Interprovider Option C for Layer 3 VPNs involves the use of Autonomous System Boundary Routers (ASBRs) to exchange labeled VPN-IPv4 routes between different Autonomous Systems (AS). This option requires BGP sessions between ASBRs, and the VPN routes are carried end-to-end using MPLS labels. Here’s a detailed analysis of the roles of different routers in this scenario:
1. **ASBR Routers**:
- ASBRs are responsible for exchanging VPN-IPv4 routes between different ASes.
- **A. ASBR routers maintain the internal routes from its own AS and the loopback addresses from the other AS PEs.**
- Correct. ASBRs maintain routes to internal destinations within their own AS, and they also need to know the loopback addresses of PEs in the other AS to set up the BGP sessions and MPLS tunnels.
2. **PE Routers**:
- PE routers are responsible for maintaining VPN routes and label information to forward VPN traffic correctly.
- **B. PE routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**
- Correct. PE routers need to maintain:
- Internal routes within their AS for routing.
- Loopback addresses of other AS PEs for establishing MPLS LSPs.
- L3VPN routes to provide end-to-end VPN connectivity.
3. **P Routers**:
- P routers are the core routers that do not participate in BGP VPN routing but forward labeled packets based on MPLS labels.
- **C. P routers only maintain the internal routes from their own AS.**
- Correct. P routers maintain the internal routing information to forward packets within the AS and use MPLS labels for forwarding VPN packets. They do not maintain VPN routes or routes from other ASes.
4. **Incorrect Statements**:
- **D. P routers maintain the internal routes from its own AS and the loopback address from the other AS PEs.**
- Incorrect. P routers do not need to maintain the loopback addresses of other AS PEs. They only maintain internal routing and MPLS label information.
- **E. ASBR routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**
- Incorrect. ASBR routers do not maintain L3VPN routes. They exchange labeled VPN-IPv4 routes with other ASBRs and forward them to PE routers.
**Conclusion**:
The correct answers are:
**A. ASBR routers maintain the internal routes from its own AS and the loopback addresses from the other AS PEs.**
**B. PE routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**
**C. P routers only maintain the internal routes from their own AS.**
**References**:
- Juniper Networks Documentation on Interprovider VPNs: [Interprovider VPN
- MPLS and VPN Architectures, CCIP Edition by Ivan Pepelnjak and Jim Guichard
