13-20
Catalyst 3750 Switch Software Configuration Guide
78-16180-02
Chapter 13 Configuring VLANs
Configuring VLAN Trunks
Note By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode
dynamic auto. If the neighboring interface supports trunking and is configured to allow trunking, the
link is a Layer 2 trunk or, if the interface is in Layer 3 mode, it becomes a Layer 2 trunk when you enter
the switchport interface configuration command. By default, trunks negotiate encapsulation. If the
neighboring interface supports ISL and 802.1Q encapsulation and both interfaces are set to negotiate the
encapsulation type, the trunk uses ISL encapsulation.
Interaction with Other Features
Trunking interacts with other features in these ways:
• A trunk port cannot be a secure port.
• Trunk ports can be grouped into EtherChannel port groups, but all trunks in the group must have the
same configuration. When a group is first created, all ports follow the parameters set for the first
port to be added to the group. If you change the configuration of one of these parameters, the switch
propagates the setting you entered to all ports in the group:
–
allowed-VLAN list.
–
STP port priority for each VLAN.
–
STP Port Fast setting.
–
trunk status: if one port in a port group ceases to be a trunk, all ports cease to be trunks.
• We recommend that you configure no more than 24 trunk ports in PVST mode and no more than 40
trunk ports in MST mode.
• If you try to enable 802.1x on a trunk port, an error message appears, and 802.1x is not enabled. If
you try to change the mode of an 802.1x-enabled port to trunk, the port mode is not changed.
• A port in dynamic mode can negotiate with its neighbor to become a trunk port. If you try to enable
802.1x on a dynamic port, an error message appears, and 802.1x is not enabled. If you try to change
the mode of an 802.1x-enabled port to dynamic, the port mode is not changed.
Configuring a Trunk Port
Beginning in privileged EXEC mode, follow these steps to configure a port as an ISL or 802.1Q
trunk port:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter the interface configuration mode and the port to be configured for
trunking.
Step 3
switchport trunk encapsulation {isl |
dot1q | negotiate}
Configure the port to support ISL or 802.1Q encapsulation or to negotiate
(the default) with the neighboring interface for encapsulation type.
You must configure each end of the link with the same encapsulation type.
To Next Page
Loading...
Need help?
General