Inter-AS Option C is a method used in MPLS (Multiprotocol Label Switching) VPN (Virtual Private Network) deployments for exchanging routing information between different Autonomous Systems (ASes). This approach is commonly used in large-scale or carrier-grade networks. Inter-AS Option C can be further divided into two subtypes: Option C1 and Option C2. These options are variations of how MP-eBGP (Multiprotocol Extensions for BGP) is used to exchange VPNv4 or VPNv6 routes between ASes.
Inter-AS Option C1 #
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Description: This is the classic implementation of Inter-AS Option C. It involves the exchange of VPNv4 or VPNv6 routes between AS Border Routers (ASBRs) of different ASes.
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Implementation:
- ASBRs of neighboring ASes establish MP-eBGP sessions directly with each other.
- These ASBRs exchange VPNv4 or VPNv6 routes, which include both the IP prefix and the MPLS label.
- The ASBRs in each AS also need to establish iBGP sessions with the Provider Edge (PE) routers within their own AS to distribute these routes.
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Use Case: Suitable for scenarios where two or more ASes need to exchange a large number of VPN routes and when scalability is a primary concern.
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Advantages: Provides scalability and efficient routing. It allows each AS to maintain control over its policies and routing decisions.
Inter-AS Option C2 #
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Description: This is a variation of Option C1, where the exchange of VPNv4 or VPNv6 routes is done not directly between the ASBRs but rather via Route Reflectors (RRs) within each AS.
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Implementation:
- Instead of establishing direct MP-eBGP sessions between ASBRs, the VPNv4 or VPNv6 routes are exchanged via internal Route Reflectors.
- The Route Reflectors in each AS establish MP-eBGP sessions with the Route Reflectors in the neighboring AS.
- This setup can reduce the number of BGP sessions on the ASBRs and centralize routing information.
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Use Case: Optimal in environments where centralizing route exchange is important and where reducing the configuration complexity on ASBRs is desired.
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Advantages: Reduces the BGP session load on ASBRs and allows for more centralized control of routing policies.
Key Considerations #
- Scalability: Both C1 and C2 offer good scalability for large networks with multiple ASes.
- Complexity: Option C2 can be more complex to implement due to the involvement of Route Reflectors.
- Control and Flexibility: Both options provide each AS with control over its routing policies.
- Network Topology: The choice between C1 and C2 may depend on the existing network topology and the roles of various routers in the network.
In summary, both Inter-AS Option C1 and C2 are designed for scenarios requiring efficient and scalable exchange of VPN routes between multiple ASes. The choice between them depends on the specific network architecture, scalability requirements, and the desire to centralize routing information.