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Inter-AS Option C with Route Reflectors
Copyright © 2010, Juniper Networks, Inc.
PE 1 learns label L5 for the route to the loopback address on ASBR 1 by means of
8.
LDP or RSVP-TE from P 1.
PE 1 learns label L1 for the VPN-IPv4 route from the multihop EBGP session with PE
9.
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 406 as follows:
The first (innermost or bottom) label, L1, is assigned by the egress PE router, PE 2.
1.
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.
The middle label, L6, is assigned by ASBR 1. This label is obtained from the MP-IBGP
2.
labeled unicast session from the ASBR. It corresponds to the /32 route to the egress
PE router, PE 2.
The top (outermost) label, L5, is assigned by the ingress PE router's IGP next hop. P
3.
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.
While the packet travels across the VPN from ingress router PE 1, labels are swapped as
follows:
P 1 swaps outermost label L5 for L7 to get to its next hop, ASBR 1.
1.
ASBR 1 pops outermost label L7 and swaps the middle label L6 for L4 to get to ASBR
2.
2.
ASBR 2 swaps outer label L4 for L3 to get to its next hop, P 2.
3.
P 2 swaps outer label L3 for L2 to get to its next hop, PE 2.
4.
PE 2 pops outer label L2 and inner label L1 and then processes the IP data packet.
5.
In contrast to the three-label stack scenario described previously, in a two-label stack
scenario, BGP labeled unicast is not used inside the autonomous system. Instead, only
LDP is used as the label distribution protocol. A PE router in one AS has a direct LSP to
a PE in another AS, achieved by using LDP labels within the AS and BGP labels across
the AS boundary.
For a two-label stack scenario to work, you must issue the mpls ldp redistribute bgp
command on the ASBRs. This command enables the BGP prefixes to be advertised by
LDP inside the autonomous systems. For more information on this command, see
"Configuring MPLS" on page 275.
When the BGP/MPLS VPN peer is a route reflector (Figure 80 on page 408), issue the
neighbor next-hop-unchanged command to prevent the RR from rewriting the BGP
next-hop attribute when the RR advertises routes to external neighbors. Issuing this
command causes the VPN RR that is multihop peering with another RR in the AS to send
the next hop unchanged for the VPN routes that it advertises.
Chapter 6: Configuring BGP-MPLS Applications
407
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