learning bridges
suppose c sends frame to d and d replies back with frame to c
c sends frame, bridge has no info about d, so floods to all other interfaces, learns c is on interface 1
d sends frame to c, bridge knows where c is, and forwards the frame there, learns d is on interface 3
modes
	filtering: src and dst on same segment, not forwarded
	forwarding: src and dst on different segments, forwarded
	flooding: dst location unknown, flooded to all but origination interface

switching loops can happen, normally self-resolve in a short time when the destination node responds
spanning tree protocol prevents this from happening by disabling redundant links until they're needed