Cross-Multi-AS E2E SR-MPLS TE

Theoretically, binding SIDs and BGP peer SIDs can be used to establish an explicit path across multiple AS domains (greater than or equal to three). AS domains, however, are subject to management of various organizations. When a path crosses multiple AS domains, the path also crosses the networks of multiple management organizations, which may hinder network deployment.

In Figure 1, the network is connected to three AS domains. If PE1 establishes an E2E SR-MPLS TE network over a multi-hop explicit path to PE3, the traffic path from AS y to AS z can be determined in AS x. AS y and AS z, however, may belong to different management organizations than AS x. In this situation, the traffic path may not be accepted by AS y or AS z, and the great depth of the label stack decreases forwarding efficiency. AS y and AS z may be connected to a controller different than AS x, leading to a difficult in establishing an E2E SR-MPLS TE network from PE1 to PE3.

To tackle the preceding problem, a restriction is implemented on a device. If the first hop of the explicit path is a binding SID, the explicit path supports a maximum of three hops. In this way, PE1 can establish an inter-AS E2E SR-MPLS TE explicit path at most to ASBR5 or ASBR6, not to AS z. The hierarchical mode can only be used to establish an inter-AS domain E2E SR-MPLS TE tunnel from AS x to AS z.

Figure 1 Cross-Multi-AS E2E SR-MPLS TE
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The process of hierarchically establishing a cross-multi-AS E2E SR-MPLS TE tunnel is as follows:

Layer 1: Establish an E2E SR-MPLS TE tunnel from AS y to AS z. Create an SR-MPLS TE tunnel within each of AS y and AS z. Set binding SIDs for the intra-AS tunnels, that is, binding SID3 and binding SID5, respectively. Configure BGP EPE between AS y and AS z to generate BGP peer SID4. The controller establishes an inter-AS E2E SR-MPLS TE Tunnel1 from AS y to AS z using the preceding SIDs. Set binding SID6 for this tunnel.
Layer 2: Establish an E2E SR-MPLS TE tunnel from AS x to AS z. Create an SR-MPLS TE tunnel within AS x. Set a binding SID for the intra-AS tunnel, that is, binding SID1. Configure BGP EPE between AS x and AS y to generate BGP peer SID2. The controller establishes a unidirectional inter-AS E2E SR-MPLS TE Tunnel2 from AS x to AS z using binding SID1, peer SID2, and binding SID6.

An E2E SR-MPLS TE tunnel across three AS domains is established. If there are more than three AS domains, a new binding SID can be allocated to E2E SR-MPLS TE Tunnel2, and the SID participates in path computation. Repeat the preceding process to construct an E2E SR-MPLS TE tunnel that spans more AS domains.