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question on how switch forwards a frame.

Folks,

From all the reading that I have done I understand that when a switch wants to find the destination mac address it simply forwards the frame out all it's switch ports except the one that it received the frame on.

I am thinking on a topology where a HUB is connected to a port, say Gi0/1 and two computers A and B are connected on this HUB.

When A wants to talk to B how will the mac learning really happen?

Regards,

Nik

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

question on how switch forwards a frame.

Hi Nik,

I understand that  when a switch wants to find the destination mac address it simply  forwards the frame out all it's switch ports except the one that it  received the frame on.

I am perhaps being too picky about the wording but let's make this statement more precise.

When a switch wants to find (i.e. look up) the destination MAC address, it tries to simply look it up in its MAC address table without first forwarding the frame. Only if the destination MAC address is not found there, the frame will be forwarded out all remaining ports. This flooding is not done in order to learn the unknown destination MAC address but simply because the switch tries to do its best to deliver the frame. The flooding has no direct relation to MAC address learning. MAC addresses are learnt as frames are received. If the destination station never responds, a switch can not learn its address because it never receives a frame from it.

When A wants to talk to B how will the mac learning really happen?

A will send a frame to B. This frame will propagate immediately through all other ports of the hub, reaching both B and the switch. B has now received the frame and it can process it. The switch sees the frame from A coming through its Gi0/1 port and learns about the sender A. Because it does not know about destination B, it floods the frame through all other ports in the same VLAN but it does not return it back to the hub.

When B responds to A, the response frame is propagated via the hub to all other connected ports including station A and the switch. Again, A is happy as it has received the response and it can process it. As for the switch, it will now receive the frame from B to A, it learns about station B on the Gi0/1 port, and because now the destination station A is also learned on the same Gi0/1 port, the switch will simply drop the frame silently.

Does this explain things a little? Please feel welcome to ask further!

Best regards,

Peter

2 REPLIES
Cisco Employee

question on how switch forwards a frame.

Hi Nik,

I understand that  when a switch wants to find the destination mac address it simply  forwards the frame out all it's switch ports except the one that it  received the frame on.

I am perhaps being too picky about the wording but let's make this statement more precise.

When a switch wants to find (i.e. look up) the destination MAC address, it tries to simply look it up in its MAC address table without first forwarding the frame. Only if the destination MAC address is not found there, the frame will be forwarded out all remaining ports. This flooding is not done in order to learn the unknown destination MAC address but simply because the switch tries to do its best to deliver the frame. The flooding has no direct relation to MAC address learning. MAC addresses are learnt as frames are received. If the destination station never responds, a switch can not learn its address because it never receives a frame from it.

When A wants to talk to B how will the mac learning really happen?

A will send a frame to B. This frame will propagate immediately through all other ports of the hub, reaching both B and the switch. B has now received the frame and it can process it. The switch sees the frame from A coming through its Gi0/1 port and learns about the sender A. Because it does not know about destination B, it floods the frame through all other ports in the same VLAN but it does not return it back to the hub.

When B responds to A, the response frame is propagated via the hub to all other connected ports including station A and the switch. Again, A is happy as it has received the response and it can process it. As for the switch, it will now receive the frame from B to A, it learns about station B on the Gi0/1 port, and because now the destination station A is also learned on the same Gi0/1 port, the switch will simply drop the frame silently.

Does this explain things a little? Please feel welcome to ask further!

Best regards,

Peter

New Member

question on how switch forwards a frame.

thanks Peter...this helps getting some basic comcepts in place..

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