Bridging/Switching Question ?

moss12

Hi guys ,


A question need explanation : Switch examines source and destination MAC address when the destination MAC address is not found in the CAM table its forwarded out all ports expect the source port , but how does the switch know the destination MAC address ok when the switch forwards unknown unicast frames how does the destination devices knows it has to respond to this request ?????.

dmaftei

moss12 wrote:

...how does the switch know the destination MAC address ok when the switch forwards unknown unicast frames how does the destination devices knows it has to respond to this request?

You may want to rephrase your question, I have no idea what you're asking... What exactly do you mean by "unknown unicast frame"?

loboernesto

the switch only cares about what port send the frame to. the switch knows (based in the CAM table) what MAC addresses are on which ports. So if it receives a frame for PCA mac address aaaa.aaaa.aaaa in port 2 or 3 it will send the frame out port 1 (see graphic)

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port 1

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port 2

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port 3

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aaaa.aaaa.aaaa | bbbb.bbbb.bbbb | cccc.cccc.cccc


An unicast frame's destination address may be unknow to the switch (in which case it forwards it out all interfaces but the once it received it ) but is still an unicast address so the destination node will know it's for it since is has its address.
example: the same switch receives a frame for PCD mac address dddd.dddd.dddd in port 2 again, the switch doesn't know in which port PCD is so it sends the frame out every port but port 2, PCD recieves the frame (as well as all the other nodes in the same segment) and it knows it's destined to itself since the destination MAC address is dddd.dddd.ddd (note that switches will forward unknown unicast frame out all ports but to do so they don't change the MAC address to a broadcast address, so the frame stays a unicast frame)
cheers.

EdTheLad

moss12 wrote:

ok when the switch forwards unknown unicast frames how does the destination devices knows it has to respond to this request ?????.

The switch will forward unknown unicast frames out all ports,these frames will have dest mac address field, all devices on the layer 2 network will receive this frame check the destination mac address and if it equals the local mac address the frame will be processed and sent to layer 3,if not it will be discarded.
The higher layers will determine the action required,for instance if it was a ping the network layer would know to respond to the source ip address in the received packet.If the packet was multicast no response would be sent.

Webmaster

Though the question has been answered a couple of times already (see also previous topics), I'll add a slightly different worded version: if the switch doesn't have the destination mac address in its forwarding table, it will act similar to a hub, and flood it out of all ports except the incoming. For the hosts it won't matter because switching is transparent. This means the destination and source devices don't know if there's a switch and the switches don't change the source/dest addresses in frames and packets. So the devices communicate with each other just as they would if they were directly connected with a cross-over cable.

moss12

thank guys