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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).


 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

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
 parent(s) of that node.  Each node has one or more parents.  Each
 node is the parent of zero or more down-stream nodes.
 For traffic engineering purposes, each router maintains a list of all
 TE-tunnels that originate at this router.  For each of those TE-
 tunnels, the router at the tail-end is known.

Shen & Smit Informational [Page 2] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

 During SPF, when a router finds the path to a new node (in other
 words, this new node is moved from the TENTative list to the PATHS
 list), the router must determine the first-hop information.  There
 are three possible ways to do this:
  1. Examine the list of tail-end routers directly reachable via a

TE-tunnel. If there is a TE-tunnel to this node, we use the

       TE-tunnel as the first-hop.
  1. If there is no TE-tunnel, and the node is directly connected,

we use the first-hop information from the adjacency database.

  1. If the node is not directly connected, and is not directly

reachable via a TE-tunnel, we copy the first-hop information

       from the parent node(s) to the new node.
 The result of this algorithm is that traffic to nodes that are the
 tail-end of TE-tunnels, will flow over those TE-tunnels.  Traffic to
 nodes that are downstream of the tail-end nodes will also flow over
 those TE-tunnels.  If there are multiple TE-tunnels to different
 intermediate nodes on the path to destination node X, traffic will
 flow over the TE-tunnel whose tail-end node is closest to node X.  In
 certain applications, there is a need to carry both the native
 adjacency and the TE-tunnel next-hop information for the TE-tunnel
 tail-end and its downstream nodes.  The head-end node may
 conditionally switch the data traffic onto TE-tunnels based on user
 defined criteria or events; the head-end node may also split flow of
 traffic towards either types of the next-hops; the head-end node may
 install the routes with two different types of next-hops into two
 separate RIBs.  Multicast protocols running over physical links may
 have to perform RPF checks using the native adjacency next-hops
 rather than the TE-tunnel next-hops.

3. Special Cases and Exceptions

 The Shortest Path First algorithm will find equal-cost parallel paths
 to destinations.  The enhancement described in this document does not
 change this.  Traffic can be forwarded over one or more native IP
 paths, over one or more TE-tunnels, or over a combination of native
 IP paths and TE-tunnels.
 A special situation occurs in the following topology:
    rtrA -- rtrB -- rtrC
             |       |
            rtrD -- rtrE

Shen & Smit Informational [Page 3] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

 Assume all links have the same cost.  Assume a TE-tunnel is set up
 from rtrA to rtrD.  When the SPF calculation puts rtrC on the
 TENTative list, it will realize that rtrC is not directly connected,
 and thus it will use the first-hop information from the parent, which
 is rtrB.  When the SPF calculation on rtrA moves rtrD from the
 TENTative list to the PATHS list, it realizes that rtrD is the tail-
 end of a TE-tunnel.  Thus rtrA will install a route to rtrD via the
 TE-tunnel, and not via rtrB.
 When rtrA puts rtrE on the TENTative list, it realizes that rtrE is
 not directly connected, and that rtrE is not the tail-end of a TE-
 tunnel.  Therefore, rtrA will copy the first-hop information from the
 parents (rtrC and rtrD) to the first-hop information of rtrE.
 Traffic to rtrE will now load-balance over the native IP path via
 rtrA->rtrB->rtrC, and the TE-tunnel rtrA->rtrD.
 In the case where both parallel native IP paths and paths over TE-
 tunnels are available, implementations can allow the network
 administrator to force traffic to flow over only TE-tunnels (or only
 over native IP paths) or both to be used for load sharing.

4. Metric Adjustment of IP Routes over TE-tunnels

 When an IGP route is installed in the routing table with a TE-tunnel
 as the next hop, an interesting question is what should be the cost
 or metric of this route?  The most obvious answer is to assign a
 metric that is the same as the IGP metric of the native IP path as if
 the TE-tunnels did not exist.  For example, rtrA can reach rtrC over
 a path with a cost of 20.  X is an IP prefix advertised by rtrC.  We
 install the route to X in rtrA's routing table with a cost of 20.
 When a TE-tunnel from rtrA to rtrC comes up, by default the route is
 still installed with metric of 20, only the next-hop information for
 X is changed.
 While this scheme works well, in some networks it might be useful to
 change the cost of the path over a TE-tunnel, to make the route over
 the TE-tunnel less or more preferred than other routes.
 For instance, when equal cost paths exist over a TE-tunnel and over a
 native IP path, by adjusting the cost of the path over the TE-tunnel,
 we can force traffic to prefer the path via the TE-tunnel, to prefer
 the native IP path, or to load-balance among them.  Another example
 is when multiple TE-tunnels go to the same or different destinations.
 Adjusting TE-tunnel metrics can force the traffic to prefer some TE-
 tunnels over others regardless of underlining IGP cost to those

Shen & Smit Informational [Page 4] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

 Setting a manual metric on a TE-tunnel does not impact the SPF
 algorithm itself.  It only affects the comparison of the new route
 with existing routes in the routing table.  Existing routes can be
 either IP routes to another router that advertises the same IP
 prefix, or it can be a path to the same router, but via a different
 outgoing interface or different TE-tunnel.  All routes to IP prefixes
 advertised by the tail-end router will be affected by the TE-tunnel
 metric.  Also, the metrics of paths to routers that are downstream of
 the tail-end router will be influenced by the manual TE-tunnel
 This mechanism is loop free since the TE-tunnels are source-routed
 and the tunnel egress is a downstream node to reach the computed
 destinations.  The end result of TE-tunnel metric adjustment is more
 control over traffic loadsharing.  If there is only one way to reach
 a particular IP prefix through a single TE-tunnel, then no matter
 what metric is assigned, the traffic has only one path to go.
 The routing table described in this section can be viewed as the
 private RIB for the IGP.  The metric is an important attribute to the
 routes in the routing table.  A path or paths with lower metric will
 be selected over other paths for the same route in the routing table.

4.1. Absolute and Relative Metrics

 It is possible to represent the TE-tunnel metric in two different
 ways: an absolute (or fixed) metric or a relative metric, which is
 merely an adjustment of the dynamic IGP metric as calculated by the
 SPF computation.  When using an absolute metric on a TE-tunnel, the
 cost of the IP routes in the routing table does not depend on the
 topology of the network.  Note that this fixed metric is not only
 used to compute the cost of IP routes advertised by the router that
 is the tail-end of the TE-tunnel, but also for all the routes that
 are downstream of this tail-end router.  For example, if we have TE-
 tunnels to two core routers in a remote POP, and one of them is
 assigned with an absolute metric of 1, then all the traffic going to
 that POP will traverse this low-metric TE-tunnel.
 By setting a relative metric, the cost of IP routes in the routing
 table is based on the IGP metric as calculated by the SPF
 computation.  This relative metric can be a positive or a negative
 number.  Not configuring a metric on a TE-tunnel is a special case of
 the relative metric scheme.  No metric is the same as a relative
 metric of 0.  The relative metric is bounded by minimum and maximum
 allowed metric values while the positive metric disables the TE-
 tunnel in the SPF calculation.

Shen & Smit Informational [Page 5] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

4.2. Examples of Metric Adjustment

 Assume the following topology.  X, Y, and Z are IP prefixes
 advertised by rtrC, rtrD, and rtrE respectively.  T1 is a TE-tunnel
 from rtrA to rtrC.  Each link in the network has an IGP metric of 10.
      ===== T1 =====>
    rtrA -- rtrB -- rtrC -- rtrD -- rtrE
         10      10  |   10  |   10  |
                     X       Y       Z
 Without TE-tunnel T1, rtrA will install IP routes X, Y, and Z in the
 routing table with metrics 20, 30, and 40 respectively.  When rtrA
 has brought up TE-tunnel T1 to rtrC, and if rtrA is configured with
 the relative metric of -5 on tunnel T1, then the routes X, Y, and Z
 will be installed in the routing table with metrics 15, 25, and 35.
 If an absolute metric of 5 is configured on tunnel T1, then rtrA will
 install routes X, Y, and Z all with metrics 5, 15, and 25

5. Security Considerations

 This document does not change the security aspects of IS-IS or OSPF.
 Security considerations specific to each protocol still apply.  For
 more information see [5] and [2].

6. Acknowledgments

 The authors would like to thank Joel Halpern and Christian Hopps for
 their comments on this document.

7. Informative References

 [1] ISO.  Information Technology - Telecommunications and Information
     Exchange between Systems - Intermediate System to Intermediate
     System Routing Exchange Protocol for Use in Conjunction with the
     Protocol for Providing the Connectionless-Mode Network Service.
     ISO, 1990.
 [2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
 [3] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
     McManus, "Requirements for Traffic Engineering Over MPLS", RFC
     2702, September 1999.
 [4] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G.
     Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209,
     December 2001.

Shen & Smit Informational [Page 6] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

 [5] Li, T. and R. Atkinson, "Intermediate System to Intermediate
     System (IS-IS) Cryptographic Authentication", RFC 3567, July
 [6] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X. Xiao,
     "Overview and Principles of Internet Traffic Engineering", RFC
     3272, May 2002.

8. Authors' Addresses

 Naiming Shen
 Redback Networks, Inc.
 300 Holger Way
 San Jose, CA 95134
 Henk Smit

Shen & Smit Informational [Page 7] RFC 3906 IGP ShortCut Over MPLS LSPs October 2004

9. Full Copyright Statement

 Copyright (C) The Internet Society (2004).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

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Shen & Smit Informational [Page 8]

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