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Catalyst 3750 Metro Switch Command Reference
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Chapter 2 Catalyst 3750 Metro Switch Cisco IOS Commands
switchport 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.
•
On an ES port, you can use the
switchport trunk dot1q ethertype
value
interface configuration
command to program a custom 802.1Q ethertype value on the interface. This feature allows the
switch to interoperate with third-party vendor switches that do not use the standard 0x8100
ethertype to identify 802.1Q-tagged frames. For example, if you set 0x1234 as the custom 802.1Q
ethertype on a trunk port, incoming frames containing that ethertype are assigned to the VLAN
contained in the tag following the ethertype, as they would be with a standard 802.1Q trunk. Frames
arriving on the trunk port containing any other ethertype are assigned to the native VLAN of that
trunk.
•
The
no
switchport trunk dot1q ethertype
command resets the ethertype value to the default.
Native VLANs:
•
All untagged traffic received on an 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 switch sends
the packet 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.
Allowed VLAN:
•
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.
Trunk pruning:
•
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.
•
VLAN 1, VLANs 1002 to 1005, and extended-range VLANs (VLANs 1006 to 4094) cannot be
pruned.
Examples
This example shows how to cause a standard port configured as a switched interface to encapsulate in
802.1Q trunking format regardless of its default trunking format in trunking mode:
Switch(config-if)#
switchport trunk encapsulation dot1q
This example shows how to configure VLAN 3 as the default port to send all untagged traffic:
Switch(config-if)#
switchport trunk native vlan 3
This example shows how to add VLANs 1, 2, 5, and 6 to the allowed list:
Switch(config-if)#
switchport trunk allowed vlan add 1,2,5,6
