< draft-ietf-isis-wg-255adj-00.txt		draft-ietf-isis-wg-255adj-01.txt >
	Internet Engineering Task Force T. Przygienda		Internet Engineering Task Force T. Przygienda
	INTERNET DRAFT Bell Labs		INTERNET DRAFT Bell Labs
	1 Nov 1998		1 Jun 1999
	Maintaining more than 255 adjacencies in IS-IS		Maintaining more than 255 circuits in IS-IS
	<draft-ietf-isis-wg-255adj-00.txt>		<draft-ietf-isis-wg-255adj-01.txt>
	Status of This Memo		Status of This Memo
	This document is an Internet Draft, and can be found as		This document is an Internet-Draft and is in full conformance with
	draft-ietf-isis-255adj-00.txt in any standard internet drafts		all provisions of Section 10 of RFC2026.
	repository. 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		Internet-Drafts are working documents of the Internet Engineering
	months. Internet Drafts may be updated, replaced, or obsoleted by		Task Force (IETF), its areas, and its working groups. Note
	other documents at any time. It is not appropriate to use Internet		that other groups may also distribute working documents as
	Drafts as reference material, or to cite them other than as a		Internet-Drafts.
	``working draft'' or ``work in progress.''		Internet-Drafts are draft documents valid for a maximum of six months
	Please check the I-D abstract listing contained in each Internet		and may be updated, replaced, or obsoleted by other documents at
	Draft directory to learn the current status of this or any other		any time. It is inappropriate to use Internet- Drafts as reference
	Internet Draft.		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.
	Abstract		Abstract
	This draft describes an optional implementation technique within		This draft describes an optional implementation technique within
	IS-IS [ISO90, Cal90a, Cal90b] used today by several ISPs for routing		IS-IS [ISO90, Cal90a, Cal90b] used today by several ISPs for routing
	within their clouds. IS-IS is an interior gateway routing protocol		within their clouds. IS-IS is an interior gateway routing protocol
	developed originally by OSI and used with IP extensions as IGP.		developed originally by OSI and used with IP extensions as IGP.
	This draft describes how to effectively bypass the problem of 255		This draft describes how to effectively bypass the problem of 255
	circuits that IS-IS allows to maintain with minor violations of the		circuits that IS-IS allows to maintain with minor violations of the
	specification that however does not prevent interoperability with		specification that however does not prevent interoperability with
	existing implementations.		existing implementations.
	1. Introduction		1. Introduction
	IS-IS reserves within its packets only one byte for a circuit ID		IS-IS reserves within its packets only one byte for a circuit ID
	and the specification requests it to be unique. This ID is used in		and the specification requests it to be unique. This ID is used in
	broadcast networks to identify a PNode. They don't seem to serve any		broadcast networks to identify a PNode. They don't seem to serve any
	particular purpose on p2p networks except for some checking in hellos		particular purpose on p2p networks except for some checking in hellos
	and tie-breaking in removal of excess adjacencies.		and tie-breaking in removal of excess adjacencies.
	2. More than 255 adjacencies on p2p links		2. More than 255 circuits for p2p links
	For all p2p links within an Intermediate System, the same circuit ID		For all p2p links within an Intermediate System, the same circuit ID
	can be chosen safely to interact with other Intermediate Systems.		can be chosen safely to interact with other Intermediate Systems.
	Even when two such systems are brought up on two ends of the link		Even when two such systems are brought up on two ends of the link
	and both pick the same value, IS-IS specification does not preclude		and both pick the same value, IS-IS specification does not preclude
	such a configuration. In case of multiple parallel links between the		such a configuration. In case of multiple parallel links between the
	systems the only obvious problem is the impact on tie-breaking in		systems the only obvious problem is the impact on tie-breaking in
	case of excessive adjacencies. However, such a configuration cannot		case of excessive adjacencies. However, such a configuration cannot
	generate forwarding loops.		generate forwarding loops.
	3. More than 255 adjacencies on broadcast links		3. More than 255 circuits for broadcast links
	More trickier a case is the one in which an intermediate system has		Trickier a case is the one in which an intermediate system has to
	to form adjacencies on a broadcast medium. The decisive insight		form adjacencies on a broadcast medium. The decisive insight is the
	is the fact that unique circuit ID on a broadcast medium is only		fact that unique circuit ID on a broadcast medium is only needed
	needed in the case where the given intermediate system is assuming		in the case where the given intermediate system is assuming the
	the role of the DIS for the LAN. As long as the intermediate system		role of the DIS for the LAN. As long as the intermediate system has
	has not elected itself DIS, it can use a non-unique circuit ID (1).		not elected itself DIS, it can use a non-unique circuit ID (1).
	Therefore, it is enough to change circuit ID in all the packets		Therefore, it is enough to change circuit ID in all the packets
	transmitted to a unique one as soon as DIS election ran and the		transmitted to a unique one as soon as DIS election ran and the
	system must become DIS. Such behavior is basically indistinguishable		system must become DIS. Such behavior is basically indistinguishable
	from a node crashing and coming immediately back with a different		from a node crashing and coming immediately back with a different
	circuit ID than the one used before the crash. Implementation		circuit ID than the one used before the crash. Implementation
	experience shows that it is unwise to tear down the adjacencies		experience shows that it is unwise to tear down the adjacencies by
	before changing circuit IDs since this can lead to ``ripple''-down		sending empty hellos before changing circuit IDs since this can lead
	behavior in DIS property on the broadcast media in case of all		to ``ripple''-down behavior in DIS in the case of all routers having
	routers having the same preference.		the same preference. However, to ensure that all involved parties
			agree on LAN ID of the media, implementations must interpret in
			subsection 8.4.5 within 10589 the specified condition
			f) the set of Intermediate system adjacency states
			as including any change in LAN ID to be an indication triggering the
			recomputation of the according DIS. Additionally, when recycling a
			previously used circuit ID and reusing it on another LAN special
			___________________________________________
			1. it is important to realize that circuit ID is not a part of
			tie-breaking rules on DIS election
			care has to be taken that the newly generated pseudonode LSPs have
			sequence numbers high enough as to prevent unnecessary flooding.
	3.1. Modification of the Behavior on Broadcast Media		3.1. Modification of the Behavior on Broadcast Media
	The exact modification of the behavior for broadcast links is		The exact modification of the behavior for broadcast links is
	given here. It is a modification of chapter 8.4.5 of the original		given here. It is a modification of chapter 8.4.5 of the original
	specification that states:		specification that states:
	___________________________________________
	1. it is important to realize that circuit ID is not a part of
	tie-breaking rules on DIS election
	When the broadcast circuit is enabled on an Intermediate system the		When the broadcast circuit is enabled on an Intermediate system the
	IS shall perform the following actions.		IS shall perform the following actions.
	a) Commence sending IIH PDUs with the LAN ID field set to the		a) Commence sending IIH PDUs with the LAN ID field set to the
	concatenation of its own SystemID and its locally assigned one		concatenation of its own SystemID and its locally assigned one
	octet Local Circuit ID.		octet Local Circuit ID.
	b) ... <omitted for clarity>		b) ... <omitted for clarity>
	c) Start listening for ISO 9542 ISH PDUs and ESH PDUs and		c) Start listening for ISO 9542 ISH PDUs and ESH PDUs and
	acquire adjacencies as appropriate. Do not run the Designated		acquire adjacencies as appropriate. Do not run the Designated
	Intermediate System election process.		Intermediate System election process.
	skipping to change at page 4, line 37 ¶		skipping to change at page 5, line 5 ¶
	LAN:		LAN:
	1) Commence sending IIH PDUs with the LAN ID field set to the		1) Commence sending IIH PDUs with the LAN ID field set to the
	concatenation of its own SystemID and a local unique circuit		concatenation of its own SystemID and a local unique circuit
	ID.		ID.
	2) go to step b.		2) go to step b.
	otherwise exhibit standard behavior.		otherwise exhibit standard behavior.
			3.2. Multiple levels
			Since election of DIS is performed indpendently at each level, the
			extensions can be applied to generate local circuit IDs only for the
			level at which the system has been elected.
	4. Acknowledgments		4. Acknowledgments
	Some smart people probably thought about most of these things before		Some smart people probably thought about most of these things before
	and the p2p case may be documented in the best current practices for		and the p2p case may be documented in the best current practices for
	IS-IS in the Internet.		IS-IS in the Internet. Mike Shand, Tony Li, Arun Satyanarayana,
			Rajesh Varadarajan and Christian Hopps provided additional comments.
	5. Security Consideration		5. Security Consideration
	ISIS security applies to the work presented. No specific security		ISIS security applies to the work presented. No specific security
	issues with the proposed solutions are known.		issues with the proposed solutions are known.
	6. Intellectual Property Considerations		6. Intellectual Property Considerations
	Lucent may seek patent or other intellectual property protection		Lucent may seek patent or other intellectual property protection
	for some or all of the technologies disclosed in this document. If		for some or all of the technologies disclosed in this document. If
	any standards arising from this document are or become protected		any standards arising from this document are or become protected
	by one or more patents assigned to Lucent, Lucent intends to		by one or more patents assigned to Lucent, Lucent intends to
	disclose those patents and license them under openly specified and		disclose those patents and license them under openly specified and
End of changes. 14 change blocks.
	35 lines changed or deleted		53 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/