11-8
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
—
Release 8.1
78-15486-01
Chapter 11 Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports
Disabling VLAN 1 on 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 a Trunk Link
On the Catalyst enterprise LAN 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 topology, the impact of broadcast storms increases. Because spanning
tree applies to the entire network topology, the possibility of spanning tree loops also increases when
VLAN 1 is enabled on all trunk links. To prevent this situation, you can disable VLAN 1 on trunk
interfaces.
When you disable VLAN 1 on a trunk interface, no user traffic is transmitted or received across that trunk
interface, but the supervisor engine will continue to transmit and receive packets from control protocols
such as Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP), Port Aggregation Protocol
(PAgP), Dynamic Trunking Protocol (DTP), and so forth.
Caution
By default, the sc0 interface management VLAN is VLAN 1. If you disable VLAN 1, you will have to
configure another VLAN to be the management VLAN for sc0.
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 4/1 1
Removing Vlan(s) 1 from allowed list.
Port 4/1 allowed vlans modified to 2-1005.
Console> (enable) show trunk 4/1
Port Mode Encapsulation Status Native vlan
-------- ----------- ------------- ------------ -----------
4/1 on isl trunking 1
Port Vlans allowed on trunk
-------- ---------------------------------------------------------------------
4/1 2-999, 1025-4094
Port Vlans allowed and active in management domain
-------- ---------------------------------------------------------------------
4/1 2-6,10,20,50,100,152,200,300,400,500,521,524,570,776,801-802,850,917,999
Task
Command
Step 1
Return the port to the default trunking type and mode
for that port type.
clear trunk mod_num/port_num
Step 2
Verify the trunking configuration.
show trunk [mod_num/port_num]
Task
Command
Step 1
Disable VLAN 1 on the trunk interface.
clear trunk mod_num/port_num [vlan-range]
Step 2
Verify the allowed VLAN list for the trunk.
show trunk [mod_num/port_num]
