Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering Tunnels
RFC 3906
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RFC - Informational
(October 2004; No errata)
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2013-03-02
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IETF
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RFC 3906 (Informational)
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Alex Zinin
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fenner@research.att.com, zinin@psg.com
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Network Working Group N. Shen
Request for Comments: 3906 Redback Networks
Category: Informational H. Smit
October 2004
Calculating Interior Gateway Protocol (IGP) Routes
Over Traffic Engineering Tunnels
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes how conventional hop-by-hop link-state
routing protocols interact with new Traffic Engineering capabilities
to create Interior Gateway Protocol (IGP) shortcuts. In particular,
this document describes how Dijkstra's Shortest Path First (SPF)
algorithm can be adapted so that link-state IGPs will calculate IP
routes to forward traffic over tunnels that are set up by Traffic
Engineering.
1. Introduction
Link-state protocols like integrated Intermediate System to
Intermediate System (IS-IS) [1] and OSPF [2] use Dijkstra's SPF
algorithm to compute a shortest path tree to all nodes in the
network. Routing tables are derived from this shortest path tree.
The routing tables contain tuples of destination and first-hop
information. If a router does normal hop-by-hop routing, the first-
hop will be a physical interface attached to the router. New traffic
engineering algorithms calculate explicit routes to one or more nodes
in the network. At the router that originates explicit routes, such
routes can be viewed as logical interfaces which supply Label
Switched Paths through the network. In the context of this document,
we refer to these Label Switched Paths as Traffic Engineering tunnels
(TE-tunnels). Such capabilities are specified in [3] and [4].
The existence of TE-tunnels in the network and how the traffic in the
network is switched over those tunnels are orthogonal issues. A node
may define static routes pointing to the TE-tunnels, it may match the
Shen & Smit Informational [Page 1]
RFC 3906 IGP ShortCut Over MPLS LSPs October 2004
recursive route next-hop with the TE-tunnel end-point address, or it
may define local policy such as affinity based tunnel selection for
switching certain traffic. This document describes a mechanism
utilizing link-state IGPs to dynamically install IGP routes over
those TE-tunnels.
The tunnels under consideration are tunnels created explicitly by the
node performing the calculation, and with an end-point address known
to this node. For use in algorithms such as the one described in
this document, it does not matter whether the tunnel itself is
strictly or loosely routed. A simple constraint can ensure that the
mechanism be loop free. When a router chooses to inject a packet
addressed to a destination D, the router may inject the packet into a
tunnel where the end-point is closer (according to link-state IGP
topology) to the destination D than is the injecting router. In
other words, the tail-end of the tunnel has to be a downstream IGP
node for the destination D. The algorithms that follow are one way
that a router may obey this rule and dynamically make intelligent
choices about when to use TE-tunnels for traffic. This algorithm may
be used in conjunction with other mechanisms such as statically
defined routes over TE-tunnels or traffic flow and QoS based TE-
tunnel selection.
This IGP shortcut mechanism assumes the TE-tunnels have already been
setup. The TE-tunnels in the network may be used for QoS, bandwidth,
redundancy, or fastreroute reasons. When an IGP shortcut mechanism
is applied on those tunnels, or other mechanisms are used in
conjunction with an IGP shortcut, the physical traffic switching
through those tunnels may not match the initial traffic engineering
setup goal. Also the traffic pattern in the network may change with
time. Some forwarding plane measurement and feedback into the
adjustment of TE-tunnel attributes need to be there to ensure that
the network is being traffic engineered efficiently [6].
2. Enhancement to the Shortest Path First Computation
During each step of the SPF computation, a router discovers the path
to one node in the network. If that node is directly connected to
the calculating router, the first-hop information is derived from the
adjacency database. If a node is not directly connected to the
calculating router, it inherits the first-hop information from the
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