Multiple VLAN considerations

Switches use a forwarding database to maintain awareness of which external devices are located on which VLANs. Some switches, such as the switches covered in this guide, have a multiple forwarding database, which means the switch allows multiple database entries of the same MAC address, with each entry showing the (different) source VLAN and source port. Other switch models have a single forwarding database, which allows only one database entry of a unique MAC address, along with the source VLAN and source port on which it is found. All VLANs on a switch use the same MAC address. Thus, connecting a multiple forwarding database switch to a single forwarding database switch where multiple VLANs exist imposes some cabling and port VLAN assignment restrictions. The following table illustrates the functional difference between the two database types.

Forwarding database content
Multiple forwarding database Single forwarding database
MAC address Destination VLAN ID Destination port MAC address Destination VLAN ID Destination port
0004ea-84d9f4 1 A5 0004ea-84d9f4 100 A9
0004ea-84d9f4 22 A12 0060b0-880af9 105 A10
0004ea-84d9f4 44 A20 0060b0-880a81 107 A17
0060b0-880a81 33 A20      
This database allows multiple destinations for the same MAC address. If the switch detects a new destination for an existing MAC entry, it just adds a new instance of that MAC to the table. This database allows only one destination for a MAC address. If the switch detects a new destination for an existing MAC entry, it replaces the existing MAC instance with a new instance showing the new destination.

All switches covered in this guide use a multiple forwarding database.