Single forwarding database operation

When a packet arrives with a destination MAC address that matches a MAC address in the switch's forwarding table, the switch tries to send the packet to the port listed for that MAC address. But if the destination port is in a different VLAN than the VLAN on which the packet was received, the switch drops the packet. This is not a problem for a switch with a multiple forwarding database, because the switch allows multiple instances of a given MAC address; one for each valid destination. However, a switch with a single forwarding database allows only one instance of a given MAC address.

If (1) two types of switches connect through multiple ports or trunks belonging to different VLANs, and (2) routing is enabled on the switch having the multiple forwarding database then, on the switch having the single forwarding database, the port and VLAN record it maintained for the connected multiple-forwarding-database switch on the switch having the single forwarding database, maintains for the connected multiple-forwarding-database switch can frequently change. This causes poor performance and the appearance of an intermittent or broken connection.