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Adding LSP-Related Routes to the inet.3 or inet6.3 Routing Table

 

By default, a host route toward the egress router is installed in the inet.3 or inet6.3 routing table. (The host route address is the one you configure in the to statement.) Installing the host route allows BGP to perform next-hop resolution. It also prevents the host route from interfering with prefixes learned from dynamic routing protocols and stored in the inet.0 or inet6.0 routing table.

Unlike the routes in the inet.0 or inet6.0 table, routes in the inet.3 or inet6.3 table are not copied to the Packet Forwarding Engine, and hence they cause no changes in the system forwarding table directly. You cannot use the ping or traceroute command through these routes. The only use for inet.3 or inet6.3 is to permit BGP to perform next-hop resolution. To examine the inet.3 or inet6.3 table, use the show route table inet.3 or show route table inet6.3 command.

To inject additional routes into the inet.3 or inet6.3 routing table, include the install statement:

You can include this statement at the following hierarchy levels:

The specified routes are installed as aliases into the routing table when the LSP is established. Installing additional routes allows BGP to resolve next hops within the specified prefix and to direct additional traffic for these next hops to a particular LSP.

Including the active option with the install statement installs the specified prefix into the inet.0 or inet6.0 routing table, which is the primary forwarding table. The result is a route that is installed in the forwarding table any time the LSP is established, which means you can ping or trace the route. Use this option with care, because this type of prefix is very similar to a static route.

You use alias routes for routers that have multiple addresses being used as BGP next hops, or for routers that are not MPLS capable. In either of these cases, the LSP can be configured to another MPLS capable system within the local domain, which then acts as a “border” router. The LSP then terminates on the border router and, from that router, Layer 3 forwarding takes the packet to the true next-hop router.

In the case of an interconnect, the domain’s border router can act as the proxy router and can advertise the prefix for the interconnect if the border router is not setting the BGP next hop to itself.

In the case of a point of presence (POP) that has routers that do not support MPLS, one router (for example, a core router) that supports MPLS can act as a proxy for the entire POP and can inject a set of prefixes that cover the POP. Thus, all routers within the POP can advertise themselves as interior BGP (IBGP) next hops, and traffic can follow the LSP to reach the core router. This means that normal IGP routing would prevail within the POP.

You cannot use the ping or traceroute commands on routes in the inet.3 or inet6.3 routing table.

For BGP next-hop resolution, it makes no difference whether a route is in inet.0/inet6.0 or inet.3/inet6.3; the route with the best match (longest mask) is chosen. Among multiple best-match routes, the one with the highest preference value is chosen.

Note

The install destination-prefix active statement is not supported on static LSPs. When the install destination-prefix active statement is configured for a static LSP, the MPLS routes do not get installed into the inet.0 routing table.

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