Objectives
The simulator ticks each one as soon as the network meets it.
Find why PE1 has no LSP to 5.5.5.5 (show bgp ipv4 unicast labels on PE1 and ABR1).
On ABR1, set next-hop-self toward PE1, so PE1’s next hop for 5.5.5.5 is ABR1 instead of ABR2, which PE1’s domain cannot reach with a label.
On PE1, exchange labels with ABR1 (neighbor 2.2.2.2 send-label).
PE1’s path to 5.5.5.5 is a BGP-LU label over LDP.
Ping 5.5.5.5 from PE1, sourced from Loopback0.
Before you start
Without next-hop-self on the ABR, the PE gets a next hop in another IGP domain. Its LDP has no label for that address, because a Level-1 PE knows only a default route. And a session carries labels only when both ends are configured for labeled unicast. The fields in show bgp ipv4 unicast labels show which: “nolabel” where a label is missing.
Concept
Unified MPLS: BGP labeled unicast across IGP domains
Large networks split their IGP into domains (IS-IS areas or separate instances) so each one stays small. LDP then only builds paths inside a domain. A Level-1 PE knows just a default route, and LDP gives no label for a default route. Unified (seamless) MPLS stitches the domains with BGP labeled unicast: each PE advertises its loopback with a label, and the border routers reflect it with next-hop-self and a label of their own.
