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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter17 Configuring VLANs
Configuring VLAN Trunks
•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 t o become a trun k
but to not generate DTP frames.
•With GigaStack GBICs, dynamic trunking is only supported when two switches are c onnected 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.
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 maint ain one instance
of spanning tree for each VLAN allowed on the trunks. Non-Cisco devices mi ght su pport 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 VLA N of th e trun k wit h the s pann ing-t ree insta nce of
the non-Cisco 802.1Q switch. However , spanni ng-tre e infor mation f or each VLAN is maintain ed b y
Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The no n- Cisco 80 2.1 Q c loud
separating the Cisco switches is treated as a single trunk link between the switches.
Table17-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode. The interface becomes
a nontrunk interface even if the neighboring interface is a trunk interface.
switchport mode dynamic
desirable 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.
switchport mode dynamic auto Makes the interface able to convert the link to a trunk link. The interface b ecomes a trunk
interface if the neighboring interface is set to trunk or desirable mode.
switchport mode trunk 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.
switchport nonegotiate 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.