Cisco 4500M Command Reference Manual page 507

Chapter2 Cisco IOS Commands for the Catalyst 4500 Series Switches
Command History Release
12.1(8a)EW
12.1(12c)EW
12.2(18)EW
Usage Guidelines
The vlan_list format is all | none | [add | remove | except] vlan_atom[,vlan_atom... ], where:
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The switchport trunk encapsulation command is supported only for platforms and interface hardware
that can support both ISL and 802.1Q formats.
If you enter the negotiate keywords, and DISL and DTP negotiation do not resolve the encapsulation
format, ISL is the selected format. The no form of the command resets the trunk encapsulation format
back to the default.
The no form of the native vlan command resets the native mode VLAN to the appropriate default VLAN
for the device.
The no form of the allowed vlan command resets the list to the default list, which allows all VLANs.
The no form of the pruning vlan command resets the list to the default list, which enables all VLANs
for VTP pruning.
The following configuration guidelines and restrictions apply when using 802.1Q trunks and impose
some limitations on the trunking strategy for a network. Keep these restrictions and suggestions in mind
when using 802.1Q trunks:
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78-16201-01
Modification
Support for this command was introduced on the Catalyst 4500 series switch.
Support for extended addressing was added.
Support for native VLAN tagging was added.
all specifies all VLANs from 1 to 4094. This keyword is not supported on commands that do not
permit all VLANs in the list to be set at the same time.
none indicates an empty list. This keyword is not supported on commands that require certain
VLANs to be set or at least one VLAN to be set.
add adds the defined list of VLANs to those currently set, instead of replacing the list.
remove removes the defined list of VLANs from those currently set, instead of replacing the list.
except lists the VLANs that should be calculated by inverting the defined list of VLANs.
vlan_atom is either a single VLAN number from 1 to 4094 or a continuous range of VLANs
described by two VLAN numbers, the lesser one first, separated by a hyphen.
When connecting Cisco switches through an 802.1Q trunk, make sure the native VLAN for an
802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk
is different from the native VLAN on the other end, spanning tree loops might result.
Disabling spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on
every VLAN in the network can cause spanning tree loops. We recommend that you leave spanning
tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree
on every VLAN in the network. Make sure your network is free of physical loops before disabling
spanning tree.
When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning tree
BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent
untagged to the reserved IEEE 802.1d spanning tree multicast MAC address (01-80-C2-00-00-00).
The BPDUs on all other VLANs on the trunk are sent tagged to the reserved SSTP multicast MAC
address (01-00-0c-cc-cc-cd).
Catalyst4500 Series SwitchCiscoIOS Command Reference—Release 12.2(18)EW
switchport trunk
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