5.1. IGP Mirroring Context Segment

One use case for a Binding segment is to provide support for an IGP node to advertise its ability to process traffic originally destined to another IGP node, called the "mirrored node" and identified by an IP address or a Node-SID, provided that a Mirroring Context segment is inserted in the segment list prior to any service segment local to the mirrored node.

When a given node B wants to provide egress node A protection, it advertises a segment identifying node's A context. Such a segment is called "Mirroring Context segment" and is identified by the Mirror SID.

The Mirror SID is advertised using the Binding segment defined in SR IGP protocol extensions [ISIS-SR-EXT].

In the event of a failure, a Point of Local Repair (PLR) diverting traffic from A to B does a PUSH of the Mirror SID on the protected traffic. When receiving the traffic with the Mirror SID as the active segment, B uses that segment and processes underlying segments in the context of A.