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Multicast Distribution Trees in a Virtual Chassis Fabric

VCF uses multicast distribution trees (MDTs) as the central mechanism for loop avoidance in forwarding of multicast, broadcast and flooding traffic inside the VCF. An MDT is calculated using the breadth-first search (BFS) algorithm rooted at any selected switch. The BFS algorithm is selected because it produces a tree that can cover the topology with the fewest number of hops, so it is conducive to shorter multicast latency. VCF uses the MDTs as bidirectional trees.

To load balance broadcast, unknown unicast, multicast, and flooding traffic evenly in any VCF topology, multiple MDTs are calculated. The number of MDTs calculated in a VCF matches the number of member devices in the VCF.

BUM and flooding traffic are assigned to a particular MDT based on VLAN ID, while known multicast traffic is load balanced to MDTs at the granularity of multicast next hops.