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Configuring a Trunk Link
Configuring an ISL/802.1Q Negotiating Trunk Port
To configure a trunk port to negotiate the trunk encapsulation type (either ISL or 802.1Q), perform this
task in privileged mode:
Task
Step 1
Configure a port to negotiate the trunk
encapsulation type.
Step 2
Verify the trunking configuration.
This example shows how to configure a port to negotiate the encapsulation type and verify the trunk
configuration. This example assumes that the neighboring port is in auto mode with encapsulation set
to isl or negotiate.
Console> (enable) set trunk 4/11 desirable negotiate
Port(s) 4/11 trunk mode set to desirable.
Port(s) 4/11 trunk type set to negotiate.
Console> (enable) show trunk 4/11
Port
--------
4/11
Port
--------
4/11
Port
--------
4/11
Port
--------
4/11
Console> (enable)
Defining the Allowed VLANs on a Trunk
When you configure a trunk port, all VLANs are added to the allowed VLANs list for that trunk.
However, you can remove VLANs from the allowed list to prevent traffic for those VLANs from passing
over the trunk.
When you first configure a port as a trunk, entering the set trunk command always adds all VLANs to
Note
the allowed VLANs list for the trunk, even if you specify a VLAN range (any specified VLAN range is
ignored). To modify the allowed VLANs list, use a combination of the clear trunk and set trunk
commands to specify the allowed VLANs.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
5-8
Mode
Encapsulation
-----------
-------------
desirable
n-isl
Vlans allowed on trunk
---------------------------------------------------------------------
1-1005,1025-4094
Vlans allowed and active in management domain
---------------------------------------------------------------------
1,5,10-32,55,101-120,998-1000
Vlans in spanning tree forwarding state and not pruned
---------------------------------------------------------------------
1,5,10-32,55,101-120,998-1000
Chapter 5
Command
set trunk mod/port [on | off | desirable | auto |
nonegotiate] negotiate
show trunk [mod/port]
Status
Native vlan
------------
-----------
trunking
1
Configuring Ethernet VLAN Trunks
OL-8978-04
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Troubleshooting
