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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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Chapter 5 Configuring Ethernet VLAN Trunks
Disabling VLAN 1 on Trunks
Disabling VLAN 1 on Trunks
On the Catalyst 6000 family switches, VLAN 1 is enabled by default to allow control protocols to
transmit and receive packets across the network topology. However, when VLAN 1 is enabled on trunk
links in a large complex network, the impact of broadcast storms increases. Because spanning tree
applies to the entire network, spanning tree loops might increase when you enable VLAN 1 on all trunk
links. To prevent this scenario, you can disable VLAN 1 on trunk interfaces.
When you disable VLAN 1 on a trunk interface, no user traffic is transmitted and received across that
trunk interface, but the supervisor engine continues to transmit and receive packets from control
protocols such as Cisco Discovery Protocol (CDP), VTP, Port Aggregation Protocol (PAgP), and DTP.
When a trunk port with VLAN 1 disabled becomes a nontrunk port, it is added to the native VLAN. If
the native VLAN is VLAN 1, the port is enabled and added to VLAN 1.
Disabling VLAN 1 on a Trunk Link
To disable VLAN 1 on a trunk interface, perform this task in privileged mode:
This example shows how to disable VLAN 1 on a trunk link and verify the configuration:
Console> (enable)
clear trunk 8/1 1
Removing Vlan(s) 1 from allowed list.
Port 8/1 allowed vlans modified to 2-1005.
Console> (enable)
show trunk 8/1
Port Mode Encapsulation Status Native vlan
-------- ----------- ------------- ------------ -----------
8/1 on isl trunking 1
Port Vlans allowed on trunk
-------- ---------------------------------------------------------------------
8/1 2-1005, 1025-4094
Port Vlans allowed and active in management domain
-------- ---------------------------------------------------------------------
8/1 2-6,10,20,50,100,152,200,300,400,500,521,524,570,776,801-802,850,917,9
99,1003,1005
Port Vlans in spanning tree forwarding state and not pruned
-------- ---------------------------------------------------------------------
8/1 2-6,10,20,50,100,152,200,300,400,500,521,524,570,776,802,850,917,999,1
003,1005
Console> (enable)
show config
Task
Command
Step 1
Disable VLAN 1 on the trunk interface.
clear trunk
mod/port
[
vlan-range
]
Step 2
Verify the allowed VLAN list for the trunk.
show trunk
[
mod/port
]