The 7750 SR, 7450 ESS, and 7950 XRS SR OS supports single-active multihoming on access LAG SAPs, regular SAPs, and spoke SDPs for a specified VPLS service.
The following SR OS procedures support EVPN single-active multihoming for a specified Ethernet-Segment:
DF (Designated Forwarder) election
As in all-active multihoming, DF election in single-active multihoming determines the forwarding for BUM traffic from the EVPN network to the Ethernet-Segment CE. Also, in single-active multihoming, DF election also determines the forwarding of any traffic (unicast/BUM) and in any direction (to/from the CE).
Backup PE
In single-active multihoming, the remote PEs do not perform aliasing to the PEs in the Ethernet-Segment. The remote PEs identify the DF based on the MAC routes and send the unicast flows for the Ethernet-Segment to the PE in the DF and program a backup PE as an alternative next-hop for the remote ESI in case of failure.
This RFC 7432 procedure is known as 'Backup PE' and is shown in Figure 1 for PE3.
Figure 1. Backup PE