Learning Modes and Filtering Databases

Addressing information the switch learns about a VLAN is stored in the filtering database assigned to that VLAN. This database contains source addresses, their source ports, and VLAN IDs, and is referred to when a switch makes a decision as to where to forward a VLAN tagged frame. Each filtering database is assigned a Filtering Database ID (FID). The FID a VLAN belongs to can be displayed using the show vlan command.

A switch learns and uses VLAN addressing information by the following modes:

  • Independent Virtual Local Area Network (VLAN) Learning (IVL): Each VLAN uses its own filtering database. Transparent source address learning performed as a result of incoming VLAN traffic is not made available to any other VLAN for forwarding purposes. This setting is useful for handling devices (such as servers) with NICs that share a common MAC address. One FID is assigned per VLAN. The FID value is the same as the VID it is assigned to. This is the default mode on Extreme Networks switches.
  • Shared Virtual Local Area Network (VLAN) Learning (SVL): Two or more VLANs are grouped to share common source address information. This setting is useful for configuring more complex VLAN traffic patterns, without forcing the switch to flood the unicast traffic in each direction. This allows VLANs to share addressing information. It enables ports or switches in different VLANs to communicate with each other (when their individual ports are configured to allow this to occur). One FID is used by two or more VLANs. The FID value defaults to the lowest VID in the filtering database.

The VLAN learning mode for the switch and the assignment of multiple VLANs to a FID are configured in VLAN constraints using the set vlan constraint command. See Appendix F of the IEEE Standardd 802.1Q 2011 standard for a detailed discussion of shared and independent VLAN learning modes.