Transmitting Information Between Autonomous Systems - Cisco CRS Configuration Manual

Transmitting Information Between Autonomous Systems

Transmitting Information Between Autonomous Systems
The following figure illustrates one MPLS VPN consisting of two separate autonomous systems. Each
autonomous system operates under different administrative control and runs a different IGP. Service providers
exchange routing information through eBGP border edge routers (ABSR1 and ASBR2).
Figure 28: eBGP Connection Between Two MPLS VPN Inter-AS Systems with ASBRs Exchanging VPN-IPv4 Addresses
This configuration uses the following process to transmit information:
1 The provider edge router (PE-1) assigns a label for a route before distributing that route. The PE router
uses the multiprotocol extensions of BGP to transmit label mapping information. The PE router distributes
the route as a VPN-IPv4 address. The address label and the VPN identifier are encoded as part of the
NLRI.
2 The two route reflectors (RR-1 and RR-2) reflect VPN-IPv4 internal routes within the autonomous system.
The border edge routers of the autonomous system (ASBR1 and ASBR2) advertise the VPN-IPv4 external
routes.
3 The eBGP border edge router (ASBR1) redistributes the route to the next autonomous system (ASBR2).
ASBR1 specifies its own address as the value of the eBGP next-hop attribute and assigns a new label. The
address ensures:
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• That the next-hop router is always reachable in the service provider (P) backbone network.
• That the label assigned by the distributing router is properly interpreted. (The label associated with
a route must be assigned by the corresponding next-hop router.)
Implementing MPLS Layer 3 VPNs