EVPN multi-homing implementation is based on the concept of the Ethernet Segment (ES). An ES is a logical structure that can be defined in one or more PEs and identifies the CE (or access network) multi-homed to the EVPN PEs. An ES is associated with port or LAG objects and shared by all the services defined on those objects. On the 7210 SAS, only the following service objects are allowed to be configured as an ES: port and LAG.
Each ES has a unique Ethernet Segment Identifier (ESI) that is 10 bytes long and is manually configured in the router.
Because the esi command is advertised in the control plane to all the PEs in the EVPN network, it is important to ensure that the 10-byte esi value is unique throughout the entire network. Single-homed CEs are assumed to be connected to an ES with esi = 0 (single-homed ESs are not explicitly configured).