Routing Area Working Group P. Sarkar, Ed.
Internet-Draft H. Gredler
Intended status: Informational S. Hegde
Expires: April 30, 2015 C. Bowers
Juniper Networks, Inc.
October 27, 2014
LFA selection for Multi-Homed Prefixes
draft-psarkar-rtgwg-multihomed-prefix-lfa-00
Abstract
This document shares experience gained from implementing algorithms
to determine Loop-Free Alternates for multi-homed prefixes. In
particular, this document provides explicit inequalities that can be
used to evaluate neighbours as a potential alternates for multi-homed
prefixes. It also provides detailed criteria for evaluating
potential alternates for external prefixes advertised by OSPF ASBRs.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. LFA inequalities for MHPs . . . . . . . . . . . . . . . . . . 3
3. LFA selection for the multi-homed external routes . . . . . . 4
3.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. Rules to select alternate ASBR . . . . . . . . . . . 4
3.2.2. Multiple ASBRs belonging different area . . . . . . . 5
3.2.3. Type 1 and Type 2 costs . . . . . . . . . . . . . . . 6
3.2.4. RFC1583compatibility is set to enabled . . . . . . . 6
3.2.5. Type 7 routes . . . . . . . . . . . . . . . . . . . . 6
3.2.6. Inequalities to be applied for alternate ASBR
selection . . . . . . . . . . . . . . . . . . . . . . 6
3.2.6.1. Forwarding address set to non zero value . . . . 6
3.2.6.2. ASBRs advertising type1 and type2 cost . . . . . 7
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The use of Loop-Free Alternates (LFA) for IP Fast Reroute is
specified in [RFC5286]. Section 6.1 of [RFC5286] describes a method
to determine loop-free alternates for a multi-homed prefixes (MHPs).
This document describes a procedure using explicit inequalities that
can be used by a computing router to evaluate a neighbour as a
potential alternate for a multi-homed prefix. The results obtained
are equivalent to those obtained using the method described in
Section 6.1 of [RFC5286]. However, some may find this formulation
useful.
Section 6.3 of [RFC5286] discusses complications associated with
computing LFAs for multi-homed prefixes in OSPF. This document
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provides detailed criteria for evaluating potential alternates for
external prefixes advertised by OSPF ASBRs, as well as explicit
inequalities.
2. LFA inequalities for MHPs
The following set of inequalities can be used to evaluate LFAs for
multi-homed prefixes.
This document proposes the following set of LFA inequalities for
selecting the most appropriate LFAs for multi-homed prefixes (MHPs).
They can be derived from the inequalities in [RFC5286] combined with
the observation that D_opt(N,P) = Min (D_opt(N,PO_i) + cost(PO_i,P))
over all PO_i
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,
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
To compute a valid LFA for a given multi-homed prefix P, a computing
router S shall evaluate for each alternate neighbor N, one of the
above set of inequalities, once for each remote node that originated
the prefix. If the inequality is satisfied by any neighbour N for
any remote prefix-originating node, router S shall choose neighbour
N, as one of the valid LFAs for the prefix P.
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3. LFA selection for the multi-homed external routes
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.
3.1. IS-IS
LFA evaluation for multi-homed external prefixes in IS-IS is similar
to the multi-homed internal prefixes. Inequalities described in sec
2 would also apply to multi-homed external prefixes as well.
3.2. OSPF
Loop free Alternates [RFC 5286] describes mechanisms to apply
inequalities to find the the loop free alternate neighbour. 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 [RFC 2328]. This
document also defines the inequalities defined in RFC [5286]
specifically for the alternate loop-free ASBR evaluation.
3.2.1. 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
concerned prefix is chosen and there is an alternate ASBR originating
same prefix.
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1. If RFC1583Compatibility is disabled
1a. if primary ASBR and alternate ASBR are intra area
non-backbone path 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 the alternate ASBR and
consider next ASBR.
2. If cost type (type1/type2) advertised by alternate
ASBR same as primary
2a. If not same skip alternate ASBR and consider next ASBR.
3. If cost type is type1
3a. If cost is same, program ECMP
3b. else go to step 5.
4 If cost type is type 2
4a. If cost is different, skip alternate ASBR and
consider next ASBR
4b. If type2 cost is same, compare type 1 cost.
4c. If type1 cost is also same program ECMP.
4d. If type 1 cost is different go to step 5.
5. If route type (type 5/type 7)
5a. If route type is same, check route p-bit,
forwarding address field for routes from both
ASBRs
match. If not skip alternate ASBR and consider
next ASBR.
5b. If route type is not same, skip ASBR
and consider next ASBR.
6. Apply inequality on the alternate ASBR.
Figure 2: Rules for selecting alternate ASBR in OSPF
3.2.2. 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 and ensured that the alternate neighbour does not loop the
traffic back.
When there are multiple ASBRs belonging to different area advertising
the same prefix, pruning rules as defined in RFC 2328 section 16.4.1
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are applied. The alternate ASBRs pruned using above rules are not
considered for LFA evaluation.
3.2.3. Type 1 and Type 2 costs
If there are multiple ASBRs not pruned via rules defined in 3.2.2,
the cost type advertised by the ASBRs is compared. ASBRs advertising
Type1 costs are preferred and the type2 costs are pruned.If two ASBRs
advertise same type2 cost, the alternate ASBRs are considered along
with their type1 cost for evaluation.If the two ASBRs with same type2
as well as type1 cost, ECMP FRR is programmed.If there are two ASBRs
with different type2 cost, the higher cost ASBR is pruned.The
inequalities for evaluating alternate ASBR for type 1 and type 2
costs are same, as the alternate ASBRs with different type2 costs are
pruned and the evaluation is based on equal type 2 cost ASBRS.
3.2.4. 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.
3.2.5. 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
ASBRs selected for LFA comparison should have same p-bit and
forwarding address attributes.
3.2.6. Inequalities to be applied for alternate ASBR selection
The alternate ASBRs selected using above mechanism described in
3.2.1, are evaluated for Loop free criteria using below inequalities.
3.2.6.1. Forwarding address set to non zero value
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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,
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 3: LFA inequality definition when forwarding address in non-
zero
3.2.6.2. ASBRs advertising type1 and type2 cost
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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,
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 4: LFA inequality definition for type1 and type 2 cost
4. Acknowledgements
Thanks to Alia Atlas and Salih K A for their useful feedback and
inputs.
5. IANA Considerations
N/A. - No protocol changes are proposed in this document.
6. Security Considerations
This document does not introduce any change in any of the protocol
specifications. It simply proposes additional inequalities for
selecting LFAs for multi-homed prefixes.
7. References
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7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
Authors' Addresses
Pushpasis Sarkar (editor)
Juniper Networks, Inc.
Electra, Exora Business Park
Bangalore, KA 560103
India
Email: psarkar@juniper.net
Hannes Gredler
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: hannes@juniper.net
Shraddha Hegde
Juniper Networks, Inc.
Electra, Exora Business Park
Bangalore, KA 560103
India
Email: shraddha@juniper.net
Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: cbowers@juniper.net
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