Which Routing Table is Used in Inter-AS L3VPN Option B?

A.

References:

In an inter-AS L3 VPN Option B, the Provider Edge (PE) routers act as Autonomous System Boundary Routers (ASBRs) and connect to the core Provider (P) routers. The PE-ASBRs exchange VPNv4 routes using Border Gateway Protocol (BGP) with each other, while the P routers only forward these routes based on their MPLS labels.

Option B refers to a scenario where each customer site has its own unique Autonomous System Number (ASN), and the service provider uses AS override to advertise the customer's routes between the PE-ASBRs. In this option, the VPN routes are exchanged between different ASes, and therefore, the PE-ASBRs need to use a separate routing table to store all the VPN routes that need to be exchanged.

The correct answer to the given question is (B) VRF routing table. In an L3 VPN, a Virtual Routing and Forwarding (VRF) instance is used to create a separate routing table for each VPN. Each VPN has its own VRF, which stores the VPN-specific routes learned from the CE routers connected to the PE-ASBRs. The PE-ASBRs use BGP to exchange the VPN routes between the different VRF tables across the different ASes.

Therefore, in an inter-AS L3 VPN Option B, the PE-ASBRs use the VRF routing table to store all the VPN routes that need to be exchanged. The global routing table contains the Internet routes learned from the P routers, while the IGP table contains the intra-AS routing information. The BGP table is used by the PE routers to store the BGP-learned routes, including the VPNv4 routes, but not specifically for storing the VPN routes that need to be exchanged between different ASes.