802.1Q Configuration Considerations - Cisco 2950 - Catalyst Switch Configuration Manual

Configuring VLAN Trunks
•
•
•
Table 17-4 Layer 2 Interface Modes
Mode
switchport mode access
switchport mode dynamic
desirable
switchport mode dynamic auto
switchport mode trunk
switchport nonegotiate

802.1Q Configuration Considerations

802.1Q trunks impose these limitations on a network:
•
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
17-16
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.
–
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.
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.
Chapter 17 Configuring VLANs
78-11380-10