Cisco 2950 Software Configuration Guide - Page 381
Defining the Allowed VLANs on a Trunk, traffic, for example, Cisco Discovery Protocol CDP
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Chapter 17 Configuring VLANs Configuring VLAN Trunks Step 8 Step 9 Command show interfaces interface-id trunk copy running-config startup-config Purpose Display the trunk configuration of the interface. (Optional) Save your entries in the configuration file. To return an interface to its default configuration, use the default interface interface-id interface configuration command. To reset all trunking characteristics of a trunking interface to the defaults, use the no switchport trunk interface configuration command. To disable trunking, use the switchport mode access interface configuration command to configure the port as a static-access port. This example shows how to configure a port as an 802.1Q trunk. The example assumes that the neighbor interface is configured to support 802.1Q trunking. Switch# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Switch(config)# interface fastethernet0/4 Switch(config-if)# switchport mode dynamic desirable Switch(config-if)# end Defining the Allowed VLANs on a Trunk By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094 when the EI is installed, and 1 to 1005 when the SI is installed, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the switchport trunk allowed vlan remove vlan-list interface configuration command to remove specific VLANs from the allowed list. To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN trunk port by removing VLAN 1 from the allowed list. This is known as VLAN 1 minimization. VLAN 1 minimization disables VLAN 1 (the default VLAN on all Cisco switch trunk ports) on an individual VLAN trunk link. As a result, no user traffic, including spanning-tree advertisements, is sent or received on VLAN 1. When you remove VLAN 1 from a trunk port, the interface continues to send and receive management traffic, for example, Cisco Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), Dynamic Trunking Protocol (DTP), and VLAN Trunking Protocol (VTP) in VLAN 1. If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the access VLAN is set to 1, the port is added to VLAN 1, regardless of the switchport trunk allowed setting. The same is true for any VLAN that has been disabled on the port. A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not become a member of the new VLAN. Beginning in privileged EXEC mode, follow these steps to modify the allowed list of an 802.1Q trunk: Step 1 Step 2 Step 3 Command configure terminal interface interface-id switchport mode trunk Purpose Enter global configuration mode. Enter interface configuration mode and the port to be configured. Configure the interface as a VLAN trunk port. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 17-19