Allowed Vlans On A Trunk - Cisco Catalyst 2960 series Configuration Manual

Configuring VLAN Trunks
Mode
switchport mode dynamic auto
switchport mode dynamic desirable
switchport mode trunk
switchport nonegotiate
Related Topics
Configuring a Trunk Port, on page 65
Trunking Modes, on page 60

Allowed VLANs on a Trunk

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, are
allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those
VLANs from passing over the trunk.
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), DTP, and VTP in VLAN 1.
If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the
access VLAN is set to 1, the port will be added to VLAN 1, regardless of the switchport trunk allowed
setting. The same is true for any VLAN that has been disabled on the port.
A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if
the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in
the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When
VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not
become a member of the new VLAN.
OL-29065
Function
Makes the interface able to convert the link to a trunk
link. The interface becomes a trunk interface if the
neighboring interface is set to trunk or desirable
mode. The default switchport mode for all Ethernet
interfaces is dynamic auto.
Makes the interface actively attempt to convert the
link to a trunk link. The interface becomes a trunk
interface if the neighboring interface is set to trunk,
desirable, or auto mode.
Puts the interface into permanent trunking mode and
negotiates to convert the neighboring link into a trunk
link. The interface becomes a trunk interface even if
the neighboring interface is not a trunk interface.
Prevents the interface from generating DTP frames.
You can use this command only when the interface
switchport mode is access or trunk. You must
manually configure the neighboring interface as a
trunk interface to establish a trunk link.
Catalyst 2960-X Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX
Allowed VLANs on a Trunk
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