Configuring IGPs and MPLS

You can use the tunnel mpls autoroute announce command to configure a tunnel to announce its endpoint to IS-IS or OSPF so that the IGP can then use the LSP as a shortcut to a destination based on the LSP’s metric.

If no tunnels are registered, the IGP calculates the shortest path to a destination by using the shortest path first (SPF) algorithm. The results are represented by the destination node, next-hop address, and output interface, where the output interface is a physical interface.

If you configure an LSP to be announced to the IGP with a certain metric, the LSP appears as a logical interface directly connected to the LSP endpoint. The IGP can consider the LSP as a potential output interface for the LSP endpoint and for destinations beyond the endpoint. In this case, the SPF computation results are represented by the destination node and the output LSP, effectively using the LSP as a shortcut through the network to the destination.

By default, IS-IS and OSPF always use the MPLS tunnel to reach the tunnel endpoint. Best paths determined by SPF calculations are not considered. You can enable the consideration of best paths by issuing the IS-IS or OSPF mpls spf-use-any-best-path command. This command causes the IGP to evaluate the LSP as it does any other path. The IGP then either forwards traffic along the best path (which might be the MPLS tunnel), or load-balances between the MPLS tunnel and another path.

The default behavior applies only to reaching the tunnel endpoint itself. For prefixes downstream of the tunnel endpoint, the value of the tunnel metric always determines whether the IGP uses the LSP or the native path, or load-balances between the native path and one or more LSPs.

The tunnel metric can be absolute or relative. An absolute metric indicates there is no relationship to the underlying IGP cost. A relative metric is added to or subtracted from the underlying IGP shortest path cost.

Example 1

The following commands announce the tunnel to OSPF and specify a relative metric of -2:

By default, the LSP is preferred to reach the tunnel endpoint. OSPF will treat this LSP as having a metric of 2 less than the shortest path metric it has calculated. The LSP is therefore also preferred over other paths to prefixes beyond the tunnel endpoint.

Example 2

The following commands announce the tunnel to OSPF, specify an absolute metric of 25, and configure OSPF to enable the consideration of SPF best paths:

OSPF uses this metric in its SPF calculations for traffic to the tunnel endpoint as well as beyond the endpoint. Traffic is routed through this LSP only when the other calculated paths have higher metrics.

Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice.