Juniper BGP, Configuration Manual

Page: 332 / 748

Example 1
The following commands announce the tunnel to OSPF and specify a relative metric
of -2:
host1(config-if)# tunnel mpls autoroute announce ospf
host1(config-if)# tunnel mpls autoroute metric relative -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:
host1(config-if)# tunnel mpls autoroute announce ospf
host1(config-if)# tunnel mpls autoroute metric absolute 25
...
host1(config)#
router ospf 1
host1(config-router)# mpls spf-use-any-best-path
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.
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.
296 ■
Configuring IGPs and MPLS
JUNOSe 11.1.x BGP and MPLS Configuration Guide