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
Summary of Contents for BGP
Page 6: ...vi ...
Page 8: ...viii JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 37: ...Part 1 Border Gateway Protocol Configuring BGP Routing on page 3 Border Gateway Protocol 1 ...
Page 38: ...2 Border Gateway Protocol JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 234: ...198 Monitoring BGP JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 236: ...200 Multiprotocol Layer Switching JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 542: ...506 Monitoring BGP MPLS VPNs JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 544: ...508 Layer 2 Services Over MPLS JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 610: ...574 Virtual Private LAN Service JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 624: ...588 VPLS References JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 680: ...644 Virtual Private Wire Service JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 724: ...688 Monitoring MPLS Forwarding Table for VPWS JUNOSe 11 1 x BGP and MPLS Configuration Guide ...
Page 725: ...Part 6 Index Index on page 691 Index 689 ...
Page 726: ...690 Index JUNOSe 11 1 x BGP and MPLS Configuration Guide ...