPF routing
	protocol packets. It is very important that the router link-state
	databases remain synchronized. For this reason, routing protocol
	packets should get preferential treatment over ordinary data
	packets, both in sending and receiving.
	Routing protocol packets are sent along adjacencies only (with the
	exception of Hello packets, which are used to discover the
	adjacencies). This means that all routing protocol packets travel a
	single IP hop, except those sent over virtual links.		single IP hop, except those sent over virtual links.
	All routing protocol packets begin with a standard header. The		All routing protocol packets begin with a standard header. The
	sections below provide details on how to fill in and verify this		sections below provide details on how to fill in and verify this
	standard header. Then, for each packet type, the section giving		standard header. Then, for each packet type, the section giving
	more details on that particular packet type's processing is listed.		more details on that particular packet type's processing is listed.
	8.1. Sending protocol packets		8.1. Sending protocol packets
	When a router sends a routing protocol packet, it fills in the		When a router sends a routing protocol packet, it fills in the
	skipping to change at page 102, line 22 ¶		skipping to change at page 102, line 22 ¶
	N Ib 0 intra-area 7 * *		N Ib 0 intra-area 7 * *
	N Ia 0 intra-area 12 RT10 *		N Ia 0 intra-area 12 RT10 *
	N N6 0 intra-area 8 RT10 *		N N6 0 intra-area 8 RT10 *
	N N7 0 intra-area 12 RT10 *		N N7 0 intra-area 12 RT10 *
	N N8 0 intra-area 10 RT10 *		N N8 0 intra-area 10 RT10 *
	N N9 0 intra-area 11 RT10 *		N N9 0 intra-area 11 RT10 *
	N N10 0 intra-area 13 RT10 *		N N10 0 intra-area 13 RT10 *
	N N11 0 intra-area 14 RT10 *		N N11 0 intra-area 14 RT10 *
	N H1 0 intra-area 21 RT10 *		N H1 0 intra-area 21 RT10 *
	R RT5 0 intra-area 6 RT5 *		R RT5 0 intra-area 6 RT5 *
			R RT7 0 intra-area 8 RT10 *
	____________________________________________________________		____________________________________________________________
	N N12 * type 1 ext. 10 RT10 RT7		N N12 * type 1 ext. 10 RT10 RT7
	N N13 * type 1 ext. 14 RT5 RT5		N N13 * type 1 ext. 14 RT5 RT5
	N N14 * type 1 ext. 14 RT5 RT5		N N14 * type 1 ext. 14 RT5 RT5
	N N15 * type 1 ext. 17 RT10 RT7		N N15 * type 1 ext. 17 RT10 RT7
	Table 12: The routing table for Router RT6		Table 12: The routing table for Router RT6
	(no configured areas).		(no configured areas).
	11.3. Sample routing table, with areas		11.3. Sample routing table, with areas
	skipping to change at page 104, line 12 ¶		skipping to change at page 104, line 12 ¶
	Each LSA begins with a standard 20-byte header. This LSA header is		Each LSA begins with a standard 20-byte header. This LSA header is
	discussed below.		discussed below.
	Type Dest Area Path Type Cost Next Adv.		Type Dest Area Path Type Cost Next Adv.
	Hops(s) Router(s)		Hops(s) Router(s)
	__________________________________________________________________		__________________________________________________________________
	N N1 1 intra-area 4 RT1 *		N N1 1 intra-area 4 RT1 *
	N N2 1 intra-area 4 RT2 *		N N2 1 intra-area 4 RT2 *
	N N3 1 intra-area 1 * *		N N3 1 intra-area 1 * *
	N N4 1 intra-area 3 RT3 *		N N4 1 intra-area 3 RT3 *
			R RT3 1 intra-area 1 * *
	__________________________________________________________________		__________________________________________________________________
	N Ib 0 intra-area 22 RT5 *		N Ib 0 intra-area 22 RT5 *
	N Ia 0 intra-area 27 RT5 *		N Ia 0 intra-area 27 RT5 *
	R RT3 0 intra-area 21 RT5 *		R RT3 0 intra-area 21 RT5 *
	R RT5 0 intra-area 8 * *		R RT5 0 intra-area 8 * *
	R RT7 0 intra-area 14 RT5 *		R RT7 0 intra-area 14 RT5 *
	R RT10 0 intra-area 22 RT5 *		R RT10 0 intra-area 22 RT5 *
	R RT11 0 intra-area 25 RT5 *		R RT11 0 intra-area 25 RT5 *
	__________________________________________________________________		__________________________________________________________________
	N N6 0 inter-area 15 RT5 RT7		N N6 0 inter-area 15 RT5 RT7
	N N7 0 inter-area 19 RT5 RT7		N N7 0 inter-area 19 RT5 RT7
	N N8 0 inter-area 18 RT5 RT7		N N8 0 inter-area 18 RT5 RT7
			N N9-N11,H1 0 inter-area 36 RT5 RT11
	__________________________________________________________________		__________________________________________________________________
	N N12 * type 1 ext. 16 RT5 RT5,RT7		N N12 * type 1 ext. 16 RT5 RT5,RT7
	N N13 * type 1 ext. 16 RT5 RT5		N N13 * type 1 ext. 16 RT5 RT5
	N N14 * type 1 ext. 16 RT5 RT5		N N14 * type 1 ext. 16 RT5 RT5
	N N15 * type 1 ext. 23 RT5 RT7		N N15 * type 1 ext. 23 RT5 RT7
	Table 13: Router RT4's routing table		Table 13: Router RT4's routing table
	in the presence of areas.		in the presence of areas.
	Type Dest Area Path Type Cost Next Adv.		Type Dest Area Path Type Cost Next Adv.
	Hop(s) Router(s)		Hop(s) Router(s)
	________________________________________________________________		________________________________________________________________
	N Ib 0 intra-area 16 RT3 *		N Ib 0 intra-area 16 RT3 *
	N Ia 0 intra-area 21 RT3 *		N Ia 0 intra-area 21 RT3 *
	R RT3 0 intra-area 1 * *		R RT3 0 intra-area 1 * *
	R RT10 0 intra-area 16 RT3 *		R RT10 0 intra-area 16 RT3 *
			R RT11 0 intra-area 19 RT3 *
	________________________________________________________________		________________________________________________________________
	N N9-N11,H1 0 inter-area 30 RT3 RT11		N N9-N11,H1 0 inter-area 30 RT3 RT11
	Table 14: Changes resulting from an		Table 14: Changes resulting from an
	additional virtual link.		additional virtual link.
	12.1. The LSA Header		12.1. The LSA Header
	The LSA header contains the LS type, Link State ID and		The LSA header contains the LS type, Link State ID and
	Advertising Router fields. The combination of these three		Advertising Router fields. The combination of these three
	skipping to change at page 106, line 47 ¶		skipping to change at page 107, line 16 ¶
	The LS type field dictates the format and function of the		The LS type field dictates the format and function of the
	LSA. LSAs of different types have different names (e.g.,		LSA. LSAs of different types have different names (e.g.,
	router-LSAs or network-LSAs). All LSA types defined by this		router-LSAs or network-LSAs). All LSA types defined by this
	memo, except the AS-external-LSAs (LS type = 5), are flooded		memo, except the AS-external-LSAs (LS type = 5), are flooded
	throughout a single area only. AS-external-LSAs are flooded		throughout a single area only. AS-external-LSAs are flooded
	throughout the entire Autonomous System, excepting stub		throughout the entire Autonomous System, excepting stub
	areas (see Section 3.6). Each separate LSA type is briefly		areas (see Section 3.6). Each separate LSA type is briefly
	described below in Table 15.		described below in Table 15.
	12.1.4. Link State ID
	This field identifies the piece of the routing domain that
	is being described by the LSA. Depending on the LSA's LS
	type, the Link State ID takes on the values listed in Table
	LS Type LSA description		LS Type LSA description
	________________________________________________		________________________________________________
	1 These are the router-LSAs.		1 These are the router-LSAs.
	They describe the collected		They describe the collected
	states of the router's		states of the router's
	interfaces. For more information,		interfaces. For more information,
			consult Section 12.4.1.
	________________________________________________		________________________________________________
	2 These are the network-LSAs.		2 These are the network-LSAs.
	They describe the set of routers		They describe the set of routers
	attached to the network. For		attached to the network. For
	more information, consult		more information, consult
	Section 12.4.2.		Section 12.4.2.
	________________________________________________		________________________________________________
	3 or 4 These are the summary-LSAs.		3 or 4 These are the summary-LSAs.
	They describe inter-area routes,		They describe inter-area routes,
	and enable the condensation of		and enable the condensation of
	routing information at area		routing information at area
	borders. Originated by area border		borders. Originated by area border
	routers, the Type 3 summary-LSAs		routers, the Type 3 summary-LSAs
	describe routes to networks while the		describe routes to networks while the
	Type 4 summary-LSAs describe routes to		Type 4 summary-LSAs describe routes to
			AS boundary routers.
	________________________________________________		________________________________________________
	5 These are the AS-external-LSAs.		5 These are the AS-external-LSAs.
	Originated by AS boundary routers,		Originated by AS boundary routers,
	they describe routes		they describe routes
	to destinations external to the		to destinations external to the
	Autonomous System. A default route for		Autonomous System. A default route for
	the Autonomous System can also be		the Autonomous System can also be
	described by an AS-external-LSA.		described by an AS-external-LSA.
	Table 15: OSPF link state advertisements (LSAs).		Table 15: OSPF link state advertisements (LSAs).
			12.1.4. Link State ID
			This field identifies the piece of the routing domain that
			is being described by the LSA. Depending on the LSA's LS
			type, the Link State ID takes on the values listed in Table
	16.		16.
	Actually, for Type 3 summary-LSAs (LS type = 3) and AS-		Actually, for Type 3 summary-LSAs (LS type = 3) and AS-
	external-LSAs (LS type = 5), the Link State ID may		external-LSAs (LS type = 5), the Link State ID may
	additionally have one or more of the destination network's		additionally have one or more of the destination network's
	"host" bits set. For example, when originating an AS-		"host" bits set. For example, when originating an AS-
	external-LSA for the network 10.0.0.0 with mask of		external-LSA for the network 10.0.0.0 with mask of
	255.0.0.0, the Link State ID can be set to anything in the		255.0.0.0, the Link State ID can be set to anything in the
	range 10.0.0.0 through 10.255.255.255 inclusive (although		range 10.0.0.0 through 10.255.255.255 inclusive (although
	10.0.0.0 should be used whenever possible). The freedom to		10.0.0.0 should be used whenever possible). The freedom to
	set certain host bits allows a router to originate separate		set certain host bits allows a router to originate separate
	LSAs for two networks having the same address but different		LSAs for two networks having the same address but different
	LS Type Link State ID
	_______________________________________________
	1 The originating router's Router ID.
	2 The IP interface address of the
	network's Designated Router.
	3 The destination network's IP address.
	4 The Router ID of the described AS
	boundary router.
	5 The destination network's IP address.
	Table 16: The LSA's Link State ID.
	masks. See Appendix E for details.		masks. See Appendix E for details.
	When the LSA is describing a network (LS type = 2, 3 or 5),		When the LSA is describing a network (LS type = 2, 3 or 5),
	the network's IP address is easily derived by masking the		the network's IP address is easily derived by masking the
	Link State ID with the network/subnet mask contained in the		Link State ID with the network/subnet mask contained in the
	body of the LSA. When the LSA is describing a router (LS		body of the LSA. When the LSA is describing a router (LS
	type = 1 or 4), the Link State ID is always the described		type = 1 or 4), the Link State ID is always the described
	router's OSPF Router ID.		router's OSPF Router ID.
	When an AS-external-LSA (LS Type = 5) is describing a		When an AS-external-LSA (LS Type = 5) is describing a
	default route, its Link State ID is set to		default route, its Link State ID is set to
	DefaultDestination (0.0.0.0).		DefaultDestination (0.0.0.0).
			LS Type Link State ID
			_______________________________________________
			1 The originating router's Router ID.
			2 The IP interface address of the
			network's Designated Router.
			3 The destination network's IP address.
			4 The Router ID of the described AS
			boundary router.
			5 The destination network's IP address.
			Table 16: The LSA's Link State ID.
	12.1.5. Advertising Router		12.1.5. Advertising Router
	This field specifies the OSPF Router ID of the LSA's		This field specifies the OSPF Router ID of the LSA's
	originator. For router-LSAs, this field is identical to the		originator. For router-LSAs, this field is identical to the
	Link State ID field. Network-LSAs are originated by the		Link State ID field. Network-LSAs are originated by the
	network's Designated Router. Summary-LSAs originated by		network's Designated Router. Summary-LSAs originated by
	area border routers. AS-external-LSAs are originated by AS		area border routers. AS-external-LSAs are originated by AS
	boundary routers.		boundary routers.
	12.1.6. LS sequence number		12.1.6. LS sequence number
	skipping to change at page 115, line 5 ¶		skipping to change at page 115, line 27 ¶
	AS-external-LSAs that it had previously originated. These		AS-external-LSAs that it had previously originated. These
	LSAs can be flushed via the premature aging procedure		LSAs can be flushed via the premature aging procedure
	specified in Section 14.1.		specified in Section 14.1.
	The construction of each type of LSA is explained in detail		The construction of each type of LSA is explained in detail
	below. In general, these sections describe the contents of the		below. In general, these sections describe the contents of the
	LSA body (i.e., the part coming after the 20-byte LSA header).		LSA body (i.e., the part coming after the 20-byte LSA header).
	For information concerning the building of the LSA header, see		For information concerning the building of the LSA header, see
	Section 12.1.		Section 12.1.
			12.4.1. Router-LSAs
			A router originates a router-LSA for each area that it
			belongs to. Such an LSA describes the collected states of
			the router's links to the area. The LSA is flooded
			throughout the particular area, and no further.
			The format of a router-LSA is shown in Appendix A (Section
			A.4.2). The first 20 bytes of the LSA consist of the
			generic LSA header that was discussed in Section 12.1.
			router-LSAs have LS type = 1. The router indicates whether
			it is willing to calculate separate routes for each IP TOS
			by setting (or resetting) the T-bit of the LSA's Options
			field.
			A router also indicates whether it is an area border router,
			or an AS boundary router, by setting the appropriate bits
			(bit B and bit E, respectively) in its router-LSAs. This
			enables paths to those types of routers to be saved in the
			routing table, for later processing of summary-LSAs and AS-
			external-LSAs. Bit B should be set whenever the router is
			actively attached to two or more areas, even if the router
	....................................		....................................
	. 192.1.2 Area 1 .		. 192.1.2 Area 1 .
	. + .		. + .
	. | .		. | .
	. | 3+---+1 .		. | 3+---+1 .
	. N1 |--|RT1|-----+ .		. N1 |--|RT1|-----+ .
	. | +---+ \ .		. | +---+ \ .
	. | \ _______N3 .		. | \ _______N3 .
	. + \/ \ . 1+---+		. + \/ \ . 1+---+
	. * 192.1.1 *------|RT4|		. * 192.1.1 *------|RT4|
	skipping to change at page 115, line 30 ¶		skipping to change at page 116, line 29 ¶
	. | |RT3|----------------|RT6|		. | |RT3|----------------|RT6|
	. + +---+ . +---+		. + +---+ . +---+
	. 192.1.3 |2 . 18.10.0.6|7		. 192.1.3 |2 . 18.10.0.6|7
	. | . |		. | . |
	. +------------+ .		. +------------+ .
	. 192.1.4 (N4) .		. 192.1.4 (N4) .
	....................................		....................................
	Figure 15: Area 1 with IP addresses shown		Figure 15: Area 1 with IP addresses shown
	12.4.1. Router-LSAs
	A router originates a router-LSA for each area that it
	belongs to. Such an LSA describes the collected states of
	the router's links to the area. The LSA is flooded
	throughout the particular area, and no further.
	The format of a router-LSA is shown in Appendix A (Section
	A.4.2). The first 20 bytes of the LSA consist of the
	generic LSA header that was discussed in Section 12.1.
	router-LSAs have LS type = 1. The router indicates whether
	it is willing to calculate separate routes for each IP TOS
	by setting (or resetting) the T-bit of the LSA's Options
	field.
	A router also indicates whether it is an area border router,
	or an AS boundary router, by setting the appropriate bits
	(bit B and bit E, respectively) in its router-LSAs. This
	enables paths to those types of routers to be saved in the
	routing table, for later processing of summary-LSAs and AS-
	external-LSAs. Bit B should be set whenever the router is
	actively attached to two or more areas, even if the router
	is not currently attached to the OSPF backbone area. Bit E		is not currently attached to the OSPF backbone area. Bit E
	should never be set in a router-LSA for a stub area (stub		should never be set in a router-LSA for a stub area (stub
	areas cannot contain AS boundary routers).		areas cannot contain AS boundary routers).
	In addition, the router sets bit V in its router-LSA for		In addition, the router sets bit V in its router-LSA for
	Area A if and only if the router is the endpoint of one or		Area A if and only if the router is the endpoint of one or
	more fully adjacent virtual links having Area A as their		more fully adjacent virtual links having Area A as their
	Transit area. The setting of bit V enables other routers in		Transit area. The setting of bit V enables other routers in
	Area A to discover whether the area supports transit traffic		Area A to discover whether the area supports transit traffic
	(see TransitCapability in Section 6).		(see TransitCapability in Section 6).
	skipping to change at page 130, line 26 ¶		skipping to change at page 131, line 4 ¶
	floods each LSA one hop further from its point of origination. To		floods each LSA one hop further from its point of origination. To
	make the flooding procedure reliable, each LSA must be acknowledged		make the flooding procedure reliable, each LSA must be acknowledged
	separately. Acknowledgments are transmitted in Link State		separately. Acknowledgments are transmitted in Link State
	Acknowledgment packets. Many separate acknowledgments can also be		Acknowledgment packets. Many separate acknowledgments can also be
	grouped together into a single packet.		grouped together into a single packet.
	The flooding procedure starts when a Link State Update packet has		The flooding procedure starts when a Link State Update packet has
	been received. Many consistency checks have been made on the		been received. Many consistency checks have been made on the
	received packet before being handed to the flooding procedure (see		received packet before being handed to the flooding procedure (see
	Section 8.2). In particular, the Link State Update packet has been		Section 8.2). In particular, the Link State Update packet has been
	associated with a particular neighbor, and a particular area. If
	the neighbor is in a lesser state than Exchange, the packet should
	be dropped without further processing.
	All types of LSAs, other than AS-external-LSAs, are associated with
	+		+
	|		|
	+---+.....|.BGP		+---+.....|.BGP
	|RTA|-----|.....+---+		|RTA|-----|.....+---+
	+---+ |-----|RTX|		+---+ |-----|RTX|
	| +---+		| +---+
	+---+ |		+---+ |
	|RTB|-----|		|RTB|-----|
	+---+ |		+---+ |
	|		|
	+---+ |		+---+ |
	|RTC|-----|		|RTC|-----|
	+---+ |		+---+ |
	|		|
	+		+
	Figure 16: Forwarding address example		Figure 16: Forwarding address example
			associated with a particular neighbor, and a particular area. If
			the neighbor is in a lesser state than Exchange, the packet should
			be dropped without further processing.
			All types of LSAs, other than AS-external-LSAs, are associated with
	a specific area. However, LSAs do not contain an area field. An		a specific area. However, LSAs do not contain an area field. An
	LSA's area must be deduced from the Link State Update packet header.		LSA's area must be deduced from the Link State Update packet header.
	For each LSA contained in a Link State Update packet, the following		For each LSA contained in a Link State Update packet, the following
	steps are taken:		steps are taken:
	(1) Validate the LSA's LS checksum. If the checksum turns out to be		(1) Validate the LSA's LS checksum. If the checksum turns out to be
	invalid, discard the LSA and get the next one from the Link		invalid, discard the LSA and get the next one from the Link
	State Update packet.		State Update packet.
	skipping to change at page 139, line 31 ¶		skipping to change at page 140, line 4 ¶
	packets is described in Table 19. The circumstances surrounding		packets is described in Table 19. The circumstances surrounding
	the receipt of the LSA are listed in the left column. The		the receipt of the LSA are listed in the left column. The
	acknowledgment action then taken is listed in one of the two		acknowledgment action then taken is listed in one of the two
	right columns. This action depends on the state of the		right columns. This action depends on the state of the
	concerned interface; interfaces in state Backup behave		concerned interface; interfaces in state Backup behave
	differently from interfaces in all other states. Delayed		differently from interfaces in all other states. Delayed
	acknowledgments must be delivered to all adjacent routers		acknowledgments must be delivered to all adjacent routers
	associated with the interface. On broadcast networks, this is		associated with the interface. On broadcast networks, this is
	accomplished by sending the delayed Link State Acknowledgment		accomplished by sending the delayed Link State Acknowledgment
	packets as multicasts. The Destination IP address used depends		packets as multicasts. The Destination IP address used depends
	on the state of the interface. If the interface state is DR or
	Backup, the destination AllSPFRouters is used. In all other
	states, the destination AllDRouters is used. On non-broadcast
	networks, delayed Link State Acknowledgment packets must be
	unicast separately over each adjacency (i.e., neighbor whose
	state is >= Exchange).
	The reasoning behind sending the above packets as multicasts is
	best explained by an example. Consider the network
	configuration depicted in Figure 15. Suppose RT4 has been
	elected as Designated Router, and RT3 as Backup Designated
	Router for the network N3. When Router RT4 floods a new LSA to
	Network N3, it is received by routers RT1, RT2, and RT3. These
	routers will not flood the LSA back onto net N3, but they still
	must ensure that their link-state databases remain synchronized
	with their adjacent neighbors. So RT1, RT2, and RT4 are waiting
	to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
	both waiting to see acknowledgments from RT1 and RT2. This is
	best achieved by sending the acknowledgments as multicasts.
	The reason that the acknowledgment logic for Backup DRs is
	slightly different is because they perform differently during
	Action taken in state		Action taken in state
	Circumstances Backup All other states		Circumstances Backup All other states
	_______________________________________________________________		_______________________________________________________________
	LSA has No acknowledgment No acknowledgment		LSA has No acknowledgment No acknowledgment
	been flooded back sent. sent.		been flooded back sent. sent.
	out receiving in-		out receiving in-
	terface (see Sec-		terface (see Sec-
			tion 13, step 5b).
	_______________________________________________________________		_______________________________________________________________
	LSA is Delayed acknowledg- Delayed ack-		LSA is Delayed acknowledg- Delayed ack-
	more recent than ment sent if adver- nowledgment sent.		more recent than ment sent if adver- nowledgment sent.
	database copy, but tisement received		database copy, but tisement received
	was not flooded from Designated		was not flooded from Designated
	back out receiving Router, otherwise		back out receiving Router, otherwise
	interface do nothing		interface do nothing
	_______________________________________________________________		_______________________________________________________________
	LSA is a Delayed acknowledg- No acknowledgment		LSA is a Delayed acknowledg- No acknowledgment
	duplicate, and was ment sent if adver- sent.		duplicate, and was ment sent if adver- sent.
	treated as an im- tisement received		treated as an im- tisement received
	plied acknowledg- from Designated		plied acknowledg- from Designated
	ment (see Section Router, otherwise		ment (see Section Router, otherwise
			13, step 7a). do nothing
	_______________________________________________________________		_______________________________________________________________
	LSA is a Direct acknowledg- Direct acknowledg-		LSA is a Direct acknowledg- Direct acknowledg-
	duplicate, and was ment sent. ment sent.		duplicate, and was ment sent. ment sent.
	not treated as an		not treated as an
	implied ack-		implied ack-
	nowledgment.		nowledgment.
	_______________________________________________________________		_______________________________________________________________
	LSA's LS Direct acknowledg- Direct acknowledg-		LSA's LS Direct acknowledg- Direct acknowledg-
	age is equal to ment sent. ment sent.		age is equal to ment sent. ment sent.
	MaxAge, and there is		MaxAge, and there is
	no current instance		no current instance
	of the LSA		of the LSA
	in the link state		in the link state
	database (see		database (see
	Section 13, step 4).		Section 13, step 4).
	Table 19: Sending link state acknowledgements.		Table 19: Sending link state acknowledgements.
			on the state of the interface. If the interface state is DR or
			Backup, the destination AllSPFRouters is used. In all other
			states, the destination AllDRouters is used. On non-broadcast
			networks, delayed Link State Acknowledgment packets must be
			unicast separately over each adjacency (i.e., neighbor whose
			state is >= Exchange).
			The reasoning behind sending the above packets as multicasts is
			best explained by an example. Consider the network
			configuration depicted in Figure 15. Suppose RT4 has been
			elected as Designated Router, and RT3 as Backup Designated
			Router for the network N3. When Router RT4 floods a new LSA to
			Network N3, it is received by routers RT1, RT2, and RT3. These
			routers will not flood the LSA back onto net N3, but they still
			must ensure that their link-state databases remain synchronized
			with their adjacent neighbors. So RT1, RT2, and RT4 are waiting
			to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
			both waiting to see acknowledgments from RT1 and RT2. This is
			best achieved by sending the acknowledgments as multicasts.
			The reason that the acknowledgment logic for Backup DRs is
			slightly different is because they perform differently during
	the flooding of LSAs (see Section 13.3, step 4).		the flooding of LSAs (see Section 13.3, step 4).
	13.6. Retransmitting LSAs		13.6. Retransmitting LSAs
	LSAs flooded out an adjacency are placed on the adjacency's Link		LSAs flooded out an adjacency are placed on the adjacency's Link
	state retransmission list. In order to ensure that flooding is		state retransmission list. In order to ensure that flooding is
	reliable, these LSAs are retransmitted until they are		reliable, these LSAs are retransmitted until they are
	acknowledged. The length of time between retransmissions is a		acknowledged. The length of time between retransmissions is a
	configurable per-interface value, RxmtInterval. If this is set		configurable per-interface value, RxmtInterval. If this is set
	too low for an interface, needless retransmissions will ensue.		too low for an interface, needless retransmissions will ensue.
	skipping to change at page 225, line 43 ¶		skipping to change at page 226, line 43 ¶
	John Moy		John Moy
	Cascade Communications Corp.		Cascade Communications Corp.
	5 Carlisle Road		5 Carlisle Road
	Westford, MA 01886		Westford, MA 01886
	Phone: 508-952-1367		Phone: 508-952-1367
	Fax: 508-692-9214		Fax: 508-692-9214
	Email: jmoy@casc.com		Email: jmoy@casc.com
	This document expires in July 1997.		This document expires in August 1997.
End of changes. 53 change blocks.
	191 lines changed or deleted		208 lines changed or added
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