Defining The Allowed Vlans On A Trunk - Cisco Catalyst 3750 Software Configuration Manual
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Also See for Catalyst 3750:
- Command reference manual (1154 pages) ,
- Hardware installation manual (231 pages) ,
- Message manual (120 pages)
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Configuring VLAN Trunks
Defining the Allowed VLANs on a Trunk
By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094,
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 command to remove specific VLANs from
the allowed list.
Note
VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a
requirement that VLAN 1 always be enabled on every trunk link. You can use the VLAN 1 minimization
feature to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including spanning
tree advertisements) is sent or received on VLAN 1.
To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN
trunk port by removing VLAN 1 from the allowed list. When you remove VLAN 1 from a trunk port,
the interface continues to sent and receive management traffic, for example, Cisco Discovery Protocol
(CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), Dynamic
Trunking Protocol (DTP), and VLAN Trunking Protocol (VTP) in VLAN 1.
If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If
the access VLAN is set to 1, the port will be added to VLAN 1, regardless of the switchport trunk
allowed setting. The same is true for any VLAN that has been disabled on the port.
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
Step 1
configure terminal
Step 2
interface interface-id
Step 3
switchport mode trunk
Step 4
switchport trunk allowed vlan {add |
all | except | remove} vlan-list
Step 5
end
Step 6
show interfaces interface-id switchport Verify your entries in the Trunking VLANs Enabled field of the display.
Step 7
copy running-config startup-config
Catalyst 3750 Metro Switch Software Configuration Guide
10-22
Purpose
Enter global configuration mode.
Enter interface configuration mode and the port to be configured.
Configure the interface as a VLAN trunk port.
(Optional) Configure the list of VLANs allowed on the trunk.
For explanations about using the add, all, except, and remove keywords,
refer to the command reference for this release.
The vlan-list parameter is either a single VLAN number from 1 to 4094
or a range of VLANs described by two VLAN numbers, the lower 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.
Return to privileged EXEC mode.
(Optional) Save your entries in the configuration file.
Chapter 10
Configuring VLANs
78-15870-01
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