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Understanding Layer 2 Learning and Forwarding for Bridge Domains

 

When you configure a bridge domain, Layer 2 address learning is enabled by default. The bridge domain learns unicast media access control (MAC) addresses to avoid flooding the packets to all the ports in the bridge domain. Each bridge domain creates a source MAC entry in its source and destination MAC tables for each source MAC address learned from packets received on the ports that belong to the bridge domain.

Note

Traffic is not flooded back onto the interface on which it was received. However, because this “split horizon” occurs at a late stage, the packet statistics displayed by commands such as show interfaces queue will include flood traffic.

You can optionally disable MAC learning either for the entire router or for a specific bridge domain or logical interface. You can also configure the following Layer 2 learning and forwarding properties:

  • Static MAC entries for logical interfaces only

  • Limit to the number of MAC addresses learned from a specific logical interface or from all the logical interfaces in a bridge domain

  • Size of the MAC address table for the bridge domain

  • MAC accounting for a bridge domain