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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 5 Configuring Ethernet VLAN Trunks
Configuring a Trunk Link
To return a port to the default trunk type and mode for that port type, perform this task in privileged
mode:
Disabling VLAN 1 on Trunks
On the Catalyst 6500 series 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.
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
Return the port to the default trunking type and
mode for that port type.
clear trunk
mod/port
Step 2
Verify the trunking configuration.
show trunk
[
mod/port
]
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
]