Configuring Inter-AS VPN Option B (ASBR Also Functioning as a PE)

In a scenario in which the backbone network spans two ASs, ASBRs need to advertise VPNv4 routes through MP-EBGP and ASBRs also need to function as PEs.

Usage Scenario

If an ASBR can manage VPN routes and the ASBR also functions as a PE for CE access, you can configure inter-AS VPN Option B (ASBR also functioning as a PE). This mode requires ASBRs to help to maintain and advertise not only the VPNv4 routes of its own VPN instances but also the VPNv4 routes of other VPN instances.

Pre-configuration Tasks

Before configuring inter-AS VPN Option B (ASBR also functioning as a PE), complete the following tasks:

Configure an IGP for the MPLS backbone network of each AS to ensure IP connectivity of the backbone network in each AS.
Configure MPLS both globally and per interface on each node of the MPLS backbone network in each AS and establish an LDP LSP or TE tunnel between MP-IBGP peers.
Configure a VPN instance on each PE and bind the interface that connects a PE to a CE to the VPN instance on that PE.
Configure an IP address for the interface connecting a CE to a PE.

Configuring MP-IBGP Between a PE and an ASBR in the Same AS: By introducing extended community attributes into BGP, MP-IBGP can advertise VPNv4 routes between the PE and ASBR.

Configuring MP-EBGP Between ASBRs in Different ASs: After an MP-EBGP peer relationship is established between ASBRs, ASBRs can exchange VPNv4 routes.

Configuring ASBRs Not to Filter VPNv4 Routes Based on VPN Targets: In inter-AS VPN Option B mode, ASBRs do not have VPN instances. If you want ASBRs to keep received VPNv4 routes, configure ASBRs not to filter VPNv4 routes based on VPN targets.

(Optional) Using a Routing Policy to Control VPNv4 Routes on ASBRs: ASBRs can use a routing policy to filter undesired VPNv4 routes.

(Optional) Configuring One-Label-per-Next-Hop Label Distribution on an ASBR: To save label resources on an ASBR, configure one-label-per-next-hop label allocation on the ASBR. One-label-per-next-hop label allocation on ASBRs and one-label-per-instance label distribution on PEs must be used together.

Configuring a VPN Instance on an ASBR: If an ASBR also functions as a PE, you need to configure a VPN instance enabled with the IPv4 address family on the ASBR to manage VPN routes.

Configuring Route Exchange Between a CE and an ASBR: The configuration of route exchange between a CE and an ASBR is similar to the configuration of route exchange between a CE and a PE on a basic BGP/MPLS IP VPN.

Configuring Route Exchange Between a CE and a PE: BGP, an IGP, or a static route (including the default route) can run between a CE and a PE. You can choose any of them as required.

Verifying the Configuration of Inter-AS VPN Option B (ASBR Also Functioning as a PE): After configuring inter-AS VPN Option B (ASBR also functioning as a PE), check the status of all BGP peer relationships and VPNv4 routing information on PEs or ASBRs.