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9-27
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 9 Creating and Maintaining VLANs
Understanding VLAN Trunks
In this example, the encapsulation method is ISL:
Switch# show interfaces gigabitethernet0/4 trunk
Port Mode Encapsulation Status Native vlan
Gi0/4 desirable n-isl trunking 1
Port Vlans allowed on trunk
Gi0/4 1-1005
Port Vlans allowed and active in management domain
Gi0/4 1,10-1000
Port Vlans in spanning tree forwarding state and not pruned
Gi0/4 1,10-1000
Defining the Allowed VLANs on a Trunk
By default, a trunk port sends traffic to and receives traffic from all VLANs in the VLAN database. All
VLANs, VLANs 1 to 1005, are allowed on each trunk. However, you can remove VLANs from the
allowed list, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a
trunk carries, use the switchport trunk allowed vlan remove vlan-list interface configuration to remove
specific VLANs from the allowed list.
A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN,
and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the
VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the
enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk
port, the trunk port does not become a member of the new VLAN.
Beginning in privileged EXEC mode, follow these steps to modify the allowed list of an ISL
or 802.1Q trunk:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter interface configuration mode and the port to be added to the
VLAN.
Step 3
switchport trunk allowed vlan {add |
except | none | remove} vlan-list
(Optional) Configure the list of VLANs allowed on the trunk.
For explanations about using the add, except, none, and remove
keywords, refer to Catalyst 3550 Multilayer Switch Command Reference
for this release.
The vlan-list parameter is either a single VLAN number from 1 to 1005
or a range of VLANs described by two VLAN numbers, the lesser one
first, separated by a hyphen. Do not enter any spaces between
comma-separated VLAN parameters or in hyphen-specified ranges.
All VLANs are allowed by default. You cannot remove any of the default
VLANs from a trunk.
Step 4
end Return to privileged EXEC.
Step 5
show interfaces interface-id switchport Verify your entries in the Trunking VLANs Enabled field of the display.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.