Outgoing VLAN traffic

This uses the MAC address of the switch as the source MAC address in packet headers. The MAC address configured on VLAN interfaces is not used on outbound VLAN traffic.

When the routing switch receives an ARP request for the IP address configured on a VLAN interface, the ARP reply uses the reconfigured MAC address in both the:

  • ARP Sender MAC address field

  • Source MAC address field in the Ethernet frame header

When proxy ARP is enabled on a VLAN interface, the ARP reply sent for an ARP request received from VLAN devices located outside the directly connected IP subnets also contains the reconfigured MAC address in both the:

  • ARP Sender MAC address field

  • Source MAC address field in the Ethernet frame header

NOTE:

The Virtual Router Redundancy Protocol (VRRP) is not supported on VLAN interfaces on which the MAC address for incoming traffic has been reconfigured.

To hosts in the network, VLAN traffic continues to be routed (using the reconfigured MAC address as destination address), but outbound VLAN traffic appears to be sent from another router attached to the same subnet (using the Switch MAC address as source address) attached to the same subnet. Although it appears as an asymmetric path to network hosts, the MAC address configuration feature enables Layer 3 VLAN migration. (A successful VLAN migration is achieved because the hosts do not verify that the source MAC address and the destination MAC address are the same when communicating with the routing switch.)