CCNP SP · UNIFIED MPLS

Troubleshoot inter-AS Unified MPLS

BGP labeled unicast is configured across ISP1 (AS 6001, IOS XE), ISP2 (AS 6002, IOS XR) and ISP3 (AS 6003, IOS XR), but PE1 and PE3 still cannot reach each other’s loopbacks over a label-switched path. One fault breaks each direction. Use the BGP label tables on the border routers to find them.

CCNP SP track · Cisco IOS XE + IOS XR · 5 objectives

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Objectives

The simulator ticks each one as soon as the network meets it.

Find why there is no BGP-LU path between PE1 and PE3 (show bgp ipv4 unicast labels on PE1 and ASBR1, show bgp ipv4 labeled-unicast on the IOS XR routers).
ASBR1 (IOS XE) must exchange labels with ISP2 on its eBGP session (neighbor 10.0.23.2 send-label).
ASBR2B (IOS XR) must send routes to ISP3: its eBGP neighbor 10.0.56.2 needs route-policy PASS out.
PE1 and PE3 have a BGP-LU path to each other’s loopback.
Ping 7.7.7.7 from PE1, sourced from Loopback0.

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Before you start

A label is missing when only one end of a session is configured for labeled unicast: the route still arrives, as “nolabel”. A missing IOS XR route-policy is quieter: the route never arrives at all, so the other ISP has no BGP route to the loopback. Follow each loopback router by router.

Concept

Inter-AS Unified MPLS: BGP-LU between providers

One LSP can cross several autonomous systems. Inside each provider, iBGP labeled unicast carries the PE loopbacks to the border routers, and LDP carries the packets between BGP next hops. Between providers, the border routers (ASBRs) peer over eBGP labeled unicast on their shared link. The next hop changes on every eBGP hop, so each ASBR allocates its own label. This is the idea behind inter-AS option C.