The standard implementation of VRRP supports only one active master device for each IP subnet. All other VRRP interfaces in a network are in backup mode.
A deficiency occurs when VRRP-enabled switches use Split MultiLink Trunking (SMLT). If VRRP switches are aggregated into two Split MultiLink Trunk switches, the end host traffic is load-shared on all uplinks to the aggregation switches (based on the Multilink Trunk traffic distribution algorithm).
However, VRRP usually has only one active routing interface enabled. All other VRRP routers are in backup mode. Therefore, all traffic that reaches the backup VRRP router is forwarded over the vIST towards the master VRRP router. In this case, the vIST does not have enough bandwidth to carry all the aggregated traffic.
To resolve this issue, assign the backup router as the backup master router. The backup master router can actively load-share the routing traffic with a master router.
When the backup master router is enabled, the incoming host traffic is forwarded over the SMLT links as usual. When the backup master router is configured along with the critical IP interface and the critical IP interface goes down, the VRRP router transitions to be the backup router with the backup master state down. In this state, the VRRP router does not forward traffic.