< draft-ietf-ospf-nssa-update-10.txt		draft-ietf-ospf-nssa-update-11.txt >
			A new Request for Comments is now available in online RFC libraries.
	Network Working Group R. Coltun		RFC 3101
	Internet Draft FORE Systems
	Expiration Date: June 2001 V. Fuller
	File name: draft-ietf-ospf-nssa-update-10.txt BBN Planet
	P. Murphy
	US Geological Survey
	December 2000
	The OSPF NSSA Option
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC2026. Internet-Drafts are working
	documents of the Internet Engineering Task Force (IETF), its areas,
	and its working groups. Note that other groups may also distribute
	working documents as Internet-Drafts.
	Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."
	The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/1id-abstracts.txt
	The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.
	Table Of Contents
	1.0 Abstract ................................................. 1
	2.0 Overview ................................................. 2
	2.1 Motivation - transit networks ............................ 2
	2.2 Motivation - corporate networks .......................... 3
	2.3 Proposed Solution ........................................ 4
	3.0 NSSA Intra-area Implementation Details ................... 6
	3.1 The N-bit ................................................ 6
	3.2 Type-7 Address Ranges .................................... 7
	3.3 Type-7 LSAs .............................................. 7
	3.4 Originating Type-7 LSAs .................................. 9
	3.5 Calculating Type-7 AS External Routes .................... 10
	3.6 Incremental Updates ...................................... 13
	3.7 Optionally Importing Summary LSAs ........................ 13
	4.0 Intra-AS implementation Details .......................... 14
	4.1 Type-7 Translator Election ............................... 14
	4.2 Translating Type-7 LSAs into Type-5 LSAs ................. 15
	4.3 Flushing Translated Type-7 LSAs .......................... 18
	5.0 Security Considerations .................................. 18
	6.0 Acknowledgments .......................................... 20
	7.0 References ............................................... 20
	8.0 Authors' Addresses ....................................... 21
	Appendix A: The Options Field ................................ 22
	Appendix B: Router-LSAs ...................................... 23
	Appendix C: Type-7 LSA Packet Format ......................... 25
	Appendix D: Configuration Parameters ......................... 26
	Appendix E: The P-bit Policy Paradox ......................... 27
	Appendix F: Differences from RFC 1587 ........................ 29
	1.0 Abstract
	This memo documents an optional type of OSPF area which is somewhat
	humorously referred to as a "not-so-stubby" area (or NSSA). NSSAs
	are similar to the existing OSPF stub area configuration option but
	have the additional capability of importing AS external routes in a
	limited fashion.
	The OSPF NSSA Option was originally defined in RFC 1587. The
	functional differences between this memo and RFC 1587 are explained
	in Appendix F. All differences, while expanding capability, are
	backward-compatible in nature. Implementations of this memo and of
	RFC 1587 will interoperate.
	Please send comments to ospf@discuss.microsoft.com.
	2.0 Overview
	2.1 Motivation - transit networks
	Wide-area transit networks often have connections to moderately-
	complex "leaf" sites. A leaf site may have multiple IP network
	numbers assigned to it. Typically, one of the leaf site's networks
	is directly connected to a router provided and administered by the
	transit network while the others are distributed throughout and
	administered by the site. From the transit network's perspective,
	all of the network numbers associated with the site make up a single
	"stub" entity. For example, BBN Planet has one site composed of a
	class-B network, 130.57.0.0, and a class-C network, 192.31.114.0.
	From BBN Planet's perspective, this configuration looks something
	like this:
	192.31.114
	|
	(cloud)
	-------------- 130.57.4
	|
	|
	------ 131.119.13 ------
	|BR18|------------|BR10|
	------ ------
	|
	V
	to BBN Planet "core" OSPF system
	where the "cloud" consists of the subnets of 130.57 and network
	192.31.114, all of which are learned by RIP on router BR18.
	Topologically, this cloud looks very much like an OSPF stub area.
	The advantages of running the cloud as an OSPF stub area are:
	1. Type-5 routes (OSPF external link state advertisements (LSAs))
	are not advertised beyond the router labeled "BR10". This is
	advantageous because the link between BR10 and BR18 may be a
	low-speed link or the router BR18 may have limited resources.
	2. The transit network is abstracted to the "leaf" router BR18 by
	advertising only a default route across the link between BR10
	and BR18.
	3. The cloud becomes a single, manageable "leaf" with respect to
	the transit network.
	4. The cloud can become, logically, a part of the transit
	network's OSPF routing system.
	However, the current definition of the OSPF protocol [OSPF] imposes
	topological limitations which restrict simple cloud topologies from
	becoming OSPF stub areas. In particular, it is illegal for a stub
	area to import routes external to OSPF; it is not possible for
	routers BR18 and BR10 to both be members of the stub area and to
	import the routes learned from RIP or other IP routing protocols as
	Type-5 (OSPF external LSAs) into the OSPF routing domain. In order
	to run OSPF out to BR18, BR18 must be a member of a non-stub area or
	the OSPF backbone before it can import routes other than its
	directly-connected network(s). Since it is not acceptable for BR18
	to maintain all of BBN Planet's external (Type-5) routes, BBN Planet
	is forced by OSPF's topological limitations to only run OSPF out to
	BR10 and to run RIP between BR18 and BR10.
	2.2 Motivation - corporate networks
	In a corporate network which supports a large corporate
	infrastructure it is not uncommon for OSPF area 0 to have a large
	non-zero area infrastructure which injects large routing tables into
	area 0. Organizations within the corporate infrastructure may
	routinely multi-home their non-0 OSPF areas to strategically located
	backbone area 0 routers, either to provide backbone redundancy or to
	increase backbone connectivity or both. Because of these large
	routing tables, OSPF aggregation via summarization is routinely used
	and recommended. Stub areas are also recommended to keep the size of
	the routing tables of non-backbone routers small. Organizations
	within the corporation are administratively autonomous and compete
	for corporate backbone resources. They also want isolation from each
	other in order to protect their own network resources within the
	organization.
	Consider a typical backbone connection, as shown on the next page,
	where routers A1, B1 and A2, B2 are connected to area 1 and area 2
	respectively, and where routers A0 and B0 are border routers which
	connect to both area 1 and area 2. Assume the 192.168.192/20 and
	192.168.208/22 clouds are subnetted with a protocol different from
	the corporate OSPF instance. These other protocols could be RIP,
	IGRP, or second and third OSPF instances separate from the corporate
	OSPF backbone instance.
	Area 1 and Area 2 would like to be stub areas to minimize the size of
	their link state data base. It is also desirable to aggregate the
	subnets of 192.168.192/20 and 192.168.208/22 each in a single
	advertisement to the backbone in such a way that the preferred path
	to 192.168.192/20 is through A0 and the preferred path to
	192.168.208/22 is through B0.
	+---A0------Area 0 cloud------B0---+
	| | | |
	| | | |
	| |T1 56kbs| |
	56kbs| | | |T1
	| | | |
	| | Area 1 cloud | |
	| A1-----192.168.192/20-----B1 |
	| |
	+---A2 B2---+
	| |
	| Area 2 cloud |
	+-----192.168.208/22------+
	The current standard OSPF stub area has no mechanism to support the
	redistribution of routes for the subnets of 192.168.192/20 and
	192.168.208/22 within a stub area or the ability to aggregate a range
	of external routes in any OSPF area. Any solution to this dilemma
	must also honor Area 1's path of choice to 192.168.192/20 through A0
	with redundancy through B0 while at the same time honoring Area 2's
	path of choice to 192.168.208/20 through B0 with redundancy through
	A0.
	2.3 Proposed Solution
	This document describes a new optional type of OSPF area, somewhat
	humorously referred to as a "not-so-stubby" area (or NSSA), which has
	the capability of importing external routes in a limited fashion.
	The OSPF specification defines two general classes of area
	configuration. The first allows Type-5 LSAs to be flooded throughout
	the area. In this configuration, Type-5 LSAs may be originated by
	routers internal to the area or flooded into the area by area border
	routers. These areas, referred to herein as Type-5 capable areas (or
	just plain areas in the OSPF specification), are distinguished by the
	fact that they can carry transit traffic. The backbone is always a
	Type-5 capable area. The second type of area configuration, called
	stub, allows no Type-5 LSAs to be propagated into/throughout the area
	and instead depends on default routing to external destinations.
	NSSAs are defined in much the same manner as existing stub areas. To
	support NSSAs, a new option bit (the "N" bit) and a new type of LSA
	(Type-7) are defined. The "N" bit ensures that routers belonging to
	an NSSA agree on its configuration. Similar to the stub area's use
	of the "E" bit, both NSSA neighbors must agree on the setting of the
	"N" bit or the OSPF neighbor adjacency will not form.
	Type-7 LSAs provide for carrying external route information within an
	NSSA. Type-7 AS external LSAs have virtually the same syntax as the
	Type-5 AS external LSAs with the obvious exception of the link-state
	type (see section 3.3 for more details). There are two major
	semantic differences between Type-5 and Type-7 LSAs.
	o Type-7 LSAs may be originated by and advertised throughout an
	NSSA; as with stub areas, Type-5 LSAs are not flooded into NSSAs
	and do not originate there.
	o Type-7 LSAs are advertised only within a single NSSA; they are
	not flooded into the backbone area or any other area by border
	routers, though the information which they contain may be
	propagated into the backbone area (see section 4.2).
	In order to allow limited exchange of external information across an
	NSSA border, NSSA border routers will translate selected Type-7 LSAs
	received from the NSSA into Type-5 LSAs. These Type-5 LSAs will be
	flooded to all Type-5 capable areas. NSSA border routers may be
	configured with address ranges so that several Type-7 LSAs may be
	aggregated into a single Type-5 LSA. The NSSA border routers which
	perform translation are configurable. In the absence of a configured
	translator one is elected.
	In addition, an NSSA border router should originate a default LSA (IP
	address of 0.0.0.0) into the NSSA. Default routes are necessary
	because NSSAs do not receive full routing information and must have a
	default route in order to route to AS-external destinations. Like
	stub areas, NSSAs may be connected to the backbone at more than one
	area border router, but may not be used as a transit area. Note that
	a Type-7 default route originated by an NSSA border router is never
	translated into a Type-5 LSA, however, a Type-7 default route
	originated by an NSSA internal AS boundary router (one that is not
	also an area border router) may be translated into a Type-5 LSA.
	Like stub areas, the importing of OSPF summary routes (Type-3 LSAs)
	into NSSAs is a configuration option. However particular care should
	be taken to ensure that OSPF internal routes are not obscured by OSPF
	external (Type-7) routes. This may happen when other IGPs, like RIP
	and ISIS, leak routing information between an OSPF NSSA and another
	OSPF area. In these cases, all OSPF summary routes should be
	imported into the effected NSSAs. The recommended default behavior
	is to import OSPF summary routes into NSSAs. Since AS-external-LSAs
	(Type-5) are not imported into NSSAs, NSSA border routers should not
	originate Type-4 summary-LSAs into their NSSAs.
	When summary routes are not imported, the default LSA originated by a
	NSSA border router into the NSSA should be a Type-3 summary LSA. The
	Type-3 summary default route insures intra-AS connectivity to the
	rest of the OSPF domain, as it takes precedence over any Type-7
	external default route which might originate on an NSSA internal
	router. This Type-3 summary default route prevents the OSPF domain's
	internal traffic, which is normally forwarded by OSPF summary routes,
	from exiting the AS via any NSSA Type-7 external default route
	originated by an NSSA internal router. Type-7 external defaults
	generated on NSSA internal routers and the no-summary option are
	mutually exclusive. When summary routes are imported into the NSSA,
	the default LSA originated by a NSSA border router into the NSSA
	should be a Type-7 LSA.
	One final note, NSSA border routers never generate Type-4 summary
	LSAs for their NSSA ASBRs as their Type-7 external advertisements are
	never flooded beyond the NSSA's borders.
	In our transit topology examples the subnets of 130.57 and network
	192.31.114 will still be learned by RIP on router BR18 but now both
	BR10 and BR18 can be in an NSSA and all of BBN Planet's external
	routes are hidden from BR18; BR10 becomes an NSSA border router and
	BR18 becomes an AS boundary router internal to the NSSA. BR18 will
	import the subnets of 130.57 and network 192.31.114 as Type-7 LSAs
	into the NSSA. BR10 then translates these routes into Type-5 LSAs
	and floods them into BBN Planet's backbone.
	In our corporate example, the subnets of 192.168.192/20 and
	192.168.208/22 are learned via their respective routing protocols,
	redistributed throughout NSSAs 1 and 2, and then aggregated during
	the translation process into a single Type-5 LSA which is flooded
	into Area 0. Area 1 may configure A0 to perform translation, while
	Area 2 configures B0 as its translator.
	3.0 NSSA Intra-area Implementation Details
	3.1 The N-bit
	The N-bit ensures that all members of an NSSA agree on the area's
	configuration. Together, the N-bit and E-bit reflect an interface's
	(and consequently the interface's associated area) external LSA
	flooding capability. As explained in section 10.5 of the OSPF
	specification, if Type-5 LSAs are not flooded into/throughout the
	area, the E-bit must be clear in the option field of the received
	Hello packets. Interfaces associated with an NSSA will not send or
	receive Type-5 LSAs on that interface but may send and receive Type-7
	LSAs. Therefore, if the N-bit is set in the options field, the E-bit
	must be clear.
	To support the NSSA option an additional check must be made in the
	function that handles the receiving of the Hello packet to verify
	that both the N-bit and the E-bit found in the Hello packet's option
	field match the value of the options that have been configured for
	the receiving interface. A mismatch in the options causes processing
	of the received Hello packet to stop and the packet to be dropped.
	3.2 Type-7 Address Ranges
	NSSA border routers may be configured with Type-7 address ranges.
	Each address range is defined as an [address,mask] pair. Many
	separate Type-7 networks may fall into a single address range, just
	as a subnetted network is composed of many separate subnets. NSSA
	border routers may aggregate Type-7 routes by advertising a single
	Type-5 LSA for each Type-7 address range. The Type-5 LSA resulting
	from a Type-7 address range match will be distributed to all Type-5
	capable areas. Section 4.2 gives the details of generating Type-5
	LSAs from Type-7 address ranges.
	A Type-7 address range includes the following configurable items.
	o An [address,mask] pair.
	o A status indication of either Advertise or DoNotAdvertise.
	o An external route tag.
	3.3 Type-7 LSAs
	External routes are imported into NSSAs as Type-7 LSAs by NSSA AS
	boundary routers. An NSSA AS boundary router (ASBR) is a router
	which has an interface associated with the NSSA and is exchanging
	routing information with routers belonging to another AS. Like OSPF
	ASBRs, an NSSA router indicates it is an NSSA ASBR by setting the E-
	bit in its router-LSA. As with Type-5 LSAs a separate Type-7 LSA is
	originated for each destination network. To support NSSAs, the
	link-state database must therefore be expanded to contain Type-7
	LSAs.
	Type-7 LSAs are identical to Type-5 LSAs except for the following
	(see section 12.4.4 "AS external links" in the OSPF specification).
	1. The type field in the LSA header is 7.
	2. Type-7 LSAs are only flooded within the originating NSSA. The
	flooding of Type-7 LSAs follows the same rules as the flooding
	of Type-1 and Type-2 LSAs.
	3. Type-7 LSAs are only listed within the OSPF area data
	structures of their respective NSSAs, making them area
	specific. Type-5 LSAs, which are flooded to all Type-5 capable
	areas, have global scope and are listed in the OSPF protocol
	data structure.
	4. At the NSSA border router, selected Type-7 LSAs are translated
	into type 5-LSAs and flooded into the OSPF domain's transit
	topology.
	5. Type-7 LSAs have a propagate (P) bit which, when set, tells an
	NSSA border router to translate the Type-7 LSA into a Type-5
	LSA. Examples of how the P-bit is used for loop avoidance are
	described in section 4.2.
	6. Those Type-7 LSAs that are to be translated into Type-5 LSAs
	must have their forwarding address set.
	Type-5 LSAs which are translations of Type-7 LSAs normally contain a
	forwarding address. The exception to this is when the Type-5 LSA is
	an aggregation of Type-7 LSAs, in which case the Type-5 LSA may not
	contain a forwarding address (see section 4.2 for details). The
	forwarding address contained in Type-5 LSAs results in more efficient
	routing to Type-7 AS external networks when there are multiple NSSA
	border routers. Moreover the forwarding address in a Type-7 LSA
	eases its translation into a Type-5 LSA, as the NSSA border router
	will not be required to compute the forwarding address.
	If the network between the NSSA AS boundary router and the adjacent
	AS is advertised into OSPF as an internal OSPF route, the forwarding
	address should be the next hop address as is currently done with
	Type-5 LSAs. If the intervening network is not advertised into OSPF
	as an internal OSPF route and the Type-7 LSA's P-bit is set, a
	forwarding address should be selected from one of the router's active
	OSPF interface addresses which belong to the NSSA. If no such
	addresses exist, then no Type-7 LSAs with the P-bit set should
	originate from this router.
	When a router is forced to pick a forwarding address for a Type-7
	LSA, precedence should be given first to the router's loopback
	addresses (provided internal addressing is supported). If a loopback
	address is not used and the selected forwarding address's interface
	transitions to a Down state (see OSPF Section 9.3), one must select a
	new forwarding address for any Type-7 LSAs which reference the
	previously selected forwarding address and then re-originate these
	Type-7 LSAs. If internal addresses are not available, preference
	should be given to the router's active OSPF stub network addresses to
	avoid the possible extra hop of a transit network's address.
	Type-5 and Type-7 metrics and path types are directly comparable.
	3.4 Originating Type-7 LSAs
	NSSA AS boundary routers may only originate Type-7 LSAs. All NSSA
	border routers must have the capability of translating Type-7 LSAs
	into Type-5 LSAs as described in Section 4.2. NSSA border routers
	must set bit E (external bit) in their router (Type-1) LSAs
	originated for directly attached non-stub areas.
	An NSSA internal AS boundary router must set the P-bit in the LSA
	header's option field of any Type-7 LSA whose path it wants
	advertised into the OSPF domain's full transit topology. The LSAs of
	these networks must have a valid non-zero forwarding address. If the
	P-bit is clear the LSA is not translated into a Type-5 LSA by NSSA
	border routers.
	When an NSSA border router originates both a Type-5 and a Type-7 LSA
	for the same network, the P-bit must be clear in the Type-7 NSSA so
	that the Type-7 LSA isn't again translated into a Type-5 LSA by
	another NSSA border router. If the border router only originates a
	Type-7 LSA, it may set the P-bit, thus allowing the network to be
	aggregated/propagated during Type-7 translation. If an NSSA
	originates a Type-5 LSA with a forwarding address which is part of
	the NSSA, it should also originate a Type-7 LSA into the NSSA. If two
	NSSA routers, both reachable from one another over the NSSA,
	originate functionally equivalent AS-external-LSAs (i.e., same
	destination, cost and non-zero forwarding address), then the router
	having the least preferred LSA should flush its LSA (See [OSPF]
	Section 12.4.4.1). Preference between two Type-7 LSAs is determined
	by the following tie breaker rules:
	1) An LSA with the p-bit set is preferred over one with the p-bit
	clear.
	2) If the p-bit settings are the same, the LSA with the higher
	router ID is preferred.
	A Type-7 default route (network 0.0.0.0) may be originated into the
	NSSA by any NSSA router. The Type-7 default route originated by the
	NSSA border router must have the P-bit clear. The Type-7 default
	route originated by an NSSA ASBR which is not an NSSA border router
	may have the P-bit set. A Type-7 default route may be installed by
	NSSA border routers if and only if its P-bit is set (see Appendix E).
	An LSA for the default destination must be originated by all the
	NSSA's border routers in order to support intra-AS routing and
	inter-AS routing. This default destination is advertised in either a
	Type-3 or Type-7 LSA, as described in the Section 3.7.
	3.5 Calculating Type-7 AS External Routes
	This calculation must be run when Type-7 LSAs are processed during
	the AS external route calculation. This calculation may process
	Type-5 LSAs, but it is written that way only for comparison sake.
	Non-default Type-7 LSAs with the P-bit clear may be installed in the
	OSPF routing table of NSSA border routers. Since these LSAs are not
	propagated throughout the OSPF domain, traffic which originates
	external to an NSSA and which passes through one of the NSSA's border
	routers may be unexpectedly diverted into the NSSA (See Appendix E).
	An NSSA border router should examine both Type-5 LSAs and Type-7 LSAs
	if either Type-5 or Type-7 routes need to be updated or recalculated.
	This is done as part of the AS external route calculation. An NSSA
	internal router should examine Type-7 LSAs when Type-7 routes need to
	be recalculated.
	What follows is only a modest modification of the OSPF Version 2
	Specification Section 16.4. Original text is suffixed with [OSPF].
	NSSA specific text is suffixed with [NSSA].
	AS external routes are calculated by examining AS-external-LSAs, be
	they Type-5 or Type-7. Each of the AS-external-LSAs is considered in
	turn. Most AS-external-LSAs describe routes to specific IP
	destinations. An AS-external-LSA can also describe a default route
	for the Autonomous System (Destination ID = DefaultDestination,
	network/subnet mask = 0x00000000). For each AS-external-LSA [~OSPF]:
	(1) If the metric specified by the LSA is LSInfinity, or if the
	age of the LSA equals MaxAge, then examine the next LSA.
	[OSPF]
	(2) If the LSA was originated by the calculating router itself,
	examine the next LSA. [OSPF]
	(3) Call the destination described by the LSA N. N's address is
	obtained by masking the LSA's Link State ID with the
	network/subnet mask contained in the body of the LSA. Look up
	the routing table entries which match the LSA's type for the
	AS boundary router (ASBR) that originated the LSA. For a
	Type-5 LSA, routing table entries may only be selected from
	each attached non-NSSA/non-stub area. Since the flooding
	scope of a Type-7 LSA is restricted to the originating NSSA,
	the routing table entry of its ASBR must be found in the
	originating NSSA. If no entries exist for the ASBR (i.e. the
	ASBR is unreachable over the transit topology for a Type-5
	LSA, or, for a Type-7 LSA, it is unreachable over its
	originating NSSA), do nothing with this LSA and consider the
	next in the list. [NSSA]
	Else if the destination is a Type-7 default route (destination
	ID = DefaultDestination) and one of the following is true,
	then do nothing with this LSA and consider the next in the
	list:
	o The calculating router is a border router and the LSA
	has its P-bit clear. Appendix E describes technique for
	border router Type-7 default installation without
	propagation. [NSSA]
	o The calculating router is suppressing the import of
	summary (Type-3) LSAs.
	Else, this LSA describes an AS external path to destination N.
	Examine the forwarding address specified in the AS-external-
	LSA. This indicates the IP address to which packets for the
	destination should be forwarded. [OSPF]
	If the forwarding address is set to 0.0.0.0 then packets
	should be sent to the ASBR itself. If the LSA is Type-5, from
	among the multiple non-NSSA routing table entries for the ASBR
	(both NSSA and non-NSSA ASBR entries might exists on an NSSA
	border router), select the preferred entry as follows [~OSPF]:
	If RFC1583Compatibility is set to "disabled", prune the
	set of routing table entries for the ASBR as described
	in OSPF Section 16.4.1. In any case, among the
	remaining routing table entries, select the routing
	table entry with the least cost; when there are multiple
	least cost routing table entries the entry whose
	associated area has the largest OSPF Area ID (when
	considered as an unsigned 32-bit integer) is chosen.
	[OSPF]
	Since a Type-7 LSA only has area-wide flooding scope, when its
	forwarding address is set to 0.0.0.0, its ASBR's routing table
	entry must be chosen from the originating NSSA. Here no
	pruning is necessary since this entry always contains non-
	backbone intra-area paths. [NSSA]
	If the forwarding address is non-zero look up the forwarding
	address in the routing table. For Type-5 LSAs the matching
	routing table entry must specify an intra-area or inter-area
	path. For Type-7 LSAs the matching routing table entry must
	specify an intra-area path through the originating NSSA. If no
	such path exists then do nothing with this LSA and consider
	the next in the list. [OSPF]
	(4) Let X be the cost specified by the preferred routing table
	entry for the ASBR/forwarding address, and Y the cost
	specified in the LSA. X is in terms of the link state metric,
	and Y is a type 1 or 2 external metric. [OSPF]
	(5) Now, look up the routing table entry for the destination N.
	If no entry exists for N, install the AS external path to N,
	with the next hop equal to the list of next hops to the
	ASBR/forwarding address, and advertising router equal to the
	ASBR. If the external metric type is 1, then the path-type is
	set to Type-1 external and the cost is equal to X + Y. If the
	external metric type is 2, the path-type is set to Type-2
	external, the link-state component of the route's cost is X,
	and the type 2 cost is Y. [OSPF]
	(6) Otherwise compare the AS external path described by the LSA
	with the existing paths in N's routing table entry. If the
	new path is preferred, it replaces the present paths in N's
	routing table entry. If the new path is of equal preference,
	it is added to N's routing table entry's list of paths. [OSPF]
	Preference is defined as follows:
	(a) Intra-area and inter-area paths are always preferred over
	AS external paths. [OSPF]
	(b) Type 1 external paths are always preferred over type 2
	external paths. When all paths are type 2 external paths,
	the paths with the smallest advertised type 2 metric are
	always preferred. [OSPF]
	(c) If the new AS external path is still indistinguishable
	from the current paths in N's routing table entry, and
	RFC1583Compatibility is set to "disabled", select the
	preferred paths based on the intra-AS paths to the
	ASBR/forwarding addresses, as specified in Section 16.4.1.
	Here intra-NSSA paths are equivalent to the intra-area
	paths of non-backbone regular OSPF areas. [NSSA]
	(d) If the new AS external path is still indistinguishable
	from the current paths in N's routing table entry, select
	the preferred path based on a least cost comparison. Type
	1 external paths are compared by looking at the sum of the
	distance to the forwarding address and the advertised type
	1 metric (X+Y). Type 2 external paths advertising equal
	type 2 metrics are compared by looking at the distance to
	the forwarding addresses. [OSPF]
	(e) If the current LSA is functionally the same as an
	installed LSA (i.e., same destination, cost and non-zero
	forwarding address) then apply the following priorities in
	deciding which LSA is preferred:
	a. A Type-7 LSA with the P-bit set.
	b. A Type-5 LSA.
	c. The LSA with the higher router ID. [NSSA]
	3.6 Incremental Updates
	Incremental updates for Type-7 LSAs should be treated the same as
	incremental updates for Type-5 LSAs (see section 16.6 of the OSPF
	specification). That is, if a new instance of a Type-7 LSA is
	received it is not necessary to recalculate the entire routing table.
	If there is already an OSPF internal route to the destination
	represented by the Type-7 LSA, no recalculation is necessary.
	Otherwise, the procedure in the proceeding section will have to be
	performed but only for the external routes (Type-5 and Type-7) whose
	networks describe the same networks as the newly received LSA.
	3.7 Optionally Importing Summary LSAs
	In order for backbone summary internal routes to be preferred over
	external Type-7 routes, all implementations must support the optional
	import of summary LSAs from the backbone into an NSSA. The default
	behavior is to import Type-3 summary LSAs. A new area configuration
	parameter, ImportSummaries, has been added. When set to enabled,
	Type-3 summary routes are imported. When set to disabled, summary
	routes are not imported. The default setting is enabled.
	When summary routes are not imported, the default LSA originated by a
	NSSA border router into the NSSA should be a Type-3 summary LSA.
	This protects the NSSA from routing intra-AS traffic out the AS via a
	Type-7 external default route originating from an internal NSSA
	router. Unlike the stub area case, when summary routes are imported
	into the NSSA, a Type-3 summary default route must not be injected
	into the NSSA, otherwise the Type-3 summary default route would be
	chosen over potentially more preferred Type-7 default routes.
	4.0 Intra-AS implementation Details
	4.1 Type-7 Translator Election
	It is not recommended that multiple NSSA border routers perform the
	translation unless the efficient routing of packets through area 0 to
	an NSSA partitioned by aggregation requires it. It is normally
	sufficient to have only one NSSA border router perform the
	translation. Excessive numbers of Type-7 translators unnecessarily
	increase the size of the OSPF link state data base.
	A new area configuration parameter, NSSATranslatorRole, is defined in
	Appendix D. It specifies whether or not an NSSA router will
	unconditionally translate Type-7 LSAs to Type-5 LSAs when acting as
	an NSSA border router. When set to Always, Type-7 LSAs are always
	translated regardless of the translator state of other NSSA border
	routers. When set to Candidate and acting as an NSSA border router,
	an NSSA router will participate in the translator election process
	described below.
	A new bit called Nt is added to the router-LSA. NSSA border routers,
	which are configured to unconditionally translate Type-7 LSAs into
	Type-5 LSAs, set bit Nt in their NSSA router-LSA. All other routers
	clear bit Nt in their NSSA router-LSAs.
	A new area parameter called the NSSATranslatorState is maintained in
	the OSPF area data structure. By default all NSSA routers initialize
	NSSATranslatorState to disabled. When an NSSA router attains border
	router status and has its NSSATranslatorRole set to Always, it sets
	NSSATranslatorState to enabled and begins the unconditional
	translation of Type-7 LSAs into Type-5 LSAs for the NSSA. When an
	NSSA border router loses its border router status,
	NSSATranslatorState is always reset to disabled and the Nt bit is
	cleared in a new router LSA.
	If an NSSA border router has its NSSATranslatorState set to disabled
	and, from the subset of NSSA border routers which are reachable over
	the NSSA and reachable as ASBRs over the AS's transit topology, no
	such router exists either with bit Nt set in its router-LSA or with
	higher router ID, then this router begins to perform the translation
	of Type-7 LSAs into Type-5 LSAs for the NSSA and it sets
	NSSATranslatorState to elected. The Nt bit of an elected translator
	is always clear. These conditions may result in more than one
	elected translator for the NSSA, should one of the NSSA border
	routers lose connectivity to area 0.
	All NSSA border routers must set the E-bit in their router-LSA to
	directly attached non-stub areas even when they are not translating.
	This allows other NSSA border routers to see their ASBR status across
	the AS's transit topology. Failure to do so may cause the election
	algorithm to elect unnecessary translators. Every NSSA border router
	is a potential translator.
	An elected translator will continue to perform translation duties
	until supplanted by a reachable NSSA border router whose Nt bit is
	set to true or whose router ID is greater. Such an event might be
	triggered by the manual setting of the NSSATranslatorState to enabled
	in one of the NSSA border routers or a topological rejoining of a
	partitioned NSSA. Any change in the membership of the reachable NSSA
	border router set, both over the NSSA and as ASBRs over the AS's
	transit topology, or a change in a router-LSA's Nt bit setting should
	force an NSSA border router to recheck its Type-7 translation status.
	If an elected translator determines its services are no longer
	required, it should continue to perform its translation duties for
	the additional time interval defined by a new area configuration
	parameter, TranslatorStabilityInterval. This minimizes excessive
	flushing of translated Type-7 LSAs and provides for a more stable
	translator transition. The default value for the
	TranslatorStabilityInterval parameter has been defined as 40 seconds
	(see Appendix D).
	Configuring the identity of the translator can be used to bias the
	routing to aggregated destinations.
	4.2 Translating Type-7 LSAs into Type-5 LSAs
	This step is performed as part of the NSSA's Dijkstra calculation
	after Type-5 and Type-7 routes have been calculated. If the
	calculating router is currently not an NSSA border router translator,
	then this translation algorithm should be skipped. Only installed
	Type-7 LSAs and those non-default Type-7 LSAs originated by the
	router itself should be examined. Locally sourced Type-7 LSAs should
	be processed first.
	Note that it is possible for a Type-5 LSA generated by translation to
	supplant a Type-5 LSA originating from a local OSPF external source.
	Future reoriginations of the locally source Type-5 LSA should be
	suppressed until the Type-5 LSA generated by translation is flushed.
	A Type-7 LSA and a Type-7 range best match one another if there does
	not exist a more specific Type-7 range which contains the Type-7 LSA.
	For each translation eligible Type-7 LSA perform the following:
	(1) If the Type-7 LSA has the P-bit clear, or its forwarding
	address is set to 0.0.0.0, or the most specific Type-7 range
	which subsumes the network has DoNotAdvertise status, then do
	nothing with this Type-7 LSA and consider the next one in the
	list. Otherwise term the LSA as translatable and proceed with
	step (2).
	(2) If the Type-7 LSA is not contained in any explicitly
	configured Type-7 address range and the calculating router has
	the highest router ID amongst NSSA translators which have
	originated a functionally equivalent Type-5 LSA (i.e. same
	destination, cost and non-zero forwarding address) and which
	are reachable over area 0 and the NSSA, then a Type-5 LSA
	should be generated if there currently is no Type-5 LSA
	originating from this router corresponding to the Type-7 LSA's
	network or there is an existing Type-5 LSA and either it
	corresponds to a local OSPF external source whose path type
	and metric is less preferred (see Section 3.5 step (6), parts
	(b) and (d)) or it doesn't and the Type-5 LSA's path type or
	cost(s) have changed (See Section 3.5 step (5)) or the
	forwarding address no longer maps to a translatable Type-7
	LSA.
	The newly originated Type-5 LSA will describe the same network
	and have the same network mask, path type, metric, forwarding
	address and external route tag as the Type-7 LSA. The
	advertising router field will be the router ID of this area
	border router. The link-state ID is equal to the LSA's
	network address (in the case of multiple originations of
	Type-5 LSAs with the same network address but different mask,
	the link-state ID can also have one or more of the range's
	"host" bits set).
	(3) Else the Type-7 LSA must be aggregated by the most specific
	Type-7 range which subsumes it. If this Type-7 range has the
	same [address,mask] pair as the Type-7 LSA's network and the
	LSA is the only Type-7 LSA which best matches this range, then
	flag the Type-7 LSA as not contained in any explicitly
	configured Type-7 address range and continue processing the
	LSA as described in step (2). Otherwise compute the path type
	and metric for this Type-7 range as described below.
	The path type and metric of the Type-7 range is determined
	from the path types and metrics of those translatable Type-7
	LSAs which best match the range plus any locally sourced
	Type-5 LSAs whose network has the same [address,mask] pair.
	If any of these LSAs have a path type of 2 the range's path
	type is 2, otherwise it is 1. If the range's path type is 1
	its metric is the highest cost amongst its LSAs; if the
	range's path type is 2 its metric is the highest Type-2 cost +
	1 amongst its LSAs (See Section 3.5 step (5)). 1 is added to
	the Type-2 cost to ensure that the translated Type-5 does not
	appear closer on the NSSA border than a translatable Type-7
	LSA whose network has the same [address,mask] pair and Type-2
	cost.
	A Type-5 LSA is generated from the Type-7 range when there
	currently is no Type-5 LSA originated by this router whose
	network has the same [address,mask] pair as the range or there
	is but either its path type or metric has changed or its
	forwarding address is non-zero.
	The newly generated Type-5 LSA will have link-state ID equal
	to the Type-7 range's address (in the case of multiple
	originations of Type-5 LSAs with the same network address but
	different mask, the link-state ID can also have one or more of
	the range's "host" bits set). The advertising router field
	will be the router ID of this area border router. The network
	mask and the external route tag are set to the Type-7 range's
	configured values. The forwarding address is set to 0.0.0.0.
	The path type and metric are set to the Type-7 range's path
	type and metric as defined above.
	The pending processing of other translation eligible Type-7
	LSAs which best match this Type-7 range is suppressed. Thus
	at most a single Type-5 LSA is originated for each Type-7
	range.
	For example, given a Type-7 range of [10.0.0.0, 255.0.0.0] which
	subsumes the following Type-7 routes:
	10.1.0.0 path type 1, metric 10
	10.2.0.0 path type 1, metric 11
	10.3.0.0 path type 2, metric 5
	a Type-5 LSA would be generated with a path type of 2 and a metric 6.
	Given a Type-7 range of [10.0.0.0, 255.0.0.0] which subsumes the
	following Type-7 routes:
	10.1.0.0 path type 1, metric 10
	10.2.0.0 path type 1, metric 11
	10.3.0.0 path type 1, metric 5
	a Type-5 LSA will be generated with a path type of 1 and a metric 11.
	These Type-7 range metric and path type rules will avoid routing
	loops in the event that path types 1 and 2 are both used within the
	area.
	As with all newly originated Type-5 LSAs, a Type-5 LSA that is the
	result of a Type-7 LSA translation or aggregation is flooded to all
	attached Type-5 capable areas.
	Like Type-3 ranges, a Type-7 range performs the dual function of
	setting propagation policy via its Advertise/DoNotAdvertise parameter
	and aggregation via its network address and mask pair. Unlike Type-3
	summary links, Type-5 translation may result in more efficient
	routing when the forwarding address is set, as is done during step
	(2) of the translation procedure.
	Another important feature of this translation process is that it
	allows a Type-7 range to apply different properties (aggregation,
	forwarding address, and Advertise/DoNotAdvertise status) for the
	Type-7 routes it subsumes, versus those Type-7 routes subsumed by
	other more specific Type-7 ranges contained by the Type-7 range.
	4.3 Flushing Translated Type-7 LSAs
	If an NSSA border router has either translated or aggregated an
	installed Type-7 LSA into a Type-5 LSA which should no longer be
	translated or aggregated, then the Type-5 LSA should either be
	flushed or reoriginated as an aggregation of other Type-7 LSAs.
	If an NSSA border router is translating Type-7 LSA's into Type-5
	LSA's with
	NSSATranslatorState = elected
	and the NSSA border router has determined that its translator
	election status has been deposed by another NSSA border router, then,
	as soon as the TranslatorStabilityInterval has expired without the
	router reelecting itself as a translator, Type-5 LSAs generated by
	translating Type-7 address ranges are flushed. Conversely Type-5
	LSAs generated by translating Type-7 LSAs are not immediately
	flushed, but are allowed to remain in the OSPF routing domain as if
	the originator is still an elected translator. This minimizes the
	impact of an NSSA which changes its translator frequently.
	5.0 Security Considerations
	There are two types of issues that need be addressed when looking at
	protecting routing protocols from misconfigurations and malicious
	attacks. The first is authentication and certification of routing
	protocol information. The second is denial of service attacks
	resulting from repetitive origination of the same router
	advertisement or origination of a large number of distinct
	advertisements resulting in database overflow. Note that both of
	these concerns exist independently of a router's support for the NSSA
	option.
	The OSPF protocol addresses authentication concerns by authenticating
	OSPF protocol exchanges. OSPF supports multiple types of
	authentication; the type of authentication in use can be configured
	on a per network segment basis. One of OSPF's authentication types,
	namely the Cryptographic authentication option, is believed to be
	secure against passive attacks and provides significant protection
	against active attacks. When using the Cryptographic authentication
	option, each router appends a "message digest" to its transmitted
	OSPF packets. Receivers then use the shared secret key and the
	received digest to verify that each received OSPF packet is
	authentic.
	The quality of the security provided by the Cryptographic
	authentication option depends completely on the strength of the
	message digest algorithm (MD5 is currently the only message digest
	algorithm specified), the strength of the key being used, and the
	correct implementation of the security mechanism in all communicating
	OSPF implementations. It also requires that all parties maintain the
	secrecy of the shared secret key. None of the standard OSPF
	authentication types provide confidentiality. Nor do they protect
	against traffic analysis. For more information on the standard OSPF
	security mechanisms, see Sections 8.1, 8.2, and Appendix D of [OSPF].
	[DIGI] describes the extensions to OSPF required to add digital
	signature authentication to Link State data and to provide a
	certification mechanism for router data. [DIGI] also describes the
	added LSA processing and key management as well as a method for
	migration from or co-existence with standard OSPF V2.
	Repetitive origination of advertisements are addressed by OSPF by
	mandating a limit on the frequency that new instances of any
	particular LSA can be originated and accepted during the flooding
	procedure. The frequency at which new LSA instances may be
	originated is set to once every MinLSInterval seconds, whose value is
	5 seconds (see Section 12.4 of [OSPF]). The frequency at which new
	LSA instances are accepted during flooding is once every MinLSArrival
	seconds, whose value is set to 1 (see Section 13, Appendix B and G.1
	of [OSPF]).
	Proper operation of the OSPF protocol requires that all OSPF routers
	maintain an identical copy of the OSPF link state database. However,
	when the size of the link state database becomes very large, some
	routers may be unable to keep the entire database due to resource
	shortages; this is termed "database overflow". When database
	overflow is anticipated, the routers with limited resources can be
	accommodated by configuring OSPF stub areas and NSSAs. [OVERFLOW]
	details a way of gracefully handling unanticipated database
	overflows.
	6.0 Acknowledgments
	This document was produced by the OSPF Working Group, chaired by John
	Moy.
	In addition, the comments of the following individuals are also
	acknowledged:
	Phani Jajjarvarpu cisco
	Dino Farinacci cisco
	Jeff Honig Cornell University
	Acee Lindem IBM
	John Moy Sycamore Networks, Inc.
	Antoni Przygienda Redback Networks, Inc
	Doug Williams IBM
	Alex Zinin cisco
	7.0 References
	[DIGI] S. Murphy, M. Badger, B. Wellington, "OSPF with Digital
	Signatures", RFC 2154, Trusted Information Systems, June 1997.
	[MUEX] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon,
	Inc., March 1994.
	[OSPF] Moy, J., "OSPF Version 2", RFC 2328, Cascade Communications
	Corp., April 1998.
	[OPAQUE] Coltun, Rob, "The OSPF Opaque LSA Option", RFC 2370, FORE
	Systems, July 1998.
	[OVERFLOW] Moy, J., "OSPF Database Overflow", RFC 1765, Cascade,
	March 1995.
	8.0 Authors' Addresses
	This update uses much of the original text from RFC 1587 authored by
	Rob Coltun
	Redback Networks, Inc.
	1195 Borregas Avenue
	Sunnyvale, CA 94089
	Phone: (408) 548-3947
	EMail: rcoltun@redback.com
	Vince Fuller
	GTE Internetworking
	3801 East Bayshore Road
	Palo Alto, California 94303
	Phone: (415) 528-7227
	EMail: vaf@BBNPlanet.com
	New sections, edits and revisions have been added by
	Pat Murphy
	US Geological Survey
	345 Middlefield Road
	Menlo Park, California 94560
	Phone: (415) 329-4044
	EMail: pmurphy@noc.doi.net
	Appendix A: The Options Field
	The OSPF options field is present in OSPF Hello packets, Database
	Description packets and all link state advertisements. See appendix A.2 in
	[OSPF] and [OPAQUE] for a description of the options field. Six bits
	are assigned but only two (the E-bit and the N/P bit) are described
	completely in this section.
	--------------------------------------
	| * | O | DC | EA | N/P | MC | E | * |
	--------------------------------------
	The Type-7 LSA options field
	E-bit: Type-5 AS external LSAs are not flooded into/through OSPF
	stub areas and NSSAs. The E-bit ensures that all members
	of a stub area agree on that area configuration. The E-
	bit is meaningful only in OSPF Hello and Database
	Description packets. When the E-bit is clear in the Hello
	packet sent out a particular interface, it means that the
	router will neither send nor receive Type-5 AS external
	LSAs on that interface (in other words, the interface
	connects to a stub area or NSSA). Two routers will not
	become neighbors unless they agree on the state of the E-
	bit.
	N-bit: The N-bit describes the router's NSSA capability. The N-
	bit is used only in Hello packets and ensures that all
	members of an NSSA agree on that area's configuration.
	When the N-bit is set in the Hello packet and sent out a
	particular interface, it means that the router will send
	and receive Type-7 LSAs on that interface. Two routers
	will not form an adjacency unless they agree on the state
	of the N-bit. If the N-bit is set in the options field,
	the E-bit must be clear.
	P-bit: The P-bit is used only in the Type-7 LSA header. It flags
	the NSSA border router to translate the Type-7 LSA into a
	Type-5 LSA. The default setting for the P-bit is clear.
	Appendix B: Router-LSAs
	Router-LSAs are the Type-1 LSAs. Each router in an area originates a
	router-LSA. The LSA describes the state and cost of the router's
	links (i.e., interfaces) to the area. All of the router's links to
	the area must be described in a single router-LSA. For details
	concerning the construction of router-LSAs, see the OSPF
	Specification, Section 12.4.1.
	0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LS age | Options | 1 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Link State ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Advertising Router |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LS sequence number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| LS checksum | length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| 0 Nt|W|V|E|B| 0 | # links |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Link ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Link Data |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Type | # TOS | TOS 0 metric |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| TOS | 0 | metric |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ... |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| TOS | 0 | metric |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Link ID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Link Data |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ... |
	In router-LSAs, the Link State ID field is set to the router's OSPF
	Router ID. The T-bit is set in the LSA's Option field if and only if
	the router is able to calculate a separate set of routes for each IP
	TOS. Router-LSAs are flooded throughout a single area only.
	bit V
	When set, the router is an endpoint of one or more fully
	adjacent virtual links having the described area as Transit
	area (V is for virtual link endpoint).
	bit E
	When set, the router is an AS boundary router (E is for
	external). ALL NSSA border routers must set bit E in the
	router-LSAs to directly attached standard areas and NSSAs.
	(See Section 4.1 for details).
	bit B
	When set, the router is an area border router (B is for
	border).
	bit W
	When set, the router is a wild-card multicast receiver (W is
	for wild).
	bit Nt
	When set, the router is an NSSA border router which is
	unconditionally translating Type-7 LSAs into Type-5 LSAs (Nt
	is for NSSA translation). Note that such routers have their
	NSSATranslatorRole area configuration parameter set to Always
	(See Appendix D and Section 4.1).
	The remainder of the router links specification is as defined in the
	OSPF Specification, Section A.4.2.
	Appendix C: Type-7 LSA Packet Format
	0 32
	------------------------------------
	| | Options | 7 |
	| -------------------
	| Link-State Header |
	| |
	------------------------------------
	| Network Mask |
	------------------------------------ ______
	|E| TOS | metric | .
	------------------------------------ . repeated for each TOS
	| Forwarding Address | .
	------------------------------------ .
	| External Route Tag | ______
	------------------------------------
	The definitions of the link-state ID, network mask, metrics and
	external route tag are the same as the definitions for Type-5 LSAs
	(see Appendix A.4.5 in the [OSPF]), except for the forwarding address
	and the N/P-bit. The Options field must have the N/P bit set as
	described in Appendix A when the originating router desires that the
	external route be propagated throughout the OSPF domain.
	Forwarding address
	Data traffic for the advertised destination will be forwarded to
	this address. If the forwarding address is set to 0.0.0.0, data
	traffic will be forwarded to the LSA's originator (i.e., the
	responsible NSSA AS boundary router). If the P-bit is set the
	forwarding address must be non-zero. If the network between the
	NSSA AS boundary router and the adjacent AS is advertised into the
	NSSA as an internal OSPF route, the forwarding address should be
	the next hop address. If the intervening network is not
	advertised into the NSSA as an internal OSPF route, the forwarding
	address should be any one of the router's active OSPF interface
	addresses (see Section 3.3 for details).
	Appendix D: Configuration Parameters
	Appendix C.2 in the OSPF specification lists the area
	configuration parameters. The area ID and the list of address
	ranges for Type-3 summary routes remain unchanged. Section 3.2 of
	this document lists the configuration parameters for Type-7
	address ranges. The following area configuration parameters have
	been added:
	NSSATranslatorRole
	Specifies whether or not the router will unconditionally
	translate Type-7 LSAs to Type-5 LSAs when acting as an NSSA
	border router. When set to Always, Type-7 LSAs are always
	translated regardless of the translator state of other NSSA
	border routers. When set to Candidate and acting as an NSSA
	border router, it participates in the translator election
	process described in Section 4.1. The default setting is
	Candidate.
	TranslatorStabilityInterval
	Defines the length of time an elected Type-7 translator will
	continue to perform its translator duties once it has
	determined that translator status has been deposed by another
	NSSA border router translator as described in Section 4.1 and
	4.3. The default setting is 40 seconds.
	ImportSummaries
	When set to enabled, Type-3 summary LSAs are imported into the
	NSSA. When set to disabled, Type-3 summary LSAs are not
	imported into the NSSA. The default setting is enabled.
	Implementations must provide a vehicle for setting the P-bit of
	external routes imported into the NSSA as Type-7 LSAs. Without
	configuration, the default setting of the P-bit is clear (see Section
	3.3 and Appendix B).
	For NSSAs the ExternalRoutingCapability area configuration parameter
	must be set to accept Type-7 external routes. Additionally there
	must be a way of configuring an NSSA border router to advertise a
	default route into the NSSA with configurable metric type (1 or 2)
	and cost.
	Appendix E: The P-bit Policy Paradox.
	Non-default Type-7 LSAs with the P-bit clear may be installed in the
	OSPF routing table of NSSA border routers (see Section 3.5). These
	LSAs are not propagated throughout the OSPF domain as translated
	Type-5 LSAs (see Section 4.2). Thus traffic which is external to an
	NSSA and which passes through one of the NSSA's border routers may be
	hijacked into the NSSA by a route installed from a Type-7 LSA with
	the P-bit clear. This may be contrary to the expected path at the
	source of the traffic. It may also violate the routing policy
	intended by the Type-7 LSA's clear P-bit. A Type-7 range configured
	with DoNotAdvertise exhibits the same paradox for any installed
	Type-7 LSAs it subsumes.
	This paradox is best illustrated by the following example. Consider
	an OSPF domain (AS 1842) with connections for default Internet
	routing and to external AS 4156. NSSA 1 and OSPF Area 2 are
	partially defined in the following diagram:
	AS 4156
	|
	Area 2 |
	|
	A2 A0 Area 0 C0-----Internet
	| | | Default
	| | |
	| | |
	+-----------------B0---------------+
	/\
	/ \
	/ \
	Internet------------A1 B1------AS 4156 (p-bit clear)
	Default (p-bit set)
	NSSA 1
	Here A0, B0, and C0 are Area 0 routers, A1 and B1 are NSSA 1 routers,
	and A2 is an Area 2 router. B0 is a border router for both NSSA 1
	and Area 2.
	If NSSA 1 routes for AS 4156 are installed on B0 so that the NSSA 1
	tree below A1 can take advantage of it, then A2's traffic to AS 4156
	is hijacked through B0 by B1, rather than its computed path through
	A0.
	The P-bit paradox can also appear with default routes. By setting
	the P-bit on a Type-7 default LSA, and configuring DoNotAdvertise for
	[0,0] in an NSSA's border routers, the default route can be installed
	but not propagated. In the example above, if A1's default is
	installed on B0 and the range [0,0] has DoNotAdvertise set, then A2's
	default bound traffic is hijacked through B0 by A1 rather than the
	computed path through C0.
	Appendix F: Differences from RFC 1587
	This section documents the differences between this memo and RFC
	1587. All differences are backward-compatible. Implementations of
	this memo and of RFC 1587 will interoperate.
	F.1 Enhancements to OSPF summary LSAs. .
	The flooding of backbone summary LSAs (Type-3 LSAs) into the NSSA
	is now optional. In RFC 1587 the flooding of backbone summary
	LSAs was mandated in order to guarantee inter-area routes are
	preferred over external routes. The current recommended default
	behavior is to import summary LSAs. When summary routes are not
	imported, the default LSA originated by a NSSA border router into
	the NSSA should be a Type-3 summary LSA.
	See Sections 2.2 and 3.4 for details.
	F.2 Changes to Type-7 LSAs.
	The setting of the forwarding address in Type-7 LSAs has been
	further refined.
	See Section 3.3 for details.
	F.3 Changes to the Type-7 AS external routing calculation.
	The Type-7 external route calculation has been revised. Most
	notably:
	o The path preference defined in OSPF Section 16.4.1 has been
	included.
	o A Type-7 default route with the P-bit clear will not be
	installed on an NSSA border router. This protects the
	default routing of other OSPF Areas. (See Appendix E.)
	See Section 3.5 for details.		Title: The OSPF Not-So-Stubby Area (NSSA) Option
			Author(s): P. Murphy
			Status: Standards Track
			Date: January 2003
			Mailbox: pmurphy@noc.usgs.net
			Pages: 33
			Characters: 76243
			Obsoletes: 1587
	F.4 Changes to translating Type-7 LSAs into Type-5 LSAs		I-D Tag: draft-ietf-ospf-nssa-update-11.txt
	The translator election algorithm of RFC 1587 has been updated to		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3101.txt
	close a bug which results when the translator with the highest
	router ID loses connectivity to the AS's transit topology. The
	default translator election process occurs only in the absence of
	an existing translator.
	The identity of the translator is optionally configurable, with		This memo documents an optional type of Open Shortest Path First
	more than one allowed. This allows the network designer to		(OSPF) area that is somewhat humorously referred to as a
	choose the most cost effective intra-AS route for NSSA originated		"not-so-stubby" area (or NSSA). NSSAs are similar to the existing
	Type-5 LSA aggregations of Type-7 LSAs.		OSPF stub area configuration option but have the additional capability
			of importing AS external routes in a limited fashion.
	Self-originated non-default Type-7 LSAs are now included in the		The OSPF NSSA Option was originally defined in RFC 1587. The
	translation process.		functional differences between this memo and RFC 1587 are explained
			in Appendix F. All differences, while expanding capability, are
			backward-compatible in nature. Implementations of this memo and of
			RFC 1587 will interoperate.
	The translation process has been strengthened to close some of		This document is a product of the Open Shortest Path First IGP Working
	the weak points of RFC 1587.		Group of the IETF.
	See Sections 4.1 and 4.2 for details.		This is now a Proposed Standard Protocol.
	F.5 Changes to flushing translated Type-7 LSAs		This document specifies an Internet standards track protocol for
			the Internet community, and requests discussion and suggestions
			for improvements. Please refer to the current edition of the
			"Internet Official Protocol Standards" (STD 1) for the
			standardization state and status of this protocol. Distribution
			of this memo is unlimited.
	An NSSA border router, which was elected by the augmented RFC		This announcement is sent to the IETF list and the RFC-DIST list.
	1587 translator selection process defined in Section 4.1 and has		Requests to be added to or deleted from the IETF distribution list
	been deposed from translation duties by another NSSA border		should be sent to IETF-REQUEST@IETF.ORG. Requests to be
	router, flushes its self-originated Type-5 LSAs that resulted		added to or deleted from the RFC-DIST distribution list should
	from the aggregation of Type-7 LSAs. This prevents these		be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
	obsolete aggregations from short circuiting the preferred path
	through the new translator(s). A deposed translator continues to
	maintain its self-originated Type-5 LSAs resulting from
	translation until they age out normally.
	See Section 4.3 for details.		Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
			an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
			help: ways_to_get_rfcs. For example:
	F.6 P-bit additions		To: rfc-info@RFC-EDITOR.ORG
			Subject: getting rfcs
	The P-bit default has been defined as clear. RFC 1587 had no		help: ways_to_get_rfcs
	default setting. ( See Appendix C)
	A discussion on the packet forwarding impact of installing Type-7		Requests for special distribution should be addressed to either the
	LSAs with the P-bit clear on NSSA border routers has been added		author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
	as Appendix E.		specifically noted otherwise on the RFC itself, all RFCs are for
			unlimited distribution.echo
			Submissions for Requests for Comments should be sent to
			RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
			Authors, for further information.
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