Active-standby DHCP relay

When VSX synchronization is enabled for DHCP relay, only the primary VSX node relays DHCP requests to the upstream DHCP server. The secondary VSX node forwards over the ISL to the primary VSX switch the DHCP requests received from the downstream endpoints. The secondary VSX switch takes over DHCP-relay service upon primary failure detection (ISL down and keepalive down). Upstream DHCP servers receive a single DHCP request. Downstream clients receive a single DHCP offer.