Layer 3 Unicast Forwarding over MCT
A simple MCT topology addresses resiliency and efficient load balancing in Layer 2 network topologies. Layer 3 technologies can run in an MCT environment too. This allows various Layer 3 technologies to function while leveraging the benefits at the Layer 2 level.
ARP Resolution
ARP resolution for the MCT client is required at the MCT cluster to forward traffic from a CEP to the CCEP. This ARP packet would normally be learned over the CCEP port. However, if the MAC address of the client is not already known on the CCEP, its ARP packet could be temporarily learned over the ICL. When the MAC Database Update Protocol (MDUP) message from the cluster peer device moves the client MAC address from the ICL to the CCEP, the ARP packet is also moved to the CCEP. During this transient time, no client ARP packet can be programmed over the ICL for a long period of time unless the local CCEP port is down.
During this transient time, the Layer 3 traffic gets forwarded toward the MCT peer. If the MCT client triggers an ARP request, it would do so for its Layer 3 next hop IP address, which generally resides on the MCT cluster devices. This address could be the default gateway on the MCT client or it could be learned through dynamic routing. If VRRP or VRRP-E is deployed on the MCT cluster devices, this IP address can be the virtual IP address.
Due to the inherent nature of LAG on the MCT client, this ARP request can reach an MCT device directly (over the CCEP) or through the MCT peer (over the ICL). In either case, the ARP response is sent out on the port where the client’s MAC address is learned. If the MAC address is already learned on the MCT device at the time of receiving the ARP request, it would be over the CCEP under normal working conditions (local CCEP is in the up state). If the client’s MAC address was not already learned when the ARP request is received, the client’s ARP could be temporarily learned over the ICL (and is moved to the CCEP when the MDUP message from the peer is received) and the ARP response could be sent over the ICL. The cluster peer then switches the ARP response further towards the MCT client.
