wdiff rfc8518.alt-original rfc8518.txt

Routing Area Working Group
Internet Engineering Task Force (IETF) P. Sarkar, Ed.
Internet-Draft
Request for Comments: 8518 Arrcus, Inc.
Updates: 5286 (if approved) U. Chunduri, Ed.
Intended status:
Category: Standards Track Huawei USA
Expires: May 25, 2019
ISSN: 2070-1721 S. Hegde
Juniper Networks, Inc.
J. Tantsura
Apstra, Inc.
H. Gredler
RtBrick, Inc.
November 21, 2018
March 2019

Selection of Loop-Free Alternates selection for Multi-Homed Prefixes
draft-ietf-rtgwg-multihomed-prefix-lfa-09

Abstract

Deployment experience gained from implementing algorithms to
determine Loop-Free Alternates (LFAs) for multi-homed prefixes (MHPs)
has revealed some avenues for potential improvement. This document
provides explicit inequalities that can be used to evaluate neighbors
as a potential alternates for multi-homed prefixes. MHPs. It also provides detailed criteria
for evaluating potential alternates for external prefixes advertised
by OSPF ASBRs. This documents document updates and
expands some of the "Routing Aspects" as specified in Section 6 of RFC 5286. 5286 by
expanding some of the routing aspects.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
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material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 25, 2019.
https://www.rfc-editor.org/info/rfc8518.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3 ...................................................3
1.2. Requirements Language ......................................4
2. LFA inequalities Inequalities for MHPs . . . . . . . . . . . . . . . . . . 4 .......................................4
3. LFA selection Selection for the multi-homed prefixes . . . . . . . . . 5 MHPs ..........................................5
3.1. Improved coverage Coverage with simplified approach Simplified Approach to MHPs . . . 7 .........7
3.2. IS-IS ATT Bit considerations . . . . . . . . . . . . . . 8 Considerations ...............................8
4. LFA selection Selection for the multi-homed external prefixes . . . . . 9 Multi-Homed External Prefixes .................9
4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 9 ......................................................9
4.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 9 .......................................................9
4.2.1. Rules to select alternate ASBR . . . . . . . . . . . 9 Select Alternate ASBRs .....................9
4.2.1.1. Multiple ASBRs belonging different area . . . . . 11 Belonging to Different Areas ..11
4.2.1.2. Type 1 and Type 2 costs . . . . . . . . . . . . . 11 Costs ......................11
4.2.1.3. RFC1583compatibility RFC1583Compatibility is set Set to enabled . . . . . 11 "Enabled" .....11
4.2.1.4. Type 7 routes . . . . . . . . . . . . . . . . . . 11 Routes ................................12
4.2.2. Inequalities to be applied Be Applied for alternate Alternate ASBR
selection . . . . . . . . . . . . . . . . . . . . . . 12
Selection ..........................................12
4.2.2.1. Forwarding address set Address Set to non-zero value . . . . 12 Non-zero Value .....12
4.2.2.2. ASBRs advertising type1 Advertising Type 1 and type2 cost . . . . . 13 Type 2 Costs ....13
5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 13 ........................................14
5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 13 .................................14
5.2. Multi Topology MT Considerations . . . . . . . . . . . . . . 14 .........................................15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 ............................................15
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. ........................................16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. .....................................................16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. ......................................16
8.2. Informative References . . . . . . . . . . . . . . . . . 16 ....................................16
Acknowledgements ..................................................18
Contributors ......................................................18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 ................................................19

1. Introduction

A framework for the development of IP fast-reroute Fast Reroute (FRR) mechanisms
is detailed in [RFC5714]. The use of LFAs for IP Fast Reroute FRR is specified in
[RFC5286]. If a prefix is advertised by more than one
router router, that
prefix is called as multi-homed a "multi-homed prefix (MHP). (MHP)". MHPs generally occur
for prefixes obtained from outside the routing domain by multiple
routers, for subnets on links where the subnet is announced from
multiple ends of the link, and for prefixes advertised by multiple
routers to provide resiliency.

Section 6.1 of [RFC5286] describes a method to determine LFAs for
MHPs. This document describes a procedure using explicit
inequalities that can be used by a computing router to evaluate a
neighbor as a potential alternate for a an MHP. The results obtained
are equivalent to those obtained using the method described in
Section 6.1 of [RFC5286].

Section 6.3 of [RFC5286] discusses complications associated with
computing LFAs for MHPs in OSPF. This document provides detailed
criteria for evaluating potential alternates for external prefixes
advertised by OSPF ASBRs, as well as explicit inequalities.

This document also provides clarifications, clarifications and additional
considerations to [RFC5286], [RFC5286] to address a few coverage and operational
observations. These observations are in concerned with 1) the area of handling IS-IS attach (ATT) ATT
(attach) bit in Level-1 the Level 1 (L1) area, 2) links provisioned with
MAX_METRIC (see Section 5.1) for traffic engineering (TE) purposes purposes,
and in the area of
Multi Topology 3) multi-topology (MT) IGP deployments. These are elaborated in
detail in Section Sections 3.2 and Section 5.

This specification uses the same terminology introduced in [RFC5714]
to represent LFA and builds on the inequalities notation for inequalities used in
[RFC5286] to compute LFAs for MHPs.

1.1. Acronyms

AF - Address Family

ATT - IS-IS Attach Bit

ECMP - Equal Cost Multi Path Equal-Cost Multipath

FRR - Fast Reroute

IGP - Interior Gateway Protocol

IS-IS - Intermediate System to Intermediate System
LFA - Loop-Free Alternate

LSP - IS-IS Link State PDU (IS-IS)

MHP - Multi-Homed Prefix

MT - Multi-Topology

OSPF - Open Shortest Path First

MHP - Multi-homed Prefix

MT - Multi Topology

SPF - Shortest Path First

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 RFC8174 [RFC2119] RFC8174 [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. LFA inequalities Inequalities for MHPs

This document proposes the following set of LFA inequalities for
selecting the most appropriate LFAs for MHPs. D_opt(X,Y) terminology Distance_opt(X,Y)
(called "D_opt(X,Y)" in this document) is defined in [RFC5714], which [RFC5714] and is
nothing but the metric sum of the shortest path from X to Y and Cost(X,Y) Y.
Cost(X,Y), introduced in this document document, is defined as the metric
value of prefix Y from the prefix advertising node X. These LFAs 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)) over
all PO_i

Link-Protection: PO_i.

Link-Protecting LFAs:
A neighbor N can provide a loop-free alternate (LFA) an LFA if and only if

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
D_opt(S,PO_best) + Cost(PO_best,P)

Link-Protection

Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:
A subset of loop-free alternates are downstream paths that must
meet a more restrictive condition that is applicable to more
complex failure scenarios scenarios.

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)

Node-Protection:

Node-Protecting LFAs:
For an alternate next-hop next hop N to protect against node failure of a
primary neighbor E for MHP P, N must be loop-free with respect to
both E and mhp MHP P. In other words, N's path to MHP P must not go
through E (where N is the neighbor providing a loop-free alternate)
alternate).

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
D_opt(E,PO_best) + Cost(PO_best,P)

Where,

Where:

P - The multi-homed prefix MHP being evaluated for computing alternates

S - The computing router

N - The alternate router being evaluated

E - The primary next-hop next hop on the shortest path from S to
prefix P. P

PO_i - The specific prefix-originating router being
evaluated.
evaluated

PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P. P

Cost(X,P) - Cost The cost of reaching the prefix P from prefix
originating node X. X

D_opt(X,Y) - Distance The distance on the shortest path from node X to
node
Y.

Figure 1: LFA inequalities for MHPs Y

3. LFA selection Selection for the multi-homed prefixes MHPs

To compute a valid LFA for a given MHP P, a computing router S MUST MUST,
for each alternate neighbor N, follow one of the appropriate
procedures below, for each alternate
neighbor N and below once for each remote node that originated the prefix
P.

Link-Protection :
=================

Link-Protecting LFAs:

1. if, If, in addition to being an alternate neighbor, N is also a prefix-originator
prefix originator of P,
1.a.

A. Select N as a an LFA for prefix P (irrespective of the metric
advertised by N for the prefix P).

2. Else, evaluate the link-protecting LFA inequality for P with
the N as
the alternate neighbor.
2.a.

A. If the LFA inequality condition is met, select N as a an LFA
for prefix P.
2.b.

B. Else, N is not a an LFA for prefix P.

Link-Protection

Link-Protecting + Downstream-paths-only :
========================================= LFAs:

1. Evaluate the link-protecting + downstream-only downstream-paths-only LFA
inequality for P with the N as the alternate neighbor.
1.a.

A. If the LFA inequality condition is met, select N as a an LFA
for prefix P.
1.b.

B. Else, N is not a an LFA for prefix P.

Node-Protection :
=================

Node-Protecting LFAs:

1. if, If, in addition to being an alternate neighbor, N is also a prefix-originator
prefix originator of P,
1.a.

A. Select N as a an LFA for prefix P (irrespective of the metric
advertised by N for the prefix P).

2. Else, evaluate the appropriate node-protecting LFA inequality for
P with the N as the alternate neighbor.
2.a.

A. If the LFA inequality condition is met, select N as a an LFA
for prefix P.
2.b.

B. Else, N is not a an LFA for prefix P.

Figure 2: Rules for selecting LFA for MHPs

In case

If an alternate neighbor N is also one of the prefix-originators prefix originators of
prefix P, N being a prefix-originator it is guaranteed that N will not loop back packets destined
for prefix P to computing router S.
So Therefore, N MUST be chosen as a
valid LFA for prefix P, P without evaluating any of the inequalities in Figure 1
Section 2 as long as a downstream-paths-only LFA is not desired. To
ensure such a neighbor N also provides a downstream-paths-only LFA,
router S MUST also evaluate the
downstream-only downstream-paths-only LFA inequality
specified in Figure 1 Section 2 for neighbor N and ensure router N satisfies
the inequality.

However, if N is not a prefix-originator prefix originator of P, the computing router
MUST evaluate one of the corresponding LFA inequalities, as mentioned inequalities defined in Figure 1,
Section 2 once for each remote node that originated the prefix.
In case If
the inequality is satisfied by the neighbor N N, router S MUST choose
neighbor N, N as one of the valid LFAs for the prefix P.

For more specific rules rules, please refer to the later sections of this
document. Section 4.

3.1. Improved coverage Coverage with simplified approach Simplified Approach to MHPs

Section 6.1 of the LFA base specification [RFC5286] Section 6.1 recommends that a
router computes the alternate next-hop next hop for an IGP MHP by considering
alternate paths via all routers that have announced that prefix and
the prefix. The
same has been elaborated with appropriate inequalities in the
above
previous section. However, [RFC5286] Section 6.1 of [RFC5286] also allows for
the router to simplify the MHP calculation by assuming that the MHP
is solely attached to the router that was its pre-failure optimal
point of attachment, at the expense of potentially lower coverage.
If an implementation chooses to simplify the MHP calculation by
assuming that the MHP is solely attached to the router that was its pre-
failure
pre-failure optimal point of attachment, the procedure described in
this memo can potentially improve coverage for equal cost multi path
(ECMP) ECMP MHPs without
incurring extra computational cost.

This document improves the above approach to provide loop-free
alternatives without any additional cost for ECMP MHPs as described
through
in the below example network presented in Figure 3. 1. The approach specified
here may also be applicable for handling default routes as explained
in Section 3.2.

5 +---+ 8 +---+ 5 +---+
+-----| S |------| A |-----| B |
| +---+ +---+ +---+
| | |
| 5 | 5 |
| | |
+---+ 5 +---+ 4 +---+ 1 +---+
| C |---| E |-----| M |-------| F |
+---+ +---+ +---+ +---+
| 10 5 |
+-----------P---------+

Figure 3: 1: MHP with same Same ECMP Next-hop Next Hop

In the above network Figure 1, a prefix P, P is advertised from both Node node E and
Node node F.
With a simplified approach taken as specified in [RFC5286] Section 6.1, 6.1 of
[RFC5286], prefix P will get only link protection a link-protecting LFA through the
neighbor C while a node protection node-protection path is available through neighbor
A. In this scenario, E and F both are pre-failure optimal points of
attachment and share the same primary next-hop. next hop. Hence, an
implementation MAY compare the kind of protection A provides to F
(link-and-node
(link and node protection) with the kind of protection C provides to
E (link protection) and inherit the better alternative to prefix P
and here it P.
In this case, the better alternative is A.

However, in the below example network presented in Figure 4, 2, prefix P has
an ECMP through both node E and node F with cost 20. Though it has 2
two pre-failure optimal points of attachment, the primary next-hop next hop to
each pre-failure optimal point of attachment is different. In this
case, prefix P MUST inherit the corresponding LFAs of each primary
next-hop
next hop calculated for the router advertising the same respectively. same. In the below diagram
Figure 2, that would be the LFA for node E's E and node F's LFA F, i.e., node N1
and node N2 N2, respectively.

4 +----+
+------------------| N2 |
| +----+
| | 4
10 +---+ 3 +---+
+------| S |----------------| B |
| +---+ +---+
| | |
| 10 | 1 |
| | |
+----+ 5 +---+ 16 +---+
| N1 |----| E |-----------------| F |
+----+ +---+ +---+
| 10 16 |
+-----------P---------+

Figure 4: 2: MHP with different Different ECMP Next-hops Next Hops

In summary, if there are multiple pre-failure points of attachment
for a MHP an MHP, and the primary next-hop next hop of a an MHP is the same as that of
the primary next-hop next hop of the router that was the pre-failure optimal
point of attachment, an implementation MAY provide a better
protection to the MHP without incurring any additional computation
cost.

3.2. IS-IS ATT Bit considerations Considerations

Per [RFC1195] [RFC1195], a default route needs to be added in Level1 the Level 1 (L1)
router to the closest reachable Level1/Level2 Level 1 / Level 2 (L1/L2) router in
the network advertising the ATT (attach) bit in its LSP-0 fragment.
All L1 routers in the area would do this during the decision process
with the next- next hop of the default route set to the adjacent router
through which the closest L1/L2 router is reachable. The base LFA base
specification [RFC5286] does not specify any procedure for computing
LFA for a default route in the IS-IS L1 area. This document specifies,
specifies that a node can consider a default route is being
advertised from the border L1/L2 router where the ATT bit is set, set and
can do LFA computation for that default route. But, when multiple
ECMP L1/L2 routers are reachable in an L1 area area, corresponding best
LFAs SHOULD be computed for each primary next-hop next hop associated with the
default route as this would be similar to the ECMP MHP example as
described in Section 3.1. Considerations as specified in Section Sections 3 and Section
3.1 are applicable for default routes, routes if the default route is
considered as an ECMP MHP. Note that, that 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 Selection for the multi-homed external prefixes Multi-Homed External Prefixes

Redistribution of external routes into IGP is required in case of 1) when two
different networks getting get merged into one or 2) during protocol
migrations. External routes could be distributed into an IGP domain
via multiple nodes to avoid a single point of failure.

During LFA calculation, alternate LFA next-hops next hops to reach the best
ASBR could be used as LFA for the routes redistributed via that ASBR.
When there is no LFA available to the best ASBR, it may be desirable
to consider the other ASBRs (referred to as alternate ASBR "alternate ASBRs"
hereafter) redistributing the external routes for LFA selection as
defined in [RFC5286] and leverage the advantage of having multiple re-
distributing
redistributing nodes in the network.

4.1. IS-IS

LFA evaluation for multi-homed external prefixes in IS-IS is the same to
as the multi-homed internal prefixes. Inequalities described in
Section 2 would also apply to multi-homed external prefixes.

4.2. OSPF

Loop Free Alternates

The LFA base specification [RFC5286] describes mechanisms to apply
inequalities to find the loop free loop-free alternate neighbor. For the
selection of alternate ASBR for LFA consideration, additional Additional
rules have to be applied in selecting the alternate ASBR for LFA
consideration due to the external route calculation rules imposed by
[RFC2328].

This document defines inequalities specifically for the alternate
loop-free loop-
free ASBR evaluation, evaluation. These inequalities are based on those in
[RFC5286].

4.2.1. Rules to select alternate ASBR Select Alternate ASBRs

The process to select an alternate ASBR is best explained using the
rules below. The below process below is applied when a primary ASBR for
the concerned prefix is chosen and there is an alternate ASBR
originating the same prefix.

1. If RFC1583Compatibility is disabled

1a. if disabled:

A. If primary ASBR and alternate ASBR belong to intra-area
non-backbone non-
backbone, go to step 2.
1b.

B. If primary ASBR and alternate ASBR belong to intra-area
backbone and/or inter-area path path, go to step 2.
1c. for

C. For other paths, skip this alternate ASBR and consider next
ASBR.

2. Compare cost types (type 1/type 1 / type 2) advertised by alternate ASBR
and
by the primary ASBR
2a. ASBR:

A. If not the same type type, skip alternate ASBR and consider next
ASBR.
2b.

B. If same the same, proceed to step 3.

3.If

3. If cost types are type 1, compare costs advertised by alternate
ASBR and by the primary ASBR
3a. ASBR:

A. If costs are the same same, then program ECMP Fast ReRoute (FRR) FRR and return.
3b. else

B. Else, go to step 5..

4 5.

4. If cost types are type 2, compare costs advertised by alternate
ASBR and by the primary ASBR
4a. ASBR:

A. If costs are different, skip alternate ASBR and consider next
ASBR.
4b.

B. If cost costs are the same, proceed to step 4c 4C to compare
cost costs to
reach ASBR/forwarding address.
4c.

C. If cost costs to reach ASBR/forwarding address are also same the same,
program ECMP FRR and return.
4d.

D. If cost costs to reach ASBR/forwarding address are different different, go
to step 5.

5. If Compare route type types (type 5/type 5 and type 7)
5a. for alternate ASBR and
primary ASBR:

A. If route type is types are the same, check if the route p-bit and the
forwarding address field for routes from both ASBRs match.
If p-bit and forwarding address matches match, proceed to step 6. If
not, skip this alternate ASBR and consider next ASBR.
5b.

B. If route type is types are not the same, skip this alternate ASBR and
consider next alternate ASBR.

6. Apply inequality on the alternate ASBR.

Figure 5: Rules for selecting alternate ASBR in OSPF

4.2.1.1. Multiple ASBRs belonging different area Belonging to Different Areas

When "RFC1583compatibility" RFC1583Compatibility is set to disabled, "disabled", OSPF [RFC2328]
defines certain rules of preference to choose the ASBRs. While
selecting an alternate ASBR for loop evaluation for LFA, these rules
should be applied to ensure that the alternate neighbor does not
cause looping.

When there are multiple ASBRs belonging to different area areas
advertising the same prefix, pruning rules as defined in [RFC2328] section Section 16.4
of [RFC2328] are applied. The alternate ASBRs pruned using above these
rules are not considered for LFA evaluation.

4.2.1.2. Type 1 and Type 2 costs Costs

If there are multiple ASBRs not pruned via the rules described in
Section 4.2.1.1, the cost type advertised by the ASBRs is compared.
ASBRs advertising type 1 costs are preferred preferred, and the type 2 costs
are pruned. If two ASBRs advertise the same type 2 cost, the
alternate ASBRs are considered along with their cost to reach the
ASBR/forwarding address for evaluation. If the two ASBRs have the
same type 2 cost as well as the same cost to reach the ASBR, ECMP FRR
is programmed. When there are multiple ASBRs advertising the same
type 2 cost for the prefix, primary Autonomous System (AS) external
route calculation calculation, as described in
[RFC2328] section Section 16.4.1 of [RFC2328],
selects the route with the lowest type 2 cost. ASBRs advertising a
different type 2 cost (higher cost) are not considered for LFA
evaluation. Alternate ASBRs advertising a type 2 cost for the prefix
but are not chosen as primary due to a higher cost to reach ASBR are
considered for LFA evaluation. The inequalities for evaluating
alternate ASBR for type 1 and type 2 costs are same, as the alternate
ASBRs with different type 2 costs are pruned and the evaluation is
based on ASBRS with equal type 2 cost ASBRS. costs.

4.2.1.3. RFC1583compatibility RFC1583Compatibility is set Set to enabled "Enabled"

When RFC1583Compatibility is set to enabled, "enabled", multiple ASBRs
belonging to different area areas advertising the same prefix are chosen
based on cost and hence are valid alternate ASBRs for the LFA
evaluation. The inequalities described in Section 4.2.2 are
applicable based on forwarding address and cost type advertised in External
the external Link State Advertisement (LSA).

4.2.1.4. Type 7 routes Routes

Type 5 routes always get preference over Type 7 type 7, and the alternate
ASBRs chosen for LFA calculation should belong to the same type.
Among
Type type 7 routes, routes with the p-bit and forwarding address set
have a higher preference than routes without these attributes.
Alternate ASBRs selected for LFA comparison should have the same
p-bit and forwarding address attributes.

4.2.2. Inequalities to be applied Be Applied for alternate Alternate ASBR selection Selection

The alternate ASBRs selected using above the mechanism described in
Section 4.2.1, 4.2.1 are evaluated for Loop free loop-free criteria using below
inequalities. the
inequalities below.

4.2.2.1. Forwarding address set Address Set to non-zero value Non-zero Value

Similar to the inequalities as defined in Figure 1, Section 2, the following
inequalities are defined when the forwarding address is a non-zero
value.

Link-Protection:

Link-Protecting LFAs:

F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
F_opt(S,PO_best) + Cost(PO_best,P)

Link-Protection

Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:

F_opt(N,PO_i)+ Cost(PO_i,P) < F_opt(S,PO_best) + Cost(PO_best,P)

Node-Protection:

Node-Protecting LFAs:

F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
F_opt(E,PO_best) + Cost(PO_best,P)

Where,

Where:

P - The multi-homed prefix MHP being evaluated for computing alternates

S - The computing router

N - The alternate router being evaluated

E - The primary next-hop next hop on the shortest path from S to
prefix P. P

PO_i - The specific prefix-originating router being
evaluated.
evaluated
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P. P

Cost(X,Y) - External The external cost for Y as advertised by X

F_opt(X,Y) - Distance The distance on the shortest path from node X to Forwarding
the forwarding address specified by ASBR Y. Y

D_opt(X,Y) - Distance The distance on the shortest path from node X to
node Y.

Figure 6: LFA inequality definition when forwarding address is non-
zero Y

4.2.2.2. ASBRs advertising type1 Advertising Type 1 and type2 cost Type 2 Costs

Similar to the inequalities as defined in Figure 1, Section 2, the following
inequlities
inequalities are defined for type1 type 1 and type2 cost.

Link-Protection: type 2 costs.

Link-Protecting LFAs:

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
D_opt(S,PO_best) + Cost(PO_best,P)

Link-Protection

Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)

Node-Protection:

Node-Protecting LFAs:

D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
D_opt(E,PO_best) + Cost(PO_best,P)

Where,

Where:

P - The multi-homed prefix MHP being evaluated for computing alternates

S - The computing router

N - The alternate router being evaluated

E - The primary next-hop next hop on the shortest path from S to
prefix P. P

PO_i - The specific prefix-originating router being
evaluated.
evaluated

PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P. P
Cost(X,Y) - External The external cost for Y as advertised by X. X

D_opt(X,Y) - Distance The distance on the shortest path from node X to
node Y.

Figure 7: LFA inequality definition for type1 and type2 cost Y

5. LFA Extended Procedures

This section explains the additional considerations in various
aspects as listed below to the base LFA base
specification [RFC5286].

5.1. Links with IGP MAX_METRIC

Section

Sections 3.5 and 3.6 of [RFC5286] describe procedures for excluding
nodes and links from use in alternate paths based on the maximum link
metric. If these procedures are strictly followed, there are
situations, as described below, where the only potential alternate
available which that satisfies the basic loop-free condition will not be
considered as alternative. This document refers to the maximum link
metric in IGPs as the MAX_METRIC. MAX_METRIC is defined for IS-IS in
[RFC5305], where it is called as "maximum link
metric" when defined for IS-IS in [RFC5305] and "MaxLinkMetric" when
defined for OSPF in [RFC6987], where it is called as "MaxLinkMetric". [RFC6987].

+---+ 10 +---+ 10 +---+
| S |------|N1 |-----|D1 |
+---+ +---+ +---+
| |
10 | 10 |
|MAX_METRIC(N2 to S) |
| |
| +---+ |
+-------|N2 |--------+
+---+
10 |
+---+
|D2 |
+---+

Figure 8: 3: Link with IGP MAX_METRIC

In the simple example network, network in Figure 3, all the link costs links have a cost
of 10 in both directions, except for the link between S and N2. The
S-N2 link has a cost of 10 in the forward direction direction, i.e., from S to
N2, and a cost of MAX_METRIC (0xffffff /2^24 - 1 for IS-IS and 0xffff
for OSPF) in the reverse direction direction, i.e., from N2 to S for a specific end-
to-end Traffic Engineering (TE)
end-to-end TE requirement of the operator. At node S, D1 is
reachable through N1 with a cost of 20, and D2 is reachable through
N2 with a cost of 20. Even though neighbor N2 satisfies the basic
loop-free condition (inequality 1 of [RFC5286]) for D1, S's neighbor
N2 could be excluded as a potential alternative because of the
current exclusions as specified in section Sections 3.5 and 3.6 procedure of [RFC5286].
But, as the primary traffic destined to D2 continues to use the link and hence link;
hence, irrespective of the reverse metric in this case, the same link
MAY be used as a potential LFA for D1.

Alternatively, the reverse metric of the link MAY be configured with
MAX_METRIC-1,
MAX_METRIC-1 so that the link can be used as an alternative while
meeting the operator's TE requirements and without having to update
the router to fix this particular issue.

5.2. Multi Topology MT Considerations

Section

Sections 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF
and IS-IS are out of scope for that specification. This memo
clarifies and describes the applicability.

In Multi Topology (MT) multi-topology IGP deployments, for each MT ID, MT-ID, a separate
shortest path tree (SPT) is built with topology specific adjacencies, topology-specific adjacencies
so the LFA principles laid out in [RFC5286] are actually applicable
for MT IS-IS [RFC5120] LFA SPF. The primary difference in this case
is,
is identifying the eligible-set eligible set of neighbors for each LFA
computation which
computation; this is done per MT ID. MT-ID. The eligible-set eligible set for each MT ID MT-ID
is determined by the presence of IGP adjacency from Source the source to the
neighboring node on that MT-ID apart from the administrative
restrictions and other checks laid out in [RFC5286]. The same is
also applicable for MT-OSPF [RFC4915] or different AFs in multi multi-
instance OSPFv3 [RFC5838].

However

However, for MT IS-IS, if a "standard unicast topology" is used with
MT-ID #0
[RFC5286] [RFC5120] and both IPv4 [RFC5305] and IPv6 routes/AFs
[RFC5308] are present, then the condition of network congruency is
applicable for LFA computation as well. Network congruency here
refers to, to having the same address families provisioned on all the
links and all the nodes of the network with MT-ID #0. Here Here, with single decision process a
single-decision process, both IPv4 and IPv6 next-hops next hops are computed
for all the prefixes in the
network and similarly network. Similarly, with one LFA
computation from all eligible neighbors per [RFC5286], all potential
alternatives can be computed.

6. IANA Considerations

This document has no actions for IANA.

9. IANA actions.

7. Security Considerations

The existing OSPF security considerations continue to apply, as do
the recommended manual key management mechanisms specified in
[RFC7474]. The existing security considerations for IS-IS also
continue to apply, as specified in [RFC5304] and [RFC5310] and
extended by [RFC7645] for KARP. Keying and Authentication for Routing
Protocols (KARP). This document does not change any of the discussed
protocol specifications [RFC1195] [RFC5120] [RFC2328]
[RFC5838], (i.e., [RFC1195], [RFC5120], [RFC2328], and
[RFC5838]); therefore, the security considerations of the LFA base
specification [RFC5286] therefore continue to apply.

10.

8. References

10.1.

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.

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>.

[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.

[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.

[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.

[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.

[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.

[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.

[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
R. Aggarwal, "Support of Address Families in OSPFv3",
RFC 5838, DOI 10.17487/RFC5838, April 2010,
<https://www.rfc-editor.org/info/rfc5838>.

[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013,
<https://www.rfc-editor.org/info/rfc6987>.

[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.

[RFC7645] Chunduri, U., Tian, A., and W. Lu, "The Keying and
Authentication for Routing Protocol (KARP) IS-IS Security
Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,
<https://www.rfc-editor.org/info/rfc7645>.

Acknowledgements

Authors

The authors acknowledge Alia Atlas and Salih K A K.A. for their useful
feedback and inputs. input. Thanks to Stewart Bryant for being document
shepherd Document
Shepherd and providing detailed review comments. Thanks to Elwyn
Davies for reviewing and providing feedback as part of Gen-art the Gen-ART
review. Thanks to Alvaro Retena, Retana, Adam Roach, Ben Campbell, Benjamin
Kaduk
Kaduk, and sponsoring Routing Area Director Martin Vigoureux for
providing detailed feedback and suggestions.

8. Contributing Authors

Contributors

The following people contributed substantially to the content of this
document and should be considered co-authors. coauthors:

Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Ave, Ave.
Sunnyvale, CA 94089, USA 94089
United States of America

Email: cbowers@juniper.net

Bruno Decraene
Orange,
Orange
France

Email: bruno.decraene@orange.com

Authors' Addresses

Pushpasis Sarkar (editor)
Arrcus, Inc.

Email: pushpasis.ietf@gmail.com

Uma Chunduri (editor)
Huawei USA
2330 Central Expressway
Santa Clara, CA 95050
USA
United States of America

Email: uma.chunduri@huawei.com

Shraddha Hegde
Juniper Networks, Inc.
Electra, Exora Business Park
Bangalore, KA 560103
India

Email: shraddha@juniper.net

Jeff Tantsura
Apstra, Inc.

Email: jefftant.ietf@gmail.com

Hannes Gredler
RtBrick, Inc.

Email: hannes@rtbrick.com