14.1.1. Single-Active Redundancy Mode

14.1.1. Single-Active Redundancy Mode

For a given ES, if the remote PE has imported the set of Ethernet A-D per ES routes from at least one PE, where the "Single-Active" flag in the ESI Label extended community is set, then the remote PE MUST deduce that the ES is operating in Single-Active redundancy mode. As such, the MAC address will be reachable only via the PE announcing the associated MAC/IP Advertisement route -- this is referred to as the primary PE. The other PEs advertising the set of Ethernet A-D per ES routes for the same ES provide backup paths for that ES, in case the primary PE encounters a failure, and are referred to as backup PEs. It should be noted that the primary PE for a given <ES, VLAN> (or <ES, VLAN bundle>) is the DF for that <ES, VLAN> (or <ES, VLAN bundle>).

If the primary PE encounters a failure, it MAY withdraw its set of Ethernet A-D per ES routes for the affected ES prior to withdrawing its set of MAC/IP Advertisement routes.

If there is only one backup PE for a given ES, the remote PE MAY use the primary PE's withdrawal of its set of Ethernet A-D per ES routes as a trigger to update its forwarding entries, for the associated MAC addresses, to point towards the backup PE. As the backup PE starts learning the MAC addresses over its attached ES, it will start sending MAC/IP Advertisement routes while the failed PE withdraws its routes. This mechanism minimizes the flooding of traffic during fail-over events.

If there is more than one backup PE for a given ES, the remote PE MUST use the primary PE's withdrawal of its set of Ethernet A-D per ES routes as a trigger to start flooding traffic for the associated MAC addresses (as long as flooding of unknown unicast packets is administratively allowed), as it is not possible to select a single backup PE.