Cisco catalyst 3750 Command Reference Manual page 891

Chapter 2 Catalyst 3750 Switch Cisco IOS Commands
Defaults The default encapsulation is negotiate.
VLAN 1 is the default native VLAN ID on the port.
The default for all VLAN lists is to include all VLANs.
Command Modes
Interface configuration
Command History Release
12.1(11)AX
12.1(14)EA1
Usage Guidelines Encapsulation:
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Native VLANs:
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Allowed VLAN:
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Trunk pruning:
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OL-8552-07
Modification
This command was introduced.
The allowed vlan vlan-list add, remove, and except keywords were modified
to accept the VLAN1 and VLANs 1002 to 1005 values.
The switchport trunk encapsulation command is supported only for platforms and interface
hardware that can support both ISL and IEEE 802.1Q formats.
You cannot configure one end of the trunk as an IEEE 802.1Q trunk and the other end as an ISL or
nontrunk port. However, you can configure one port as an ISL trunk and a different port on the same
switch as an IEEE 802.1Q trunk.
If you enter the negotiate keywords and DTP negotiation does not resolve the encapsulation format,
ISL is the selected format. The no form of the command resets the trunk encapsulation format to the
default.
The no form of the encapsulation command resets the encapsulation format to the default.
All untagged traffic received on an IEEE 802.1Q trunk port is forwarded with the native VLAN
configured for the port.
If a packet has a VLAN ID that is the same as the sending-port native VLAN ID, the packet is sent
without a tag; otherwise, the switch sends the packet with a tag.
The no form of the native vlan command resets the native mode VLAN to the appropriate default
VLAN for the device.
To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual
VLAN trunk port by removing VLAN 1 from the allowed list. When you remove VLAN 1 from a
trunk port, the interface continues to send and receive management traffic, for example, Cisco
Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol
(LACP), Dynamic Trunking Protocol (DTP), and VLAN Trunking Protocol (VTP) in VLAN 1.
The no form of the allowed vlan command resets the list to the default list, which allows all VLANs.
The pruning-eligible list applies only to trunk ports.
Each trunk port has its own eligibility list.
If you do not want a VLAN to be pruned, remove it from the pruning-eligible list. VLANs that are
pruning-ineligible receive flooded traffic.
Catalyst 3750 Switch Command Reference
switchport trunk
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