Tuesday, September 25, 2012

Unknown unicast: router vs switch.

Unknown unicast - it is a frame with an unknown destination mac-address, which means that the switch/router doesn't have this mac-address in its' CAM. Thus the switch/router has no idea where to forward this frame.
The behavior of switches and routers is different here.
Switches flood this frame over all its' ports. If a frame belongs to some vlan - it is being flooded over the ports in this particular vlan.
Routers operate on layer 3, therefore this frame will be decapsulated in order to get the destination IP address. Before the encapsulation of this packet to L2 a router will send ARP request to match mac-address with destination IP-address. This causes some additional delays, e.g. - .!!!!

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