Specifications

CHAPTER 2

VLAN Implementation

Configure a port for trunking at the interface configuration mode:

(config-if)#switchport mode {dynamic {auto | desirable} | trunk}

If dynamic mode is used, DTP negotiates the trunking state and encapsulation. If trunk mode is used, you must specify

encapsulation, and you can disable all DTP negotiation:

(config-if)#switchport trunk encapsulation {isl | dot1q | negotiate}

(config-if)# switchport nonnegotiate

If you use 802.1Q, specify a native VLAN for the trunk link with the command:

(config-if)# switchport trunk native vlan vlan-no

Frames from the native VLAN are sent over the trunk link untagged. Native VLAN must match on both sides of the trunk

link. VLAN 1 is the default native VLAN for all ports, but best practice is to set the native VLAN to one not assigned to

users. This practice also decreases the danger of having a large spanning tree instance in VLAN1.

VLANs Allowed on the Trunk

By default, a trunk carries traffic for all VLANs. You can change that behavior for a particular trunk link by giving the

following command at the interface config mode:

switchport trunk allowed vlan vlans

Make sure that both sides of a trunk link enable the same VLANs.

Verifying a Trunk Link

Two commands you can use to verify your trunk configuration are

# show running-config

# show interfaces [interface no.] switchport | trunk

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

9781587140112.qxd 11/23/09 11:34 AM Page 19