BUM

networking

Stands for: broadcast, unknown unicast, and multicast

Switching shorthand for the three traffic types a switch cannot deliver to a single known port and therefore floods: broadcast, unknown unicast, and multicast.

A switch forwards a frame out one port only when its MAC table says where the destination lives; broadcasts never qualify, multicasts map to groups rather than ports, and an unknown unicast is simply a destination the table has not learned yet. All three get flooded to every port in the VLAN, which is harmless at small scale and a scaling problem at large scale. The term earns its keep in overlay networks: VXLAN fabrics must decide how to carry BUM traffic across the underlay, by ingress replication or by a multicast underlay, and storm-control features exist to rate-limit BUM before a loop or a chatty host turns flooding into a broadcast storm.

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