DHCP relay backup

When the two VSX switches are configured for DHCP relay on their VLAN interfaces, only the primary switch actively relays DHCP client requests to the upstream server. The secondary switch acts as a backup. If the primary VSX switch goes down, the secondary switch takes over, such in the case with ISL and keepalive both down. Even though both primary and secondary switches receive the DHCP request, the primary switch takes precedence.

The secondary VSX node forwards over the ISL to the primary VSX switch the DHCP requests received from the downstream endpoints. The upstream DHCP servers receive a single DHCP request. The downstream clients receive a single DHCP offer.

Both devices do not end up relaying DHCP requests to the server as duplicates. That scenario is usually the case with typical aggregation switches running VRRP-based redundancy.