Internet DRAFT - draft-ietf-rtgwg-multihomed-prefix-lfa
draft-ietf-rtgwg-multihomed-prefix-lfa
Routing Area Working Group P. Sarkar, Ed.
Internet-Draft Arrcus, Inc.
Updates: 5286 (if approved) U. Chunduri, Ed.
Intended status: Standards Track Huawei USA
Expires: May 25, 2019 S. Hegde
Juniper Networks, Inc.
J. Tantsura
Apstra, Inc.
H. Gredler
RtBrick, Inc.
November 21, 2018
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 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. It also provides
detailed criteria for evaluating potential alternates for external
prefixes advertised by OSPF ASBRs. This documents updates and
expands some of the "Routing Aspects" as specified in Section 6 of
RFC 5286.
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.
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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
Sarkar, et al. Expires May 25, 2019 [Page 1]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 25, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LFA inequalities for MHPs . . . . . . . . . . . . . . . . . . 4
3. LFA selection for the multi-homed prefixes . . . . . . . . . 5
3.1. Improved coverage with simplified approach to MHPs . . . 7
3.2. IS-IS ATT Bit considerations . . . . . . . . . . . . . . 8
4. LFA selection for the multi-homed external prefixes . . . . . 9
4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Rules to select alternate ASBR . . . . . . . . . . . 9
4.2.1.1. Multiple ASBRs belonging different area . . . . . 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 and type2 cost . . . . . 13
5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 13
5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 13
5.2. Multi Topology Considerations . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
Sarkar, et al. Expires May 25, 2019 [Page 2]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
A framework for the development of IP fast-reroute mechanisms is
detailed in [RFC5714]. The use of LFAs for IP Fast Reroute is
specified in [RFC5286]. If a prefix is advertised by more than one
router that prefix is called as 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 a 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, additional considerations
to [RFC5286], to address a few coverage and operational observations.
These observations are in the area of handling IS-IS attach (ATT) bit
in Level-1 (L1) area, links provisioned with MAX_METRIC (see
Section 5.1) for traffic engineering (TE) purposes and in the area of
Multi Topology (MT) IGP deployments. These are elaborated in detail
in Section 3.2 and Section 5.
This specification uses the same terminology introduced in [RFC5714]
to represent LFA and builds on the inequalities notation used in
[RFC5286] to compute LFAs for MHPs.
1.1. Acronyms
AF - Address Family
ATT - IS-IS Attach Bit
ECMP - Equal Cost Multi Path
IGP - Interior Gateway Protocol
Sarkar, et al. Expires May 25, 2019 [Page 3]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
IS-IS - Intermediate System to Intermediate System
LFA - Loop-Free Alternate
LSP - IS-IS Link State PDU
OSPF - Open Shortest Path First
MHP - Multi-homed Prefix
MT - Multi Topology
SPF - Shortest Path First
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
is defined in [RFC5714], which is nothing but the metric sum of the
shortest path from X to Y and 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
Sarkar, et al. Expires May 25, 2019 [Page 4]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
Link-Protection:
A neighbor N can provide a loop-free alternate (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 + Downstream-paths-only:
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:
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 multi-homed prefix being evaluated for
computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on 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) - Cost of reaching the prefix P from prefix
originating node X.
D_opt(X,Y) - Distance on the shortest path from node X to node
Y.
Figure 1: LFA inequalities for MHPs
3. LFA selection for the multi-homed prefixes
To compute a valid LFA for a given MHP P, a computing router S MUST
follow one of the appropriate procedures below, for each alternate
neighbor N and once for each remote node that originated the prefix
P.
Sarkar, et al. Expires May 25, 2019 [Page 5]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
Link-Protection :
=================
1. if, in addition to being an alternate neighbor, N is also a prefix-originator of P,
1.a. Select N as a 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. If LFA inequality condition is met,
select N as a LFA for prefix P.
2.b. Else, N is not a LFA for prefix P.
Link-Protection + Downstream-paths-only :
=========================================
1. Evaluate the link-protecting + downstream-only LFA inequality
for P with the N as the alternate neighbor.
1.a. If LFA inequality condition is met,
select N as a LFA for prefix P.
1.b. Else, N is not a LFA for prefix P.
Node-Protection :
=================
1. if, in addition to being an alternate neighbor, N is also a prefix-originator of P,
1.a. Select N as a 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. If LFA inequality condition is met,
select N as a LFA for prefix P.
2.b. Else, N is not a LFA for prefix P.
Figure 2: Rules for selecting LFA for MHPs
In case an alternate neighbor N is also one of the 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 N MUST be chosen as a valid LFA for prefix P, without evaluating
any of the inequalities in Figure 1 as long as 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 LFA inequality specified in Figure 1 for neighbor N
and ensure router N satisfies the inequality.
However, if N is not a prefix-originator of P, the computing router
MUST evaluate one of the corresponding LFA inequalities, as mentioned
in Figure 1, once for each remote node that originated the 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.
Sarkar, et al. Expires May 25, 2019 [Page 6]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
For more specific rules please refer to the later sections of this
document.
3.1. Improved coverage with simplified approach to MHPs
LFA base specification [RFC5286] Section 6.1 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 and
the same has been elaborated with appropriate inequalities in the
above section. However, [RFC5286] Section 6.1 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 equal cost multi path
(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 the below example network presented in Figure 3. 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: MHP with same ECMP Next-hop
In the above network a prefix P, is advertised from both Node E and
Node F. With simplified approach taken as specified in [RFC5286]
Section 6.1, prefix P will get only link protection 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
Sarkar, et al. Expires May 25, 2019 [Page 7]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
E (link protection) and inherit the better alternative to prefix P
and here it is A.
However, in the below example network presented in Figure 4, prefix P
has an ECMP through both node 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 optimal point of attachment is different. In
this case, prefix P MUST inherit corresponding LFAs of each primary
next-hop calculated for the router advertising the same respectively.
In the below diagram that would be node E's and node F's LFA 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 4: MHP with different ECMP Next-hops
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
primary next-hop of the router that was pre-failure optimal point of
attachment, an implementation MAY provide a better protection to MHP
without incurring any additional computation cost.
3.2. IS-IS ATT Bit considerations
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
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-
hop of the default route set to the adjacent router through which the
closest L1/L2 router is reachable. The base LFA specification
[RFC5286] does not specify any procedure for computing LFA for a
default route in IS-IS L1 area. This document specifies, a node can
consider a default route is being advertised from the border L1/L2
Sarkar, et al. Expires May 25, 2019 [Page 8]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
router where 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 default route as this would be
similar to ECMP MHP example as described in Section 3.1.
Considerations as specified in Section 3 and Section 3.1 are
applicable for default routes, if the default route is considered as
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 the multi-homed external prefixes
Redistribution of external routes into IGP is required in case of two
different networks getting merged into one or 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 ASBR hereafter)
redistributing the external routes for LFA selection as defined in
[RFC5286] and leverage the advantage of having multiple re-
distributing nodes in the network.
4.1. IS-IS
LFA evaluation for multi-homed external prefixes in IS-IS is same to
the multi-homed internal prefixes. Inequalities described in
Section 2 would also apply to multi-homed external prefixes.
4.2. OSPF
Loop Free Alternates [RFC5286] describes mechanisms to apply
inequalities to find the loop free alternate neighbor. For the
selection of alternate ASBR for LFA consideration, additional rules
have to be applied in selecting the alternate ASBR due to the
external route calculation rules imposed by [RFC2328].
This document defines inequalities specifically for the alternate
loop-free ASBR evaluation, based on those in [RFC5286].
4.2.1. Rules to select alternate ASBR
The process to select an alternate ASBR is best explained using the
rules below. The below process is applied when primary ASBR for the
Sarkar, et al. Expires May 25, 2019 [Page 9]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
concerned prefix is chosen and there is an alternate ASBR originating
same prefix.
1. If RFC1583Compatibility is disabled
1a. if primary ASBR and alternate ASBR belong to intra-area
non-backbone go to step 2.
1b. If primary ASBR and alternate ASBR belong to
intra-area backbone and/or inter-area path go
to step 2.
1c. for other paths, skip this alternate ASBR and
consider next ASBR.
2. Compare cost types (type 1/type 2) advertised by alternate ASBR and
by the primary ASBR
2a. If not the same type skip alternate ASBR and
consider next ASBR.
2b. If same proceed to step 3.
3.If cost types are type 1, compare costs advertised by alternate ASBR
and by the primary ASBR
3a. If costs are the same then program ECMP Fast ReRoute (FRR) and return.
3b. else go to step 5..
4 If cost types are type 2, compare costs advertised by alternate ASBR
and by the primary ASBR
4a. If costs are different, skip alternate ASBR and
consider next ASBR.
4b. If cost are the same, proceed to step 4c to compare
cost to reach ASBR/forwarding address.
4c. If cost to reach ASBR/forwarding address are also same
program ECMP FRR and return.
4d. If cost to reach ASBR/forwarding address are different
go to step 5.
5. If route type (type 5/type 7)
5a. If route type is 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
proceed to step 6.
If not, skip this alternate ASBR and consider
next ASBR.
5b. If route type is not 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
Sarkar, et al. Expires May 25, 2019 [Page 10]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
4.2.1.1. Multiple ASBRs belonging different area
When "RFC1583compatibility" is set to disabled, OSPF [RFC2328]
defines certain rules of preference to choose the ASBRs. While
selecting 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 advertising
the same prefix, pruning rules as defined in [RFC2328] section 16.4
are applied. The alternate ASBRs pruned using above 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 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 same type 2 cost, the alternate ASBRs
are considered along with their cost to reach ASBR/forwarding address
for evaluation. If the two ASBRs have same type 2 cost as well as
same cost to reach ASBR, ECMP FRR is programmed. When there are
multiple ASBRs advertising same type 2 cost for the prefix, primary
Autonomous System (AS) external route calculation as described in
[RFC2328] section 16.4.1 selects the route with lowest type 2 cost.
ASBRs advertising different type 2 cost (higher cost) are not
considered for LFA evaluation. Alternate ASBRs advertising type 2
cost for the prefix but are not chosen as primary due to 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 equal type 2 cost ASBRS.
4.2.1.3. RFC1583compatibility is set to enabled
When RFC1583Compatibility is set to enabled, multiple ASBRs belonging
to different area advertising 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 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 same type. Among
Type 7 routes, routes with p-bit and forwarding address set have
higher preference than routes without these attributes. Alternate
Sarkar, et al. Expires May 25, 2019 [Page 11]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
ASBRs selected for LFA comparison should have same p-bit and
forwarding address attributes.
4.2.2. Inequalities to be applied for alternate ASBR selection
The alternate ASBRs selected using above mechanism described in
Section 4.2.1, are evaluated for Loop free criteria using below
inequalities.
4.2.2.1. Forwarding address set to non-zero value
Similar to inequalities as defined in Figure 1, the following
inequalities are defined when forwarding address is a non-zero value.
Link-Protection:
F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
F_opt(S,PO_best) + Cost(PO_best,P)
Link-Protection + Downstream-paths-only:
F_opt(N,PO_i)+ Cost(PO_i,P) < F_opt(S,PO_best) + Cost(PO_best,P)
Node-Protection:
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 multi-homed prefix being evaluated for
computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on 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) - External cost for Y as advertised by X
F_opt(X,Y) - Distance on the shortest path from node X to Forwarding
address specified by ASBR Y.
D_opt(X,Y) - Distance on the shortest path from node X to node Y.
Figure 6: LFA inequality definition when forwarding address is non-
zero
Sarkar, et al. Expires May 25, 2019 [Page 12]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
4.2.2.2. ASBRs advertising type1 and type2 cost
Similar to inequalities as defined in Figure 1, the following
inequlities are defined for type1 and type2 cost.
Link-Protection:
D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
D_opt(S,PO_best) + Cost(PO_best,P)
Link-Protection + Downstream-paths-only:
D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)
Node-Protection:
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 multi-homed prefix being evaluated for
computing alternates
S - The computing router
N - The alternate router being evaluated
E - The primary next-hop on 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) - External cost for Y as advertised by X.
D_opt(X,Y) - Distance on the shortest path from node X to node Y.
Figure 7: LFA inequality definition for type1 and type2 cost
5. LFA Extended Procedures
This section explains the additional considerations in various
aspects as listed below to the base LFA specification [RFC5286].
5.1. Links with IGP MAX_METRIC
Section 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 satisfies the basic loop-free condition will not be
considered as alternative. This document refers the maximum link
metric in IGPs as the MAX_METRIC. MAX_METRIC is defined for IS-IS in
Sarkar, et al. Expires May 25, 2019 [Page 13]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
[RFC5305], where it is called as "maximum link metric" and defined
for OSPF in [RFC6987], where it is called as "MaxLinkMetric".
+---+ 10 +---+ 10 +---+
| S |------|N1 |-----|D1 |
+---+ +---+ +---+
| |
10 | 10 |
|MAX_METRIC(N2 to S) |
| |
| +---+ |
+-------|N2 |--------+
+---+
10 |
+---+
|D2 |
+---+
Figure 8: Link with IGP MAX_METRIC
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
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 Traffic Engineering (TE) requirement of the operator. At node
S, D1 is reachable through N1 with cost 20, and D2 is reachable
through N2 with cost 20. Even though neighbor N2 satisfies 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 3.5 and 3.6 procedure of
[RFC5286]. But, as the primary traffic destined to D2 continues to
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
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 Considerations
Section 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.
Sarkar, et al. Expires May 25, 2019 [Page 14]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
In Multi Topology (MT) IGP deployments, for each 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 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
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 topology" is used with MT-ID #0
[RFC5286] 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
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
IPv4 and IPv6 next-hops are computed for all the prefixes in the
network and 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.
7. Acknowledgements
Authors acknowledge Alia Atlas and Salih K A for their useful
feedback and inputs. 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 Gen-art
review. Thanks to Alvaro Retena, Adam Roach, Ben Campbell, Benjamin
Kaduk and sponsoring Routing Area Director Martin Vigoureux for
providing detailed feedback and suggestions.
8. Contributing Authors
The following people contributed substantially to the content of this
document and should be considered co-authors.
Sarkar, et al. Expires May 25, 2019 [Page 15]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Ave,
Sunnyvale, CA 94089, USA
Email: cbowers@juniper.net
Bruno Decraene
Orange,
France
Email: bruno.decraene@orange.com
9. 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. This document does not change any of
the discussed protocol specifications [RFC1195] [RFC5120] [RFC2328]
[RFC5838], and the security considerations of the LFA base
specification [RFC5286] therefore continue to apply.
10. References
10.1. 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. 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>.
Sarkar, et al. Expires May 25, 2019 [Page 16]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
[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>.
Sarkar, et al. Expires May 25, 2019 [Page 17]
Internet-DraftLoop-Free Alternates selection for Multi-HomeNovember 2018
[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>.
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
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
Sarkar, et al. Expires May 25, 2019 [Page 18]
