CCIE Pursuit Blog

March 7, 2008

spanning-tree link-type

I ran into the ‘spanning-tree link-type’ command while reviewing switching this week:

spanning-tree link-type

Use the spanning-tree link-type interface configuration command to override the default link-type setting, which is determined by the duplex mode of the interface, and to enable rapid spanning-tree transitions to the forwarding state. Use the no form of this command to return to the default setting.

spanning-tree link-type {point-to-point | shared}
no spanning-tree link-type

Syntax Description
 point-to-point
 Specify that the link type of an interface is point-to-point. 
 shared
 Specify that the link type of an interface is shared.

Defaults
The switch derives the link type of an interface from the duplex mode. A full-duplex interface is considered a point-to-point link, and a half-duplex interface is considered a shared link.

Usage Guidelines
You can override the default setting of the link type by using the spanning-tree link-type command. For example, a half-duplex link can be physically connected point-to-point to a single interface on a remote switch running the Multiple Spanning Tree Protocol (MSTP) or the rapid per-VLAN spanning-tree plus (rapid-PVST+) protocol and be enabled for rapid transitions.

Examples
This example shows how to specify the link type as shared (regardless of the duplex setting) and to prevent rapid transitions to the forwarding state:

Switch(config-if)# spanning-tree link-type shared

This is an interesting command that I had never encountered before.  Chances are pretty slim that I’ll ever use it in the wild, but I sure made a note of it for my CCIE studies.  When I encounter odd commands like this, I like to create a list of tasks that could be written that would require the command:

“Ensure that half-duplex spanning-tree ports are treated as point-to-point ports.”

I don’t need to memorize the command or its parameters, but I do like to know that it exists and to be familiar enough with its function that if I saw the above task, it would turn on a light in my head and I would know where to look in the DOCCD to find the command.

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