Inter-As Option A; Inter-As Option B; Figure 77: Inter-As Topology With Vrfs On Each As Boundary Router - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - BGP AND MPLS CONFIGURATION GUIDE 2010-10-12 Configuration Manual

JunosE 11.3.x BGP and MPLS Configuration Guide

Inter-AS Option A

Figure 77: Inter-AS Topology with VRFs on Each AS Boundary Router

Inter-AS Option B

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Figure 77 on page 404 illustrates the first method, where you create a VRF for each VPN
on each AS boundary router.
Within each AS, routes are announced by internal MP-BGP and the data packets are
forwarded across an MPLS tunnel. You create a logical connection such as an ATM VC
between each pair of VRFs (on separate AS boundary routers); these logical connections
can share the same physical connection. The following factors limit the scalability of this
method:
All inter-AS VPN routes (potentially a very large number) must be stored in the BGP
RIBs and IP routing tables on the AS boundary routers.
You must configure VRFs on each AS boundary router.
MPLS tunnels are unidirectional; Figure 77 on page 404 shows only the tunnels established
to carry traffic from ASBR 2 to PE 1 and from PE 4 to ASBR 3. Note that ASBR 2 and ASBR
3 are both also PE routers. In that sense, ASBR 2 treats ASBR 3 as a CE router, and ASBR
3 treats ASBR 2 as a CE router.
The second method is known as inter-AS option B or 2547bis option B, after IETF RFC
4364—BGP/MPLS IP Virtual Private Networks (VPNs) (February 2006). This method
uses BGP to signal VPN labels between the AS boundary routers (Figure 78 on page 405).
The base MPLS tunnels are local to each AS. Stacked tunnels run from end to end
between PE routers on the different ASs. This method provides greater scalability, because
only the BGP RIBs store all the inter-AS VPN routes.
Copyright © 2010, Juniper Networks, Inc.