These are my notes for the CCNP SWITCH, 300-115, Objective 1.4.b dot1Q.
Dot1Q is IEEE 802.1Q, the standard for trunking encapsulation. On Cisco switches, you configure dot1q on trunk ports which allow tagged frames to be transported on a trunk link, allowing multiple VLANs to traverse through one link. This extends the VLANs across the network.
When configuring trunk ports, both ends of the link must have the same native VLAN. If the native VLAN does not match, an error message will be displayed on the console.
An interface supports different trunking modes:
- dynamic auto – Interface is able to convert to a trunk link if the other end of the link is set to trunk or desirable mode. This is the default switchport mode.
- dynamic desirable – Interface actively tries to convert to a trunk link. The link becomes a trunk if the other end is set to trunk, desirable, or auto mode.
- trunk – Permanently places the interface in trunk mode. Interface becomes a trunk even if the other end is not a trunk interface.
- nonegotiate – Prevents the interface from generating DTP frames.
When an interface is in trunk mode, it is able to send and receive traffic from all VLANs. It is possible to allow only specific VLANs to traverse over the trunk.
Priorities can be set on a parallel trunk port so that the port carries all traffic for a given VLAN. A trunk port with the higher priority (lower value) for a VLAN is forwarding traffic for that VLAN. The trunk port with the lower priority (higher value) for that same VLAN is in the blocking state for that VLAN.
Trunk ports can be configured into an EtherChannel port group with all trunk interfaces with the same configuration. Any changes to the port group will propagate the change to all ports part of the group. [Read more…] about CCNP SWITCH – Dot1Q