Octet

Say this first

The host's links to two or more leaves form an Ethernet segment, identified by a 10-byte ESI configured the same on each leaf, plus a common LACP system ID so the host sees one partner. Leaves discover each other with type 4 routes and elect a designated forwarder per VLAN for BUM toward the host. Type 1 routes give mass withdraw and aliasing. There is no peer link and no state sync, it is standards-based and multi-vendor. MLAG and vPC are vendor-specific pairs with a peer link.

Go deeper

What each piece does

Type 4 Ethernet segment route. Leaves with the same ESI find each other and elect a designated forwarder (DF) per VLAN, so only one leaf sends BUM traffic down to the host.
Type 1 per segment. If a leaf loses its link, one withdrawal tells every remote VTEP to stop using that leaf for all MACs on the segment (mass withdraw).
Type 1 per EVI. Aliasing: remote leaves load-balance to every leaf on the segment, even if only one of them learned the MAC.
Split horizon. With VXLAN, local bias: BUM from a host is delivered to the segment by the leaf it arrived on, and peers on the same segment do not send it back.

Config on EOS

Arista EOS

interface Port-Channel10
   switchport mode trunk
   switchport trunk allowed vlan 10
   evpn ethernet-segment
      identifier 0000:0000:0102:0304:0506
      route-target import 01:02:03:04:05:06
   lacp system-id 0102.0304.0506

The same block, with the same ESI and system ID, goes on the other leaf.

	MLAG or vPC	EVPN ESI multihoming
Peer link	Required	None
Leaves per segment	Two	Two or more (vendor limits apply)
Standards	Vendor-specific	RFC 7432, RFC 8365
State sync	Over the peer link	Through BGP
Typical home	NX-OS fabrics, classic pairs	Arista, Junos, multi-vendor fabrics

They will ask next

What goes wrong if the LACP system IDs differ?
The host sees two different partners and cannot bundle the links. Depending on the OS, one link stays out of the bundle or the bond flaps.

Traps

Forgetting that BGP convergence now drives multihoming failover, so the overlay must be healthy.

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