LDP and Static Routes
Load isis.cfg
Configure LDP on all routers using LDP IGP autoconfig.
Configure a static route for 20.20.20.21/32 via R2 on R1. On R2, configure a static route for 20.20.20.21/32 via R3.
Do both routers advertise a label mapping for this prefix? If so, do they install an entry in the LFIB?
Answer
Explanation
Static routes “count” as IGP routes to LDP. So labels are locally allocated for static routes, advertised as label bindings to all peers, and entries are installed in the forwarding table.
On R1, we can see a locally allocated label for this prefix, and an outgoing label which was received from R2:
On R2, we see R1’s label binding, but it is not used because R1 is not the nexthop for the static route. R3 has no label binding for this prefix, so R2’s LFIB entry is an outgoing label action of “no label.”
Note that this process is the same for connected prefixes, even if the connected prefix is not advertised into the IGP. The local router allocates a label binding (which is just imp-null), and advertises the FEC/binding mapping to its LDP neighbors. So we can say in general, that LDP allocates labels and advertises bindings for all prefixes in the table except for BGP-learned prefixes.
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