< draft-ietf-rtgwg-multihomed-prefix-lfa-02.txt		draft-ietf-rtgwg-multihomed-prefix-lfa-03.txt >
	Routing Area Working Group P. Sarkar, Ed.		Routing Area Working Group P. Sarkar, Ed.
	Internet-Draft Individual		Internet-Draft Arrcus, Inc.
	Updates: 5286 (if approved) S. Hegde		Updates: 5286 (if approved) S. Hegde
	Intended status: Standards Track C. Bowers		Intended status: Standards Track C. Bowers
	Expires: January 26, 2018 Juniper Networks, Inc.		Expires: May 3, 2018 Juniper Networks, Inc.
	U. Chunduri, Ed.		U. Chunduri, Ed.
	Huawei Technologies		Huawei Technologies
	J. Tantsura		J. Tantsura
	Individual		Individual
	B. Decraene		B. Decraene
	Orange		Orange
	H. Gredler		H. Gredler
	RtBrick, Inc.		RtBrick, Inc.
	July 25, 2017		October 30, 2017
	LFA selection for Multi-Homed Prefixes		LFA selection for Multi-Homed Prefixes
	draft-ietf-rtgwg-multihomed-prefix-lfa-02		draft-ietf-rtgwg-multihomed-prefix-lfa-03
	Abstract		Abstract
	This document shares experience gained from implementing algorithms		This document shares experience gained from implementing algorithms
	to determine Loop-Free Alternates for multi-homed prefixes. In		to determine Loop-Free Alternates for multi-homed prefixes. In
	particular, this document provides explicit inequalities that can be		particular, this document provides explicit inequalities that can be
	used to evaluate neighbors as a potential alternates for multi-homed		used to evaluate neighbors as a potential alternates for multi-homed
	prefixes. It also provides detailed criteria for evaluating		prefixes. It also provides detailed criteria for evaluating
	potential alternates for external prefixes advertised by OSPF ASBRs.		potential alternates for external prefixes advertised by OSPF ASBRs.
	This documents updates and expands some of the "Routing Aspects" as		This documents updates and expands some of the "Routing Aspects" as
	skipping to change at page 1, line 46 ¶		skipping to change at page 1, line 46 ¶
	document are to be interpreted as described in RFC2119 [RFC2119].		document are to be interpreted as described in RFC2119 [RFC2119].
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 26, 2018.		This Internet-Draft will expire on May 3, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. LFA inequalities for MHPs . . . . . . . . . . . . . . . . . . 4		2. LFA inequalities for MHPs . . . . . . . . . . . . . . . . . . 4
	3. LFA selection for the multi-homed prefixes . . . . . . . . . 4		3. LFA selection for the multi-homed prefixes . . . . . . . . . 4
	3.1. Improved coverage with simplified approach to MHPs . . . 6		3.1. Improved coverage with simplified approach to MHPs . . . 6
	3.2. IS-IS ATT Bit considerations . . . . . . . . . . . . . . 7		3.2. IS-IS ATT Bit considerations . . . . . . . . . . . . . . 8
	4. LFA selection for the multi-homed external prefixes . . . . . 8		4. LFA selection for the multi-homed external prefixes . . . . . 8
	4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 8		4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 8
	4.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 8		4.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 8
	4.2.1. Rules to select alternate ASBR . . . . . . . . . . . 8		4.2.1. Rules to select alternate ASBR . . . . . . . . . . . 9
	4.2.2. Multiple ASBRs belonging different area . . . . . . . 9		4.2.2. Multiple ASBRs belonging different area . . . . . . . 10
	4.2.3. Type 1 and Type 2 costs . . . . . . . . . . . . . . . 10		4.2.3. Type 1 and Type 2 costs . . . . . . . . . . . . . . . 11
	4.2.4. RFC1583compatibility is set to enabled . . . . . . . 10		4.2.4. RFC1583compatibility is set to enabled . . . . . . . 11
	4.2.5. Type 7 routes . . . . . . . . . . . . . . . . . . . . 10		4.2.5. Type 7 routes . . . . . . . . . . . . . . . . . . . . 11
	4.2.6. Inequalities to be applied for alternate ASBR		4.2.6. Inequalities to be applied for alternate ASBR
	selection . . . . . . . . . . . . . . . . . . . . . . 10		selection . . . . . . . . . . . . . . . . . . . . . . 11
	4.2.6.1. Forwarding address set to non-zero value . . . . 10		4.2.6.1. Forwarding address set to non-zero value . . . . 11
	4.2.6.2. ASBRs advertising type1 and type2 cost . . . . . 11		4.2.6.2. ASBRs advertising type1 and type2 cost . . . . . 12
	5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 12		5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 13
	5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 12		5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 13
	5.2. Multi Topology Considerations . . . . . . . . . . . . . . 13		5.2. Multi Topology Considerations . . . . . . . . . . . . . . 14
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 14		7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
	8.1. Normative References . . . . . . . . . . . . . . . . . . 14		8.1. Normative References . . . . . . . . . . . . . . . . . . 15
	8.2. Informative References . . . . . . . . . . . . . . . . . 14		8.2. Informative References . . . . . . . . . . . . . . . . . 16
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
	1. Introduction		1. Introduction
	The use of Loop-Free Alternates (LFA) for IP Fast Reroute is		The use of Loop-Free Alternates (LFA) for IP Fast Reroute is
	specified in [RFC5286]. Section 6.1 of [RFC5286] describes a method		specified in [RFC5286]. Section 6.1 of [RFC5286] describes a method
	to determine loop-free alternates for a multi-homed prefixes (MHPs).		to determine loop-free alternates for a multi-homed prefixes (MHPs).
	This document describes a procedure using explicit inequalities that		This document describes a procedure using explicit inequalities that
	can be used by a computing router to evaluate a neighbor as a		can be used by a computing router to evaluate a neighbor as a
	potential alternate for a multi-homed prefix. The results obtained		potential alternate for a multi-homed prefix. The results obtained
	are equivalent to those obtained using the method described in		are equivalent to those obtained using the method described in
	skipping to change at page 3, line 45 ¶		skipping to change at page 3, line 45 ¶
	AF - Address Family		AF - Address Family
	ATT - IS-IS Attach Bit		ATT - IS-IS Attach Bit
	ECMP - Equal Cost Multi Path		ECMP - Equal Cost Multi Path
	IGP - Interior Gateway Protocol		IGP - Interior Gateway Protocol
	IS-IS - Intermediate System to Intermediate System		IS-IS - Intermediate System to Intermediate System
			LSP - IS-IS Link State PDU
	OSPF - Open Shortest Path First		OSPF - Open Shortest Path First
	MHP - Multi-homed Prefix		MHP - Multi-homed Prefix
	MT - Multi Topology		MT - Multi Topology
	SPF - Shortest Path First PDU		SPF - Shortest Path First PDU
	2. LFA inequalities for MHPs		2. LFA inequalities for MHPs
	This document proposes the following set of LFA inequalities for		This document proposes the following set of LFA inequalities for
	selecting the most appropriate LFAs for multi-homed prefixes (MHPs).		selecting the most appropriate LFAs for multi-homed prefixes (MHPs).
	They can be derived from the inequalities in [RFC5286] combined with		They can be derived from the inequalities in [RFC5286] combined with
	the observation that D_opt(N,P) = Min (D_opt(N,PO_i) + cost(PO_i,P))		the observation that D_opt(N,P) = Min (D_opt(N,PO_i) + cost(PO_i,P))
	over all PO_i		over all PO_i
	skipping to change at page 4, line 25 ¶		skipping to change at page 4, line 26 ¶
	D_opt(S,PO_best) + cost(PO_best,P)		D_opt(S,PO_best) + cost(PO_best,P)
	Link-Protection + Downstream-paths-only:		Link-Protection + Downstream-paths-only:
	D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost(PO_best,P)		D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost(PO_best,P)
	Node-Protection:		Node-Protection:
	D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +		D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
	D_opt(E,PO_best) + cost(PO_best,P)		D_opt(E,PO_best) + cost(PO_best,P)
	Where,		Where,
			P - The multi-homed prefix being evaluated for
			computing alternates
	S - The computing router		S - The computing router
	N - The alternate router being evaluated		N - The alternate router being evaluated
	E - The primary next-hop on shortest path from S to		E - The primary next-hop on shortest path from S to
	prefix P.		prefix P.
	PO_i - The specific prefix-originating router being		PO_i - The specific prefix-originating router being
	evaluated.		evaluated.
	PO_best - The prefix-originating router on the shortest path		PO_best - The prefix-originating router on the shortest path
	from the computing router S to prefix P.		from the computing router S to prefix P.
	Cost (X,P) - Cost of reaching the prefix P from prefix		Cost (X,P) - Cost of reaching the prefix P from prefix
	originating node X.		originating node X.
	D_opt(X,Y) - Distance on the shortest path from node X to node		D_opt(X,Y) - Distance on the shortest path from node X to node
	Y.		Y.
	Figure 1: LFA inequalities for MHPs		Figure 1: LFA inequalities for MHPs
	3. LFA selection for the multi-homed prefixes		3. LFA selection for the multi-homed prefixes
	To compute a valid LFA for a given multi-homed prefix P, through an		To compute a valid LFA for a given multi-homed prefix P, a computing
	alternate neighbor N a computing router S MUST follow one of the		router S MUST follow one of the appropriate procedures below, for
	appropriate procedures below.		each alternate neighbor N.
	Link-Protection :		Link-Protection :
	=================		=================
	1. If alternate neighbor N is also prefix-originator of P,		1. If alternate neighbor N is also prefix-originator of P,
	1.a. Select N as a LFA for prefix P (irrespective of		1.a. Select N as a LFA for prefix P (irrespective of
	the metric advertised by N for the prefix P).		the metric advertised by N for the prefix P).
	2. Else, evaluate the link-protecting LFA inequality for P with		2. Else, evaluate the link-protecting LFA inequality for P with
	the N as the alternate neighbor.		the N as the alternate neighbor.
	2.a. If LFA inequality condition is met,		2.a. If LFA inequality condition is met,
	select N as a LFA for prefix P.		select N as a LFA for prefix P.
	skipping to change at page 5, line 38 ¶		skipping to change at page 5, line 38 ¶
	the metric advertised by N for the prefix P).		the metric advertised by N for the prefix P).
	2. Else, evaluate the appropriate node-protecting LFA inequality		2. Else, evaluate the appropriate node-protecting LFA inequality
	for P with the N as the alternate neighbor.		for P with the N as the alternate neighbor.
	2.a. If LFA inequality condition is met,		2.a. If LFA inequality condition is met,
	select N as a LFA for prefix P.		select N as a LFA for prefix P.
	2.b. Else, N is not a LFA for prefix P.		2.b. Else, N is not a LFA for prefix P.
	Figure 2: Rules for selecting LFA for MHPs		Figure 2: Rules for selecting LFA for MHPs
	In case an alternate neighbor N is also one of the prefix-originators		In case an alternate neighbor N is also one of the prefix-originators
	of prefix P, N MAY be selected as a valid LFA for P.		of prefix P, N MAY be selected as a valid LFA for P since being a
			prefix-originator it is guaranteed that N will not loop back packets
			destined for prefix P to computing router S.
	However, if N is not a prefix-originator of P, the computing router		However, if N is not a prefix-originator of P, the computing router
	SHOULD evaluate one of the corresponding LFA inequalities, as		SHOULD evaluate one of the corresponding LFA inequalities, as
	mentioned in Figure 1, once for each remote node that originated the		mentioned in Figure 1, once for each remote node that originated the
	prefix. In case the inequality is satisfied by the neighbor N router		prefix. In case the inequality is satisfied by the neighbor N router
	S MUST choose neighbor N, as one of the valid LFAs for the prefix P.		S MUST choose neighbor N, as one of the valid LFAs for the prefix P.
	When computing a downstream-only LFA, in addition to being a prefix-		When computing a downstream-only LFA, in addition to being a prefix-
	originator of P, router N MUST also satisfy the downstream-only LFA		originator of P, router N MUST also satisfy the downstream-only LFA
	inequality specified in Figure 1.		inequality specified in Figure 1.
	skipping to change at page 6, line 25 ¶		skipping to change at page 6, line 28 ¶
	lower coverage. If an implementation chooses to simplify the multi-		lower coverage. If an implementation chooses to simplify the multi-
	homed prefix calculation by assuming that the MHP is solely attached		homed prefix calculation by assuming that the MHP is solely attached
	to the router that was its pre-failure optimal point of attachment,		to the router that was its pre-failure optimal point of attachment,
	the procedure described in this memo can potentially improve coverage		the procedure described in this memo can potentially improve coverage
	for equal cost multi path (ECMP) MHPs without incurring extra		for equal cost multi path (ECMP) MHPs without incurring extra
	computational cost.		computational cost.
	The approach specified in [RFC5286] Section 6.1 last paragraph, is to		The approach specified in [RFC5286] Section 6.1 last paragraph, is to
	simplify the MHP as solely attached to the router that was its pre-		simplify the MHP as solely attached to the router that was its pre-
	failure optimal point of attachment; though it is a scalable approach		failure optimal point of attachment; though it is a scalable approach
	and simplifies computation, [RFC5286] notes this may result in little		and simplifies computation, [RFC5286] notes this MAY result in little
	less coverage.		less coverage.
	This document improves the above approach to provide loop-free		This document improves the above approach to provide loop-free
	alternatives without any additional cost for equal cost multi path		alternatives without any additional cost for ECMP MHPs as described
	MHPs as described through the below example network. The approach		through the below example network. The approach specified here MAY
	specified here MAY also be applicable for handling default routes as		also be applicable for handling default routes as explained in
	explained in Section 3.2.		Section 3.2.
	5 +---+ 8 +---+ 5 +---+		5 +---+ 8 +---+ 5 +---+
	+-----| S |------| A |-----| B |		+-----| S |------| A |-----| B |
	| +---+ +---+ +---+		| +---+ +---+ +---+
	| | |		| | |
	| 5 | 5 |		| 5 | 5 |
	| | |		| | |
	+---+ 5 +---+ 4 +---+ 1 +---+		+---+ 5 +---+ 4 +---+ 1 +---+
	| C |---| E |-----| M |-------| F |		| C |---| E |-----| M |-------| F |
	+---+ +---+ +---+ +---+		+---+ +---+ +---+ +---+
	skipping to change at page 7, line 15 ¶		skipping to change at page 7, line 20 ¶
	attachment and share the same primary next-hop. Hence, an		attachment and share the same primary next-hop. Hence, an
	implementation MAY compare the kind of protection A provides to F		implementation MAY compare the kind of protection A provides to F
	(link-and-node protection) with the kind of protection C provides to		(link-and-node protection) with the kind of protection C provides to
	E (link protection) and inherit the better alternative to prefix p		E (link protection) and inherit the better alternative to prefix p
	and here it is A.		and here it is A.
	However, in the below network prefix p has an ECMP through both node		However, in the below network prefix p has an ECMP through both node
	E and node F with cost 20. Though it has 2 pre-failure optimal		E and node F with cost 20. Though it has 2 pre-failure optimal
	points of attachment, the primary next-hop to each pre-failure		points of attachment, the primary next-hop to each pre-failure
	optimal point of attachment is different. In this case, prefix p		optimal point of attachment is different. In this case, prefix p
	shall inherit corresponding LFA to each primary next-hop calculated		MUST inherit corresponding LFAs of each primary next-hop calculated
	for the router advertising the same respectively (node E's and node		for the router advertising the same respectively. In the below
	F's LFA).		diagram that would be node E's and node F's LFA i.e., node N1 and
			node N2 respectively.
	+---+ 3 +---+		4 +----+
	| S |----------------| B |		+------------------| N2 |
	+---+ +---+		| +----+
	| |		| | 4
	10 | 1 |		10 +---+ 3 +---+
	| |		+------| S |----------------| B |
	+---+ 6 +---+		| +---+ +---+
	| E |-----------------| F |		| | |
	+---+ +---+		| 10 | 1 |
			| | |
			+----+ 5 +---+ 16 +---+
			| N1 |----| E |-----------------| F |
			+----+ +---+ +---+
	| 10 16 |		| 10 16 |
	+-----------p---------+		+-----------p---------+
	Figure 4: MHP with different ECMP Next-hops		Figure 4: MHP with different ECMP Next-hops
	In summary, if there are multiple pre-failure points of attachment		In summary, if there are multiple pre-failure points of attachment
	for a MHP and primary next-hop of a MHP is same as that of the		for a MHP and primary next-hop of a MHP is same as that of the
	primary next-hop of the router that was pre-failure optimal point of		primary next-hop of the router that was pre-failure optimal point of
	attachment, an implementation MAY provide the better protection to		attachment, an implementation MAY provide the better protection to
	MHP without incurring any additional computation cost.		MHP without incurring any additional computation cost.
	3.2. IS-IS ATT Bit considerations		3.2. IS-IS ATT Bit considerations
	Per [RFC1195] a default route needs to be added in Level1 (L1) router		Per [RFC1195] a default route needs to be added in Level1 (L1) router
	to the closest reachable Level1/Level2 (L1/L2) router in the network		to the closest reachable Level1/Level2 (L1/L2) router in the network
	advertising ATT (attach) bit in its LSP-0 fragment. All L1 routers		advertising ATT (attach) bit in its LSP-0 fragment. All L1 routers
	in the area would do this during the decision process with the next-		in the area would do this during the decision process with the next-
	hop of the default route set to the adjacent router through which the		hop of the default route set to the adjacent router through which the
	closest L1/L2 router is reachable. The base LFA specification		closest L1/L2 router is reachable. The base LFA specification
	[RFC5286] does not specify any procedure for computing LFA for a		[RFC5286] does not specify any procedure for computing LFA for a
	default route in IS-IS L1 area. Potentially one MAY consider a		default route in IS-IS L1 area. This document specifies, potentially
	default route is being advertised from the border L1/L2 router where		a node MAY consider a default route is being advertised from the
	ATT bit is set and can do LFA computation for the default route.		border L1/L2 router where ATT bit is set and can do LFA computation
			for the default route. But, when multiple ECMP L1/L2 routers are
	But, when multiple ECMP L1/L2 routers are reachable in an L1 area		reachable in an L1 area corresponding best LFAs SHOULD be given for
	corresponding best LFAs SHOULD be given for each primary next-hop		each primary next-hop associated with default route. Considerations
	associated with default route. Considerations as specified in		as specified in Section 3 and Section 3.1 are applicable for default
	Section 3 and Section 3.1 are applicable for default routes, if the		routes, if the default route is considered as ECMP MHP. Note that,
	default route is considered as ECMP MHP.		this document doesn't alter any ECMP handling rules or computation of
			LFAs for ECMP in general as laid out in [RFC5286].
	4. LFA selection for the multi-homed external prefixes		4. LFA selection for the multi-homed external prefixes
	Redistribution of external routes into IGP is required in case of two		Redistribution of external routes into IGP is required in case of two
	different networks getting merged into one or during protocol		different networks getting merged into one or during protocol
	migrations. External routes could be distributed into an IGP domain		migrations. External routes could be distributed into an IGP domain
	via multiple nodes to avoid a single point of failure.		via multiple nodes to avoid a single point of failure.
	During LFA calculation, alternate LFA next-hops to reach the best		During LFA calculation, alternate LFA next-hops to reach the best
	ASBR could be used as LFA for the routes redistributed via that ASBR.		ASBR could be used as LFA for the routes redistributed via that ASBR.
	skipping to change at page 8, line 34 ¶		skipping to change at page 8, line 48 ¶
	distributing nodes in the network.		distributing nodes in the network.
	4.1. IS-IS		4.1. IS-IS
	LFA evaluation for multi-homed external prefixes in IS-IS is similar		LFA evaluation for multi-homed external prefixes in IS-IS is similar
	to the multi-homed internal prefixes. Inequalities described in sec		to the multi-homed internal prefixes. Inequalities described in sec
	2 would also apply to multi-homed external prefixes as well.		2 would also apply to multi-homed external prefixes as well.
	4.2. OSPF		4.2. OSPF
	Loop free Alternates [RFC 5286] describes mechanisms to apply		Loop free Alternates [RFC5286] describes mechanisms to apply
	inequalities to find the loop free alternate neighbor. For the		inequalities to find the loop free alternate neighbor. For the
	selection of alternate ASBR for LFA consideration, additional rules		selection of alternate ASBR for LFA consideration, additional rules
	have to be applied in selecting the alternate ASBR due to the		have to be applied in selecting the alternate ASBR due to the
	external route calculation rules imposed by [RFC 2328].		external route calculation rules imposed by [RFC2328].
	This document also defines the inequalities defined in RFC [5286]		This document also defines the inequalities defined in [RFC5286]
	specifically for the alternate loop-free ASBR evaluation.		specifically for the alternate loop-free ASBR evaluation.
	4.2.1. Rules to select alternate ASBR		4.2.1. Rules to select alternate ASBR
	The process to select an alternate ASBR is best explained using the		The process to select an alternate ASBR is best explained using the
	rules below. The below process is applied when primary ASBR for the		rules below. The below process is applied when primary ASBR for the
	concerned prefix is chosen and there is an alternate ASBR originating		concerned prefix is chosen and there is an alternate ASBR originating
	same prefix.		same prefix.
	1. If RFC1583Compatibility is disabled		1. If RFC1583Compatibility is disabled
	1a. if primary ASBR and alternate ASBR are intra area		1a. if primary ASBR and alternate ASBR are intra area
	non-backbone path go to step 2.		non-backbone path go to step 2.
	1b. If primary ASBR and alternate ASBR belong to		1b. If primary ASBR and alternate ASBR belong to
	intra-area backbone and/or inter-area path go		intra-area backbone and/or inter-area path go
	to step 2.		to step 2.
	1c. for other paths, skip the alternate ASBR and		1c. for other paths, skip this alternate ASBR and
	consider next ASBR.		consider next ASBR.
	2. If cost type (type1/type2) advertised by alternate		2. If cost type (type1/type2) advertised by alternate
	ASBR same as primary		ASBR same as primary
	2a. If not same skip alternate ASBR and consider next ASBR.		2a. If not, same skip alternate ASBR and consider
			next ASBR.
			2b. If same proceed to step 3.
	3. If cost type is type1		3. If cost type is type1
	3a. If cost is same, program ECMP		3a. If cost is same, program ECMP and return.
	3b. else go to step 5.		3b. else go to step 5.
	4 If cost type is type 2		4 If cost type is type 2
	4a. If cost is different, skip alternate ASBR and		4a. If cost is different, skip alternate ASBR and
	consider next ASBR		consider next ASBR.
	4b. If type2 cost is same, compare type 1 cost.		4b. If type2 cost is same, proceed to step 4c to compare
	4c. If type1 cost is also same program ECMP.		compare type 1 cost.
			4c. If type1 cost is also same program ECMP and return.
	4d. If type 1 cost is different go to step 5.		4d. If type 1 cost is different go to step 5.
	5. If route type (type 5/type 7)		5. If route type (type 5/type 7)
	5a. If route type is same, check route p-bit,		5a. If route type is same, check route p-bit,
	forwarding address field for routes from both		forwarding address field for routes from both
	ASBRs		ASBRs match. If p-bit matches proceed to step 6.
	match. If not skip alternate ASBR and consider		If not, skip this alternate ASBR and consider
	next ASBR.		next ASBR.
	5b. If route type is not same, skip ASBR		5b. If route type is not same, skip this alternate ASBR
	and consider next ASBR.		and consider next alternate ASBR.
	6. Apply inequality on the alternate ASBR.		6. Apply inequality on the alternate ASBR.
	Figure 5: Rules for selecting alternate ASBR in OSPF		Figure 5: Rules for selecting alternate ASBR in OSPF
	4.2.2. Multiple ASBRs belonging different area		4.2.2. Multiple ASBRs belonging different area
	When "RFC1583compatibility" is set to disabled, OSPF[RFC2328] defines		When "RFC1583compatibility" is set to disabled, OSPF [RFC2328]
	certain rules of preference to choose the ASBRs. While selecting		defines certain rules of preference to choose the ASBRs. While
	alternate ASBR for loop evaluation for LFA, these rules should be		selecting alternate ASBR for loop evaluation for LFA, these rules
	applied and ensured that the alternate neighbor does not loop the		should be applied and ensured that the alternate neighbor does not
	traffic back.		loop the traffic back.
	When there are multiple ASBRs belonging to different area advertising		When there are multiple ASBRs belonging to different area advertising
	the same prefix, pruning rules as defined in RFC 2328 section 16.4.1		the same prefix, pruning rules as defined in [RFC2328] section 16.4.1
	are applied. The alternate ASBRs pruned using above rules are not		are applied. The alternate ASBRs pruned using above rules are not
	considered for LFA evaluation.		considered for LFA evaluation.
	4.2.3. Type 1 and Type 2 costs		4.2.3. Type 1 and Type 2 costs
	If there are multiple ASBRs not pruned via rules defined in 3.2.2,		If there are multiple ASBRs not pruned via rules defined in 3.2.2,
	the cost type advertised by the ASBRs is compared. ASBRs advertising		the cost type advertised by the ASBRs is compared. ASBRs advertising
	Type1 costs are preferred and the type2 costs are pruned. If two		Type1 costs are preferred and the type2 costs are pruned. If two
	ASBRs advertise same type2 cost, the alternate ASBRs are considered		ASBRs advertise same type2 cost, the alternate ASBRs are considered
	along with their type1 cost for evaluation. If the two ASBRs with		along with their type1 cost for evaluation. If the two ASBRs with
	skipping to change at page 13, line 24 ¶		skipping to change at page 14, line 24 ¶
	+---+		+---+
	10 |		10 |
	+---+		+---+
	|D2 |		|D2 |
	+---+		+---+
	Figure 8: Link with IGP MAX_METRIC		Figure 8: Link with IGP MAX_METRIC
	In the simple example network, all the link costs have a cost of 10		In the simple example network, all the link costs have a cost of 10
	in both directions, except for the link between S and N2. The S-N2		in both directions, except for the link between S and N2. The S-N2
	link has a cost of 10 in the direction from S to N2, and a cost of		link has a cost of 10 in the forward direction i.e., from S to N2,
	MAX_METRIC in the direction from N2 to S (0xffffff /2^24 - 1 for IS-		and a cost of MAX_METRIC (0xffffff /2^24 - 1 for IS-IS and 0xffff for
	IS and 0xffff for OSPF) for a specific end to end Traffic Engineering		OSPF) in the reverse direction i.e., from N2 to S for a specific end-
	(TE) requirement of the operator. At node S, D1 is reachable through		to-end Traffic Engineering (TE) requirement of the operator. At node
	N1 with cost 20, and D2 is reachable through N2 with cost 20. Even		S, D1 is reachable through N1 with cost 20, and D2 is reachable
	though neighbor N2 satisfies basic loop-free condition (inequality 1		through N2 with cost 20. Even though neighbor N2 satisfies basic
	of [RFC5286]) for D1 this could be excluded as potential alternative		loop-free condition (inequality 1 of [RFC5286]) for D1, S's neighbor
	because of the current exclusions as specified in section 3.5 and 3.6		N2 could be excluded as a potential alternative because of the
	procedure of [RFC5286]. But, as the primary traffic destined to D2		current exclusions as specified in section 3.5 and 3.6 procedure of
	continue to use the link and hence irrespective of the reverse metric		[RFC5286]. But, as the primary traffic destined to D2 continue to
	in this case, the same link MAY be used as a potential LFA for D1.		use the link and hence irrespective of the reverse metric in this
			case, same link MAY be used as a potential LFA for D1.
	Alternatively, reverse metric of the link MAY be configured with		Alternatively, reverse metric of the link MAY be configured with
	MAX_METRIC-1, so that the link can be used as an alternative while		MAX_METRIC-1, so that the link can be used as an alternative while
	meeting the TE requirements.		meeting the operator's TE requirements and without having to update
			the router to fix this particular issue.
	5.2. Multi Topology Considerations		5.2. Multi Topology Considerations
	Section 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF and		Section 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF and
	ISIS are out of scope for that specification. This memo clarifies		ISIS are out of scope for that specification. This memo clarifies
	and describes the applicability.		and describes the applicability.
	In Multi Topology (MT) IGP deployments, for each MT ID, a separate		In Multi Topology (MT) IGP deployments, for each MT ID, a separate
	shortest path tree (SPT) is built with topology specific adjacencies,		shortest path tree (SPT) is built with topology specific adjacencies,
	the LFA principles laid out in [RFC5286] are actually applicable for		the LFA principles laid out in [RFC5286] are actually applicable for
	MT IS-IS [RFC5120] LFA SPF. The primary difference in this case is,		MT IS-IS [RFC5120] LFA SPF. The primary difference in this case is,
	identifying the eligible-set of neighbors for each LFA computation		identifying the eligible-set of neighbors for each LFA computation
	which is done per MT ID. The eligible-set for each MT ID is		which is done per MT ID. The eligible-set for each MT ID is
	determined by the presence of IGP adjacency from Source to the		determined by the presence of IGP adjacency from Source to the
	neighboring node on that MT-ID apart from the administrative		neighboring node on that MT-ID apart from the administrative
	restrictions and other checks laid out in [RFC5286]. The same is		restrictions and other checks laid out in [RFC5286]. The same is
	also applicable for OSPF [RFC4915] [MT-OSPF] or different AFs in		also applicable for MT-OSPF [RFC4915] or different AFs in multi
	multi instance OSPFv3 [RFC5838].		instance OSPFv3 [RFC5838].
	However for MT IS-IS, if a default topology is used with MT-ID 0		However for MT IS-IS, if a "standart topology" is used with MT-ID #0
	[RFC5286] and both IPv4 [RFC5305] and IPv6 routes/AFs [RFC5308] are		[RFC5286] and both IPv4 [RFC5305] and IPv6 routes/AFs [RFC5308] are
	present, then the condition of network congruency is applicable for		present, then the condition of network congruency is applicable for
	LFA computation as well. Network congruency here refers to, having		LFA computation as well. Network congruency here refers to, having
	same address families provisioned on all the links and all the nodes		same address families provisioned on all the links and all the nodes
	of the network with MT-ID 0. Here with single decision process both		of the network with MT-ID #0. Here with single decision process both
	IPv4 and IPv6 next-hops are computed for all the prefixes in the		IPv4 and IPv6 next-hops are computed for all the prefixes in the
	network and similarly with one LFA computation from all eligible		network and similarly with one LFA computation from all eligible
	neighbors per [RFC5286], all potential alternatives can be computed.		neighbors per [RFC5286], all potential alternatives can be computed.
	6. Acknowledgements		6. Acknowledgements
	Thanks to Alia Atlas and Salih K A for their useful feedback and		Thanks to Alia Atlas and Salih K A for their useful feedback and
	inputs.		inputs. Thanks to Stewart Bryant for being document shepherd and
			providing detailed review comments.
	7. Security Considerations		7. Security Considerations
	This document does not introduce any change in any of the protocol		This document does not introduce any change in any of the protocol
	specifications and also this does not introduce any new security		[RFC1195] [RFC5120] [RFC2328] [RFC5838] specifications discussed here
	issues other than as noted in the LFA base specification [RFC5286].		and also this does not introduce any new security issues other than
			as noted in the LFA base specification [RFC5286].
	8. References		8. References
	8.1. Normative References		8.1. Normative References
	[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
	dual environments", RFC 1195, DOI 10.17487/RFC1195,
	December 1990, <http://www.rfc-editor.org/info/rfc1195>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<http://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
			[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
			IP Fast Reroute: Loop-Free Alternates", RFC 5286,
			DOI 10.17487/RFC5286, September 2008,
			<https://www.rfc-editor.org/info/rfc5286>.
	8.2. Informative References		8.2. Informative References
			[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
			dual environments", RFC 1195, DOI 10.17487/RFC1195,
			December 1990, <https://www.rfc-editor.org/info/rfc1195>.
			[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
			DOI 10.17487/RFC2328, April 1998,
			<https://www.rfc-editor.org/info/rfc2328>.
	[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.		[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
	McPherson, "OSPF Stub Router Advertisement", RFC 3137,		McPherson, "OSPF Stub Router Advertisement", RFC 3137,
	DOI 10.17487/RFC3137, June 2001,		DOI 10.17487/RFC3137, June 2001,
	<http://www.rfc-editor.org/info/rfc3137>.		<https://www.rfc-editor.org/info/rfc3137>.
	[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.		[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
	Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",		Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
	RFC 4915, DOI 10.17487/RFC4915, June 2007,		RFC 4915, DOI 10.17487/RFC4915, June 2007,
	<http://www.rfc-editor.org/info/rfc4915>.		<https://www.rfc-editor.org/info/rfc4915>.
	[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi		[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
	Topology (MT) Routing in Intermediate System to		Topology (MT) Routing in Intermediate System to
	Intermediate Systems (IS-ISs)", RFC 5120,		Intermediate Systems (IS-ISs)", RFC 5120,
	DOI 10.17487/RFC5120, February 2008,		DOI 10.17487/RFC5120, February 2008,
	<http://www.rfc-editor.org/info/rfc5120>.		<https://www.rfc-editor.org/info/rfc5120>.
	[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
	IP Fast Reroute: Loop-Free Alternates", RFC 5286,
	DOI 10.17487/RFC5286, September 2008,
	<http://www.rfc-editor.org/info/rfc5286>.
	[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic		[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
	Engineering", RFC 5305, DOI 10.17487/RFC5305, October		Engineering", RFC 5305, DOI 10.17487/RFC5305, October
	2008, <http://www.rfc-editor.org/info/rfc5305>.		2008, <https://www.rfc-editor.org/info/rfc5305>.
	[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,		[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
	DOI 10.17487/RFC5308, October 2008,		DOI 10.17487/RFC5308, October 2008,
	<http://www.rfc-editor.org/info/rfc5308>.		<https://www.rfc-editor.org/info/rfc5308>.
	[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and		[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
	R. Aggarwal, "Support of Address Families in OSPFv3",		R. Aggarwal, "Support of Address Families in OSPFv3",
	RFC 5838, DOI 10.17487/RFC5838, April 2010,		RFC 5838, DOI 10.17487/RFC5838, April 2010,
	<http://www.rfc-editor.org/info/rfc5838>.		<https://www.rfc-editor.org/info/rfc5838>.
	[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
	Horneffer, M., and P. Sarkar, "Operational Management of
	Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
	July 2016, <http://www.rfc-editor.org/info/rfc7916>.
	Authors' Addresses		Authors' Addresses
	Pushpasis Sarkar (editor)		Pushpasis Sarkar (editor)
	Individual		Arrcus, Inc.
	Email: pushpasis.ietf@gmail.com		Email: pushpasis.ietf@gmail.com
	Shraddha Hegde		Shraddha Hegde
	Juniper Networks, Inc.		Juniper Networks, Inc.
	Electra, Exora Business Park		Electra, Exora Business Park
	Bangalore, KA 560103		Bangalore, KA 560103
	India		India
	Email: shraddha@juniper.net		Email: shraddha@juniper.net
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