In dual homing, a redundant interface must be created. A redundant interface is a Layer 3 spoke SDP-based interface that allows delivery of packets between the two nodes. The redundant interface is required to allow a node with a failed link to deliver packets destined for subscribers behind that link to the redundant node. Because subscriber subnets can span multiple ports it is not possible to stop advertising the subnet, therefore, without this interface the node would black hole.
The redundant interface is associated with one or more group interfaces. An interface in backup state uses the redundant interface to send traffic to the active interface (in the active node). The SAP structure under the group interface must be the same on both nodes as the synchronization of subscriber information is enabled on a group interface basis. Traffic can be forwarded through the redundant interface during normal operation even when there are no failed paths. See Figure: Dual homing example.