viral_thakkar
04-15-2008, 05:15 AM
Allowed, Active, and Pruned VLANs
Although a trunk can support VLANs 1–4094, several mechanisms reduce the actual number of
VLANs whose traffic flows over the trunk. First, VLANs can be administratively forbidden from
existing over the trunk using the switchport trunk allowed interface subcommand. Also, any
allowed VLANs must be configured on the switch before they are considered active on the trunk.
Finally, VTP can prune VLANs from the trunk, with the switch simply ceasing to forward frames
from that VLAN over the trunk.
The show interface trunk command lists the VLANs that fall into each category, as shown in the
last command in Example 2-5. The categories are summarized as follows:
■ Allowed VLANs—Each trunk allows all VLANs by default. However, VLANs can be
removed or added to the list of allowed VLANs by using the switchport trunk allowed
command.
■ Allowed and active—To be active, a VLAN must be in the allowed list for the trunk (based
on trunk configuration), and the VLAN must exist in the VLAN configuration on the switch.
With PVST+, an STP instance is actively running on this trunk for the VLANs in this list.
■ Active and not pruned—This list is a subset of the “allowed and active” list, with any
VTP-pruned VLANs removed.
Although a trunk can support VLANs 1–4094, several mechanisms reduce the actual number of
VLANs whose traffic flows over the trunk. First, VLANs can be administratively forbidden from
existing over the trunk using the switchport trunk allowed interface subcommand. Also, any
allowed VLANs must be configured on the switch before they are considered active on the trunk.
Finally, VTP can prune VLANs from the trunk, with the switch simply ceasing to forward frames
from that VLAN over the trunk.
The show interface trunk command lists the VLANs that fall into each category, as shown in the
last command in Example 2-5. The categories are summarized as follows:
■ Allowed VLANs—Each trunk allows all VLANs by default. However, VLANs can be
removed or added to the list of allowed VLANs by using the switchport trunk allowed
command.
■ Allowed and active—To be active, a VLAN must be in the allowed list for the trunk (based
on trunk configuration), and the VLAN must exist in the VLAN configuration on the switch.
With PVST+, an STP instance is actively running on this trunk for the VLANs in this list.
■ Active and not pruned—This list is a subset of the “allowed and active” list, with any
VTP-pruned VLANs removed.