Configuring Inter-AS VPN Option B (Basic Networking)
In a scenario in which the backbone network spans two ASs, ASBRs need to advertise VPNv4 routes through MP-EBGP.
Usage Scenario
If an ASBR can manage VPN routes but there are not enough interfaces for all inter-AS VPNs, inter-AS VPN Option B can be used. Inter-AS VPN Option B requires ASBRs to help to maintain and advertise VPNv4 routes, and you do not need to create VPN instances on the ASBRs. In the basic networking of inter-AS VPN Option B, an ASBR cannot play other roles, such as the PE or RR, and an RR is not required in each AS.
On the network shown in Figure 1, the interfaces connected between ASBRs do not need to be bound to the VPN. A single-hop MP-EBGP peer relationship is set up between the ASBRs to transmit all inter-AS VPN routing information.
Pre-configuration Tasks
Before configuring inter-AS VPN Option B (basic networking), 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 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.
Configuring a VPN Instance on the PE connected to the CE 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 that connects 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 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.
- 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 (Basic Networking)
- After configuring inter-AS VPN Option B (basic networking), check the status of all BGP peer relationships and VPNv4 routing information on PEs or ASBRs.