Cisco WS-C2960-24TC-S Software Guide - Page 268
Defining the Allowed VLANs on a Trunk, CDP, Port Aggregation Protocol PAgP
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Configuring VLAN Trunks Chapter 12 Configuring VLANs 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, 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. Note VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a requirement that VLAN 1 always be enabled on every trunk link. You can use the VLAN 1 minimization feature to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including spanning-tree advertisements) is sent or received on VLAN 1. 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. When you remove VLAN 1 from a trunk port, the interface continues to sent and receive management traffic, for example, Cisco Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), DTP, and 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 will be 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 a trunk: Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Command Purpose configure terminal Enter global configuration mode. interface interface-id Specify the port to be configured, and enter interface configuration mode. switchport mode trunk Configure the interface as a VLAN trunk port. switchport trunk allowed vlan {add | all | except | remove} vlan-list (Optional) Configure the list of VLANs allowed on the trunk. For explanations about using the add, all, except, and remove keywords, see the command reference for this release. The vlan-list parameter is either a single VLAN number from 1 to 4094 or a range of VLANs described by two VLAN numbers, the lower one first, separated by a hyphen. Do not enter any spaces between comma-separated VLAN parameters or in hyphen-specified ranges. All VLANs are allowed by default. end Return to privileged EXEC mode. show interfaces interface-id switchport Verify your entries in the Trunking VLANs Enabled field of the display. copy running-config startup-config (Optional) Save your entries in the configuration file. 12-18 Catalyst 2960 Switch Software Configuration Guide OL-8603-04
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