LVTEP is used as a redundancy mechanism for VTEP. For implementing LVTEP, the same IP address is configured on two (or more) MCT nodes. These MCT peers are connected with each other with Inter Chassis Links (ICL). To achieve split horizon for BUM traffic received from remote VTEP, a backup tunnel (referred to as Overlay-ICL or VX-ICL tunnel) is also created.
Designated Forwarder (DF) for a given VLAN/Broadcast Domain (BD) is the MCT node responsible for forwarding the Broadcast, Unknown-unicast, Multicast (BUM) traffic received in that flooding domain. This method is used for VxLAN tunnels so that the BUM traffic can be load-balanced towards the Fabric. In fabrics with Dual-Homing, when the tunnel state is UP on both the MCT peers, one of these MCT nodes becomes the DF for all odd VLANs/BDs and the other one becomes the DF for all even VLANs/BDs.
When Local Bias for LVTEP is not enabled and if a VxLAN tunnel becomes DOWN on one side, its corresponding tunnel on the peer MCT node then becomes the DF for all VLANs/BDs that is extended on it. As LVTEP uses MGID for controlling DF membership, it must be updated to the peer MCT node and the BUM traffic can only be resumed once the MGIDs are updated. This causes delay in resuming BUM traffic on the working MCT node.
With local bias forwarding, all VLAN/BD BUM traffic is locally forwarded to the VxLAN tunnel clients. On the peer node, traffic received on the ICL is not forwarded to any VxLAN tunnel clients.
There is no DF election for forwarding BUM traffic towards the Fabric – both MCT peers are DF for all VLANs/BDs.
The BUM from AC side will be forwarded to Fabric at the same node (Local Bias) irrespective of the DF status.
The BUM from both AC and Fabric sides will be forwarded over AC-ICL instead of Vx-ICL; and dropped at the MCT peer to avoid duplication of traffic.
This won‘t support singly homed LVTEP client (spine uplink DOWN in one of the MCT nodes).
It is important to have a backup routing over ICL to reach the spines in case of uplink failure.