Cisco WS-C2960-24TC-L Software Guide - Page 238
Disabling VLAN 1 on a Trunk Link
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Disabling VLAN 1 on a Trunk Link Chapter 11 Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports To return a port to the default trunk type and mode for that port type, perform this task in privileged mode: Step 1 Step 2 Task Command Return the port to the default trunking type and mode clear trunk mod_num/port_num for that port type. Verify the trunking configuration. show trunk [mod_num/port_num] 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: Step 1 Step 2 Task Disable VLAN 1 on the trunk interface. Verify the allowed VLAN list for the trunk. Command clear trunk mod_num/port_num [vlan-range] show trunk [mod_num/port_num] 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 11-8 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 78-15486-01