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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:
Task
Step 1
Return the port to the default trunking type and
mode for that port type.
Step 2
Verify the trunking configuration.
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:
Task
Step 1
Disable VLAN 1 on the trunk interface.
Step 2
Verify the allowed VLAN list for the trunk.
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
Console> (enable) show trunk 8/1
Port
--------
8/1
Port
--------
8/1
Port
--------
8/1
99,1003,1005
Port
--------
8/1
003,1005
Console> (enable) show config
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
5-10
8/1 allowed vlans modified to 2-1005.
Mode
Encapsulation
-----------
-------------
on
isl
Vlans allowed on trunk
---------------------------------------------------------------------
2-1005, 1025-4094
Vlans allowed and active in management domain
---------------------------------------------------------------------
2-6,10,20,50,100,152,200,300,400,500,521,524,570,776,801-802,850,917,9
Vlans in spanning tree forwarding state and not pruned
---------------------------------------------------------------------
2-6,10,20,50,100,152,200,300,400,500,521,524,570,776,802,850,917,999,1
Chapter 5
Command
clear trunk mod/port
show trunk [mod/port]
Command
clear trunk mod/port [vlan-range]
show trunk [mod/port]
Status
Native vlan
------------
-----------
trunking
1
Configuring Ethernet VLAN Trunks
OL-8978-04
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Troubleshooting
