How Switches Learn Addresses
Posted on Tuesday, November 24, 2009
This article was posted in Networking Basics
How Switches Learn Addresses – A switch uses its bridge forwarding table (called a MAC table in Catalyst) address table when forwarding frames to devices. With an enpty bridge forwarding table, the switch must flood frames to all ports other than the one it arrivied on. This is the least-efficient way to transmit data.
Initially, the switch MAC address table is empty. Then station A with the MAC address sends a frame to station C. When the switch receives this frame, it does the following:
- Because the MAC table is empty, the switch must flood the frame to all other ports except on the originating port ( E0)
- The switch notes the source address of the originating device and associates it with port E0 in its MAC address table entry. Note that the table uses the source address to populate the table, not the destination address.
The swtich continues to learn address in this manner. continually updating the table. As the MAC table becomes more complete, the switching becomes more efficient, becuase frames are filtered to specific ports rather than being flooded out all ports.