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The current implementation of VRRP allows you to have one active master switch per IP subnet. All other VRRP interfaces in a network are in backup mode.
A deficiency occurs when Split-MLT is used. Users that access switches which are aggregated into two Split-MLT switches, send their traffic load shared (based on the MLT traffic distribution algorithm) on all uplinks towards the Split-MLT aggregation switches.
VRRP however only has one active routing interface enabled. All other interfaces are in backup mode and therefore in standby mode. In this case all traffic is forwarded over the Inter Switch Trunk (IST) link towards the master VRRP switch. Potentially there will be not enough bandwidth on the IST to carry all the aggregated riser traffic.
An enhancement in VRRP overcomes this deficiency. The enhancement makes sure that the IST trunk is not used in such a case.