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Understanding Layer 2 Learning and Forwarding

On MX Series routers only, you can configure Layer 2 MAC address and VLAN learning and forwarding properties in support of Layer 2 bridging. The router learns unicast media access control (MAC) addresses to avoid flooding the packets to all the ports in a bridge domain. The MX Series router creates a source MAC entry in its source and destination MAC tables for each MAC address learned from packets received on ports that belong to the bridge domain. If the bridge domain receives a control protocol data unit (PDU) which does not have a corresponding protocol configured, then the control PDU is considered as an unknown multicast data packet and the packets are flooded across all the ports that are part of the same bridge domain. If the bridge domain has the protocol corresponding to the PDU configured , then the control PDU is considered as a control packet and is processed by the routing engine.

By default, Layer 2 address learning is enabled. You can disable MAC learning for the router or for a specific bridge domain or logical interfaces. You can also configure the following Layer 2 forwarding properties for an MX Series router:

  • Timeout interval for MAC entries

  • MAC accounting

  • A limit to the number of MAC addresses learned from the logical interfaces