Internet Engineering Task Force (IETF) P. Sarkar, Ed.
Request for Comments: 8518 Arrcus, Inc.
Updates: 5286 U. Chunduri, Ed.
Category: Standards Track Huawei USA
ISSN: 2070-1721 S. Hegde
Juniper Networks, Inc.
J. Tantsura
Apstra, Inc.
H. Gredler
RtBrick, Inc.
February 2019
Selection of Loop-Free Alternates for Multihomed Multi-Homed Prefixes
Abstract
Deployment experience gained from implementing algorithms to
determine Loop-Free Alternates (LFAs) for multihomed multi-homed prefixes (MHPs)
has revealed some avenues for potential improvement. This document
provides explicit inequalities that can be used to evaluate neighbors
as potential alternates for MHPs. It also provides detailed criteria
for evaluating potential alternates for external prefixes advertised
by OSPF ASBRs. This document updates Section 6 of RFC 5286 by
expanding some of the routing aspects.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8518.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. LFA Inequalities for MHPs . . . . . . . . . . . . . . . . . . 4
3. LFA Selection for MHPs . . . . . . . . . . . . . . . . . . . 5
3.1. Improved Coverage with Simplified Approach to MHPs . . . 6
3.2. IS-IS ATT Bit Considerations . . . . . . . . . . . . . . 8
4. LFA Selection for Multihomed Multi-Homed External Prefixes . . . . . . . 9
4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Rules to Select Alternate ASBRs . . . . . . . . . . . 9
4.2.1.1. Multiple ASBRs Belonging to Different Areas . . . 11
4.2.1.2. Type 1 and Type 2 Costs . . . . . . . . . . . . . 11
4.2.1.3. RFC1583Compatibility is Set to "Enabled" . . . . 11
4.2.1.4. Type 7 Routes . . . . . . . . . . . . . . . . . . 11
4.2.2. Inequalities to Be Applied for Alternate ASBR
Selection . . . . . . . . . . . . . . . . . . . . . . 12
4.2.2.1. Forwarding Address Set to Non-zero Value . . . . 12
4.2.2.2. ASBRs Advertising type1 Type 1 and type2 Type 2 Costs . . . . . 13
5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 14
5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 14
5.2. MT Considerations . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
A framework for the development of IP Fast Reroute (FRR) mechanisms
is detailed in [RFC5714]. The use of LFAs for IP FRR is specified in
[RFC5286]. If a prefix is advertised by more than one router, that
prefix is called a "multihomed "multi-homed prefix (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 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 and additional
considerations to [RFC5286] to address a few coverage and operational
observations. These observations are in the area of handling concerned with 1) the IS-
IS IS-IS ATT
(attach) bit in the Level 1 (L1) area, in 2) links provisioned with
MAX_METRIC (see Section 5.1) for traffic engineering (TE) purposes,
and in 3) multi-topology (MT) IGP deployments. These are elaborated in
detail in Sections 3.2 and 5.
This specification uses the same terminology introduced in [RFC5714]
to represent LFA and builds on the 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 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 - Multihomed Multi-Homed Prefix
MT - Multi-Topology
OSPF - Open Shortest Path First
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. LFA 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.
Cost(X,Y), introduced in this 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:
Link-Protecting LFAs:
A neighbor N can provide 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.
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 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 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).
D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
D_opt(E,PO_best) + Cost(PO_best,P)
Where:
P - The MHP being evaluated for computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next hop on the shortest path from S to
prefix P
PO_i - The specific prefix-originating router being
evaluated
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P
Cost(X,P) - The cost of reaching the prefix P from prefix
originating node X
D_opt(X,Y) - The distance on the shortest path from node X to
node Y
3. LFA Selection for 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 once for each remote node that originated the prefix
P.
Link-Protection:
Link-Protecting LFAs:
1. If, in addition to being an alternate neighbor, N is also a
prefix-originator
prefix originator of P,
A. Select N as 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 N as
the alternate neighbor.
A. If the LFA inequality condition is met, select N as an LFA
for prefix P.
B. Else, N is not an LFA for prefix P.
Link-Protection
Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:
1. Evaluate the link-protecting + downstream-only downstream-paths-only LFA
inequality for P with N as the alternate neighbor.
A. If the LFA inequality condition is met, select N as an LFA
for prefix P.
B. Else, N is not an LFA for prefix P.
Node-Protection:
Node-Protecting LFAs:
1. If, in addition to being an alternate neighbor, N is also a
prefix-originator
prefix originator of P,
A. Select N as 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 N as the alternate neighbor.
A. If the LFA inequality condition is met, select N as an LFA
for prefix P.
B. Else, N is not an LFA for prefix P.
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. Therefore, N MUST be chosen as a
valid LFA for prefix P without evaluating any of the inequalities in
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 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, inequalities defined in
Section 2, 2 once for each remote node that originated the prefix. If
the inequality is satisfied by the neighbor N, router S MUST choose
neighbor N as one of the valid LFAs for the prefix P.
For more specific rules, please refer to the later sections of this
document. Section 4.
3.1. Improved Coverage with Simplified Approach to MHPs
Section 6.1 of the LFA base specification [RFC5286] recommends that a
router computes the alternate next hop for an IGP MHP by considering
alternate paths via all routers that have announced that prefix. The
same has been elaborated with appropriate inequalities in the
previous section. However, 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 optimal point of attachment, the procedure described in
this memo can potentially improve coverage for 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
in the example network presented in Figure 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 1: MHP with Same ECMP Next Hop
In Figure 1, a prefix P is advertised from both node E and node F.
With a simplified approach taken as specified in Section 6.1 of
[RFC5286], prefix P will get only a link-protection link-protecting LFA through the
neighbor C while a 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. Hence, an
implementation MAY compare the kind of protection A provides to F
(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 example network presented in Figure 2, prefix P has
an ECMP through both node E and node F with cost 20. Though it has
two pre-failure optimal points of attachment, the primary 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 calculated for the router advertising the same. In
Figure 2, that would be the LFA for node E and node F, i.e., node N1
and node N2, respectively.
4 +----+
+------------------| N2 |
| +----+
| | 4
10 +---+ 3 +---+
+------| S |----------------| B |
| +---+ +---+
| | |
| 10 | 1 |
| | |
+----+ 5 +---+ 16 +---+
| N1 |----| E |-----------------| F |
+----+ +---+ +---+
| 10 16 |
+-----------P---------+
Figure 2: MHP with Different ECMP Next Hops
In summary, if there are multiple pre-failure points of attachment
for an MHP, and the primary next hop of an MHP is the same as that of
the primary 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
Per [RFC1195], a default route needs to be added in the Level 1 (L1)
router to the closest reachable 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 hop of the default route set to the adjacent router
through which the closest L1/L2 router is reachable. The 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 that a node can consider a default route is being
advertised from the border L1/L2 router where the ATT bit is set and
can do LFA computation for that default route. But, when multiple
ECMP L1/L2 routers are reachable in an L1 area, corresponding best
LFAs SHOULD be computed for each primary next hop associated with the
default route as this would be similar to the ECMP MHP example
described in Section 3.1. Considerations specified in Sections 3 and
3.1 are applicable for default routes if the default route is
considered an ECMP MHP. Note 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 for Multihomed 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 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 ASBRs"
hereafter) redistributing the external routes for LFA selection as
defined in [RFC5286] and leverage the advantage of having multiple
redistributing nodes in the network.
4.1. IS-IS
LFA evaluation for multihomed multi-homed external prefixes in IS-IS is the same
as the multihomed multi-homed internal prefixes. Inequalities described in
Section 2 would also apply to multihomed 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 alternate neighbor. For the
selection of alternate ASBRs 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 alternate loop-
free ASBR evaluation. These inequalities are based on those in
[RFC5286].
4.2.1. Rules to Select Alternate ASBRs
The process to select an alternate ASBR is best explained using the
rules below. The 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:
A. If primary ASBR and alternate ASBR belong to intra-area non-
backbone, go to step 2.
B. If primary ASBR and alternate ASBR belong to intra-area
backbone and/or inter-area path, go to step 2.
C. For other paths, skip this alternate ASBR and consider next
ASBR.
2. Compare cost types (type 1 / type 2) advertised by alternate ASBR
and primary ASBR:
A. If not the same type, skip alternate ASBR and consider next
ASBR.
B. If the same, proceed to step 3.
3. If cost types are type 1, compare costs advertised by alternate
ASBR and primary ASBR:
A. If costs are the same, then program ECMP FRR and return.
B. Else, go to step 5.
4. If cost types are type 2, compare costs advertised by alternate
ASBR and primary ASBR:
A. If costs are different, skip alternate ASBR and consider next
ASBR.
B. If costs are the same, proceed to step 4C to compare costs to
reach ASBR/forwarding address.
C. If costs to reach ASBR/forwarding address are also the same,
program ECMP FRR and return.
D. If costs to reach ASBR/forwarding address are different, go
to step 5.
5. If Compare route type types (type 5 / and type 7): 7) for alternate ASBR and
primary ASBR:
A. If route type is types are the same, check if route p-bit and
forwarding address field for routes from both ASBRs match.
If p-bit and forwarding address match, proceed to step 6. If
not, skip this alternate ASBR and consider next ASBR.
B. If route type is types are not the same, skip this alternate ASBR and
consider next alternate ASBR.
6. Apply inequality on alternate ASBR.
4.2.1.1. Multiple ASBRs Belonging to Different Areas
When RFC1583Compatibility is set to "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 areas
advertising the same prefix, pruning rules as defined in Section 16.4
of [RFC2328] are applied. The alternate ASBRs pruned using the above these
rules are not considered for LFA evaluation.
4.2.1.2. Type 1 and Type 2 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, 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, as described in 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 not chosen as primary due to a higher cost to reach ASBR are
considered for LFA evaluation. The inequalities for evaluating
alternate ASBRs ASBR for type 1 and type 2 costs are the 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 is Set to "Enabled"
When RFC1583Compatibility is set to "enabled", multiple ASBRs
belonging to different 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
the external Link State Advertisement (LSA).
4.2.1.4. Type 7 Routes
Type 5 routes always get preference over type 7, and the alternate
ASBRs chosen for LFA calculation should belong to the same type.
Among 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 for Alternate ASBR Selection
The alternate ASBRs selected using the mechanism described in
Section 4.2.1 are evaluated for loop-free criteria using the
inequalities below.
4.2.2.1. Forwarding Address Set to Non-zero Value
Similar to the inequalities defined in 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:
P - The MHP being evaluated for computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next hop on the shortest path from S to
prefix P
PO_i - The specific prefix-originating router being
evaluated
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P
Cost(X,Y) - The external cost for Y as advertised by X
F_opt(X,Y) - The distance on the shortest path from node X to
the forwarding address specified by ASBR Y
D_opt(X,Y) - The distance on the shortest path from node X to
node Y
4.2.2.2. ASBRs Advertising type1 Type 1 and type2 Type 2 Costs
Similar to the inequalities defined in Section 2, the following
inequalities are defined for type1 type 1 and type2 type 2 costs.
Link-Protection:
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:
P - The MHP being evaluated for computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next hop on the shortest path from S to
prefix P
PO_i - The specific prefix-originating router being
evaluated
PO_best - The prefix-originating router on the shortest path
from the computing router S to prefix P
Cost(X,Y) - The external cost for Y as advertised by X
D_opt(X,Y) - The distance on the shortest path from node X to
node Y
5. LFA Extended Procedures
This section explains additional considerations to the LFA base
specification [RFC5286].
5.1. Links with IGP MAX_METRIC
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, described below, where the only potential alternate
available 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 called "maximum link
metric" when defined for IS-IS in [RFC5305] where it is called "maximum link metric" and "MaxLinkMetric" when
defined for OSPF in
[RFC6987] where it is called "MaxLinkMetric". [RFC6987].
+---+ 10 +---+ 10 +---+
| S |------|N1 |-----|D1 |
+---+ +---+ +---+
| |
10 | 10 |
|MAX_METRIC(N2 to S) |
| |
| +---+ |
+-------|N2 |--------+
+---+
10 |
+---+
|D2 |
+---+
Figure 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 directions, except for the link between S and N2. The
S-N2 link has a cost of 10 in the forward 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, i.e., from N2 to S for a specific
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 specified in Sections 3.5 and 3.6 of [RFC5286].
But, as the primary traffic destined to D2 continues to use the link 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 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. MT Considerations
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 IGP deployments, for each MT ID, MT-ID, a separate
shortest path tree (SPT) is built with 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 identifying the eligible set of neighbors for each LFA
computation; this is done per MT ID. MT-ID. The eligible set for each MT ID MT-ID
is determined by the presence of IGP adjacency from 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-
instance OSPFv3 [RFC5838].
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 having the same address families provisioned on all the
links and all the nodes of the network with MT-ID #0. Here, with a
single-decision process, both IPv4 and IPv6 next hops are computed
for all the prefixes in the 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 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 Keying and Authentication for Routing
Protocols (KARP). This document does not change any of the discussed
protocol specifications (i.e., [RFC1195], [RFC5120], [RFC2328], and
[RFC5838]); therefore, the security considerations of the LFA base
specification [RFC5286] continue to apply.
8. References
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>.
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
The authors acknowledge Alia Atlas and Salih K A K.A. for their useful
feedback and input. Thanks to Stewart Bryant for being Document
Shepherd and providing detailed review comments. Thanks to Elwyn
Davies for reviewing and providing feedback as part of the Gen-ART
review. Thanks to Alvaro Retana, Adam Roach, Ben Campbell, Benjamin
Kaduk, and sponsoring Routing Area Director Martin Vigoureux for
providing detailed feedback and suggestions.
Contributors
The following people contributed substantially to the content of this
document and should be considered coauthors:
Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
United States of America
Email: cbowers@juniper.net
Bruno Decraene
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
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