Cisco 2950 Software Configuration Guide - Page 378
Q Configuration Considerations, Function
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Configuring VLAN Trunks Chapter 17 Configuring VLANs • If you do not intend to trunk across those links, use the switchport mode access interface configuration command to disable trunking. • To enable trunking to a device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration commands to cause the interface to become a trunk but to not generate DTP frames. • With GigaStack GBICs, dynamic trunking is only supported when two switches are connected by a single GigaStack GBIC link. If trunking is required when more than two switches in a stack are connected by GigaStack GBIC links, you must manually configure trunking in this manner: - Manually shut down the GigaStack port by using the shutdown interface configuration command. - Manually configure trunk mode on the GigaStack port by using the switchport mode trunk interface configuration command on both GBIC interfaces to cause the interfaces to become trunks. - Use the no shutdown interface configuration command to bring up the GigaStack port. Table 17-4 Layer 2 Interface Modes Mode switchport mode access switchport mode dynamic desirable switchport mode dynamic auto switchport mode trunk switchport nonegotiate Function Puts the interface (access port) into permanent nontrunking mode. The interface becomes a nontrunk interface even if the neighboring interface is a trunk interface. Makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. The default switch-port mode for all Ethernet interfaces is dynamic desirable. Makes the interface able to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. Puts the interface into permanent trunking mode and negotiates to convert the link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link. 802.1Q Configuration Considerations 802.1Q trunks impose these limitations on a network: • In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree for each VLAN allowed on the trunks. Non-Cisco devices might support one spanning-tree instance for all VLANs. When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch. However, spanning-tree information for each VLAN is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches. 17-16 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10