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.
8. PE 1 learns label L5 for the route to the loopback address on ASBR 1 by means
of LDP or RSVP-TE from P 1.
9. PE 1 learns label L1 for the VPN-IPv4 route from the multihop EBGP session with
PE 2.
Because the routes to the PE routers are unknown to all P routers other than the
ASBRs, the ingress PE must push a three-label stack on packets received from the
VPN end users. This is illustrated in Figure 79 on page 402 as follows:
1. The first (innermost or bottom) label, L1, is assigned by the egress PE router,
PE 2. This label is obtained from the multihop MP-EBGP session. It corresponds
to the packet's destination address in a particular VRF at the remote PE router.
2. The middle label, L6, is assigned by ASBR 1. This label is obtained from the
MP-IBGP labeled unicast session from the ASBR. It corresponds to the /32 route
to the egress PE router, PE 2.
3. The top (outermost) label, L5, is assigned by the ingress PE router’s IGP next
hop. P 1. This label is obtained from an LDP or RSVP-TE session with the next
hop. It corresponds to the /32 route to ASBR 1.
Providing IPv4 VPN Services Across Multiple Autonomous Systems â– 403
Chapter 5: Configuring BGP-MPLS Applications
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