which ports does a switch forward a unkown unicast or multicast frame when there are several vlans spanned over several switches?
Lets say a switch received a unknowns unicast frame on a port that is part of vlan 20, this switch has several vlans, some other switches connected on trunks and also a router. So will this switch forward the frame on all vlan 20 ports on this switch and on the trunk ports as well?
thanks for your help.
Yes, for an unknown unicast, the switch will forward the frame to all ports on the VLAN, including any trunks, unless the port is in Spanning Tree blocking state, or unless the trunk does not have the VLAN on its "allowed" list.
For an unknown multicast, the situation is slightly different. If it is an IP multicast (but not in the range 224.0.0.x), and you have IGMP snooping or CGMP enabled on the switch, then the frame is only forwarded to those hosts that have actually expressed an interest in that multicast by sending an IGMP report for that address.