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.