Juniper JUNOSE, Configuration Manual

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The labels that are generated to be sent to the inter-AS BGP peers are generated for
each next-hop PE router/received label tuple. Scaling is improved when all routes
advertised from a given VRF have the same label; this is the default E Series router
behavior. You can disable this behavior by issuing the
ip mpls forwarding-mode
label-switched command for the VRF.
Inter-AS Option C
The third method of configuring inter-AS services and inter-AS VPNs is known as
inter-AS option C or 2547bis option C. This method is described in RFC
4364—BGP/MPLS IP Virtual Private Networks (VPNs) (February 2006).
Inter-AS option C, similarly to the carrier-of-carriers configuration, requires a
label-switched path from a packet's ingress PE router to its egress PE router. Option C
introduces multihop EBGP redistribution of labeled VPN-IPv4 routes between source
and destination autonomous systems. Labeled IPv4 routes are redistributed by EBGP
between neighboring autonomous systems. Inter-AS option C uses BGP as the label
distribution protocol.
In an inter-AS option C network, ASBRs do not maintain or distribute VPN-IPv4 routes.
Each ASBR maintains labeled IPv4 /32 routes to the PE routers within its AS. The
ASBR distributes these routes to other autonomous systems with EBGP. If transit
autonomous systems are included in the topology, their ASBRs must also distribute
the labeled /32 routes with EBGP. This configuration creates a label-switched path
from the ingress PE router to the egress PE router. This configuration enables the PE
routers in different autonomous systems to establish multihop EBGP connections to
each other, and to exchange VPN-IPv4 routes over those connections.
Two different configuration scenarios are possible with option C, one employing a
two-label stack and the other a three-label stack.
Figure 79 on page 396 illustrates the three-label stack scenario. PHP is not used in
this example.
Figure 79: Topology for Three-label Stack Configuration for Inter-AS Option C
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Providing IPv4 VPN Services Across Multiple Autonomous Systems
JUNOSe 11.0.x BGP and MPLS Configuration Guide