phases in building mac

Unanswered Question
scottmac Sun, 02/18/2007 - 19:46

A switch is (in basic operation) just a bridge ... pretty much anything you can read about bridge operation applies.

The short story is that as the switch/bridge receives the frame, it notes the ingress port and the *source* MAC.

It builds the table based on "I heard this MAC address on that port"

Later, when another frame enters the switch destined for that MAC, it finds the MAC, sees which port is associated with the MAC, and forwards the frame out that port.

If a frame is received and the destination MAC is unknown (i.e., not in the forwarding table), the switch/bridge FLOODS (not broadasts) the frame out all ports except the one it was received on.

The difference between a flood and a broadast is that a broadcast frame has the destination MAC set to all ones (ff.ff.ff.ff.ff.ff), where a flooded frame keeps the same source and destination MAC address.

Good Luck

Scott

Richard Burts Mon, 02/19/2007 - 06:26

Aksher

You asked this question before - see this thread:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=Getting%20Started%20with%20LANs&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddd9601

To restate the answer the ARP table is used for layer 3 when the switch management interface wants to send IP packets to other addresses. The MAC table is used for layer 2 forwarding. So the ARP table and the MAC table are not really related.

HTH

Rick

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