Cisco Catalyst 3030 Command Reference Manual page 542

switchport trunk
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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
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Usage Guidelines
Encapsulation:
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Native VLANs:
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Allowed VLAN:
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Cisco Catalyst Blade Switch 3030 Command Reference
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Separate nonconsecutive VLAN IDs with a comma; use a hyphen to designate a range of IDs.
except lists the VLANs that should be calculated by inverting the defined list of VLANs. (VLANs
are added except the ones specified.) Valid IDs are from 1 to 1005. Separate nonconsecutive VLAN
IDs with a comma; use a hyphen to designate a range of IDs.
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.
Modification
This command was introduced.
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.
Chapter 2 Cisco Catalyst Blade Switch 3030 Cisco IOS Commands
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