Configuring Inter-AS VPN Option B (Spanning More Than Two ASs)
In a scenario in which the backbone network spans more than two ASs, ASBRs need to advertise VPNv4 routes through MP-EBGP.
Usage Scenario
If the L3VPN needs to span more than two ASs, you can configure inter-AS VPN Option B (spanning more than two ASs). On the network shown in Figure 1, the L3VPN needs to span three ASs to transmit VPN routes.
Pre-configuration Tasks
Before configuring inter-AS VPN Option B (spanning more than two ASs), 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. OSPF or IS-IS is used in most cases.
Configure MPLS and MPLS LDP 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, an ASBR can advertise the VPNv4 routes of its AS to the other ASBR.
- Configuring MP-IBGP Between ASBRs in the Same AS
- After the MP-IBGP peer relationship is established between the ASBRs in the same AS, 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) 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.
- (Optional) Using a Routing Policy to Control VPNv4 Routes on ASBRs
- ASBRs can use a routing policy to filter undesired VPNv4 routes.
- Verifying the Configuration of Inter-AS VPN Option B (Spanning More Than Two ASs)
- After configuring inter-AS VPN Option B (spanning more than two ASs), you can view the status of all BGP peer relationships and VPNv4 routing information on PEs or ASBRs.