draft-ietf-rtgwg-lfa-applicability-01.txt   draft-ietf-rtgwg-lfa-applicability-02.txt 
Network Working Group Clarence Filsfils Network Working Group Clarence Filsfils
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Expires: September 14, 2011 Pierre Francois Expires: November 5, 2011 Pierre Francois
UCLouvain UCLouvain
Mike Shand Mike Shand
Cisco Systems Cisco Systems
Bruno Decraene Bruno Decraene
France Telecom France Telecom
James Uttaro James Uttaro
ATT ATT
Nicolai Leymann Nicolai Leymann
Martin Horneffer Martin Horneffer
Deutsche Telekom Deutsche Telekom
March 13, 2011 May 4, 2011
LFA applicability in SP networks LFA applicability in SP networks
draft-ietf-rtgwg-lfa-applicability-01 draft-ietf-rtgwg-lfa-applicability-02
Abstract Abstract
In this draft, we analyze the applicability of LoopFree Alternates in In this draft, we analyze the applicability of LoopFree Alternates in
both core and access parts of Service Provider networks. We provide both core and access parts of Service Provider networks. We provide
design guides to favor their applicability where relevant, typically design guides to favor their applicability where relevant, typically
in the access part of the network. in the access part of the network.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Access Network . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Access Network . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Triangle . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Triangle . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. E1C1 failure . . . . . . . . . . . . . . . . . . . . . 8 3.1.1. E1C1 failure . . . . . . . . . . . . . . . . . . . . . 8
3.1.2. C1E1 failure . . . . . . . . . . . . . . . . . . . . . 9 3.1.2. C1E1 failure . . . . . . . . . . . . . . . . . . . . . 9
3.1.3. uLoop . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.3. uLoop . . . . . . . . . . . . . . . . . . . . . . . . 9
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emphasize the independence between protection schemes used in the emphasize the independence between protection schemes used in the
core and at the access level of the network. Finally we discuss the core and at the access level of the network. Finally we discuss the
key benefits of LFA which stem from its simplicity and we draw some key benefits of LFA which stem from its simplicity and we draw some
conclusions. conclusions.
2. Terminology 2. Terminology
In this document, we assume that all links to be protected are point- In this document, we assume that all links to be protected are point-
to-point. to-point.
We use ISIS as reference. The analysis is equally applicable to We use ISIS as reference. It is assumed that normal routing (i.e.,
OSPF. when traffic not being fast re-routed around a failure) occurs along
the shortest path. The analysis is equally applicable to OSPF.
A per-prefix LFA at a destination D for a node S is a precomputed A per-prefix LFA for a destination D at a node S is a precomputed
backup IGP nexthop for that destination. This backup IGP nexthop can backup IGP nexthop for that destination. This backup IGP nexthop can
be link protecting or node protecting. be link protecting or node protecting.
Link-protecting: A neighbor N is a link-protecting per-prefix LFA for Link-protecting: A neighbor N is a link-protecting per-prefix LFA for
S's route to D if equation eq1 is satisfied, with eq1 == ND < NS + SD S's route to D if equation eq1 is satisfied, with eq1 == ND < NS + SD
where XY refers to the IGP distance from X to Y. This is in line with where XY refers to the IGP distance from X to Y. This is in line with
the definition of an LFA in [RFC5714]. the definition of an LFA in [RFC5714].
Node-protecting: A Neighbor N is a node-protecting LFA for S's route Node-protecting: A Neighbor N is a node-protecting LFA for S's route
to D, with initial IGP nexthop F if N is a link-protecting LFA for D to D, with initial IGP nexthop F if N is a link-protecting LFA for D
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o A prefix P models an important IGP destination that is not present o A prefix P models an important IGP destination that is not present
in the local access region. The igp metric from C1 to P is x and in the local access region. The igp metric from C1 to P is x and
the metric from C2 to P is x+e. the metric from C2 to P is x+e.
o We analyze LFA coverage against all link and node failures within o We analyze LFA coverage against all link and node failures within
the access region. the access region.
o WxYz refers to the link from Wx to Yz. o WxYz refers to the link from Wx to Yz.
o We assume that c < d + u and a < d + u (commonly agreed design o We assume that c < d + u and a < d + u (commonly agreed design
rule). rule).
o In the square access design, we assume that c < a (commonly agreed o In the square access design, we assume that c < a (commonly agreed
design rule). design rule).
o We analyze the most frequent topologies found in an access region.
o We analyze the most frequent topologies found in an access region.
o We first analyze per-prefix LFA applicability and then per-link. o We first analyze per-prefix LFA applicability and then per-link.
o The topologies are symmetric with respect to a vertical axe and o The topologies are symmetric with respect to a vertical axe and
hence we only detail the logic for the link and node failures of hence we only detail the logic for the link and node failures of
the left half of the topology. the left half of the topology.
o We do not consider SRLGs. Future revisions of the draft will o We do not consider SRLGs. Future revisions of the draft will
address this topic. address this topic.
3.1. Triangle 3.1. Triangle
We describe the LFA applicability for the failures of each direction We describe the LFA applicability for the failures of each direction
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