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Figure 79: Topology for Three-label Stack Configuration for Inter-AS Option C

In this topology, you can use either LDP or RSVP-TE to establish an LSP between each

ASBR router and the PE router in an autonomous system. A labeled MP-IBGP session

exists between the ASBR and the PE router in each autonomous system. A labeled

MP-EBGP session exists between the two ASBR routers. The ASBR routers advertise the

loopback IP addresses of their PE routers and associates the prefixes with labels.

When PE 1 learns the PE 2 loopback address and PE 2 learns the PE 1 loopback address,

these PE routers can establish a multihop MP-EBGP session in order to exchange

VPN-IPv4 routes. Because VPN-IPv4 routes are only exchanged between end PE routers,

no other router on the path from PE 1 to PE 2 needs to keep or install VPN routes in its

RIB or FIB.

1. P 2 learns label L2 for the route to the loopback address on PE 2 by means of LDP

or RSVP-TE from PE 2.

2. ASBR 2 learns label L3 for the route to the loopback address on PE 2 by means of

LDP or RSVP-TE from P 2.

Each ASBR builds its own MPLS forwarding table with the received and advertised

routes and labels. ASBR 2 uses its own IP address as the next hop.

3. ASBR 2 uses an MP-EBGP labeled unicast session to advertise label L4 for the route

to the loopback address on PE 2 to neighboring ASBR 1.

4. ASBR 1 receives this route and the associated label L4.

5. ASBR 1 assigns label L6 to the route to the loopback address on PE 2 and changes

the next-hop address to its own address.

6. ASBR 1 then uses an MP-IBGP session to advertise that address to PE 1. PE 1 therefore

has an update with the label information and a next hop to ASBR 1.

7. P 1 learns label L7 for the route to the loopback address on ASBR 1 by means of LDP

or RSVP-TE from ASBR 1.

JunosE 11.2.x BGP and MPLS Configuration Guide

Juniper JUNOSE 11.2.X BGP AND MPLS User Manual

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