While the packet travels across the VPN from ingress router PE 1, labels are swapped
as follows:
1. P 1 swaps outermost label L5 for L7 to get to its next hop, ASBR 1.
2. ASBR 1 pops outermost label L7 and swaps the middle label L6 for L4 to get to
ASBR 2.
3. ASBR 2 swaps outer label L4 for L3 to get to its next hop, P 2.
4. P 2 swaps outer label L3 for L2 to get to its next hop, PE 2.
5. PE 2 pops outer label L2 and inner label L1 and then processes the IP data packet.
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 267.
Inter-AS Option C with Route Reflectors
When the BGP/MPLS VPN peer is a route reflector (Figure 80 on page 404), 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.
Figure 80: Topology for Inter-AS Option C with Route Reflectors
neighbor next-hop-unchanged
404 ■ Providing IPv4 VPN Services Across Multiple Autonomous Systems
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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