The Designated Forwarder (DF) is a Provider Edge (PE) router that is responsible for sending Broadcast, Multicast, and Unicast (BUM) traffic to multihomed Customer Edge (CE) devices on a particular VLAN on a particular Ethernet Segment (ES). DF election happens using an algorithm that uses the number of PEs in an ES and the VLAN value to decide the DF. This algorithm was found to have various inefficiencies that needed to be addressed. A new algorithm was defined in the RFC8584 that aims to improve the behaviour of DF elections on PEs.
This new algorithm is backward compatible to the existing RFC7432 compatible devices, wherein, these devices can choose to ignore the new implementation of the DF election algorithm.
The existing algorithm was modified by removing from consideration any candidate PEs in a particular ES that could not forward traffic on the specific AC that belonged to the broadcast domain (BD). The current implementation in SLXOS re-triggers the DF election whenever there is a change in the configuration. This satisfies some of the clauses defined in RFC8584.
Note
Highest Random Weight (HRW) algorithm as defined in RFC8584 is not supported in the current implementation.
By default, SLXOS does not advertise AC-DF capabilities to peer nodes in the same ES. This might cause black-holing of traffic due to peers continuing to use RFC7432 algorithm. This issue is resolved by the RFC8584 using the new DF Election Extended community in the ES route.
This is manually enabled using the advertise capabilities ac-influenced-df-election command. This command can only be used when ESI or LACP Auto is already configured on the Ethernet Segment. By default, advertising AC influenced DF capability is disabled.