Preference-based DF election for EVPN and PBB-EVPN

The designated forwarder (DF) election in a multihomed Ethernet VPN (EVPN) environment can be controlled using an administrative preference value for an Ethernet segment identifier (ESI). 

Currently, the DF election (as specified in RFC 7432) is performed randomly by all the multihoming devices using the modulo operation. With the preference-based DF election, the DF is elected manually using interface configuration options, such as the preference value and the router ID or loopback address. This method of DF election is useful when there is a need to choose the DF based on interface attributes like bandwidth associated with the interface. To enable preference-based DF election, include the df-election-type preference value statements at the [edit interfaces interface-name esi] hierarchy level.