STP Convergence Mechanisms – Part 2

Loop Guard, Uplink Fast, Backbone Fast and UDLD

In this lesson, we will learn other STP Convergence Mechanisms; Loop Guard, Uplink Fast, Backbone Fast and UDLD. Now, let’s learn each of these mechanisms and their roles one by one.

 

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Loop Guard

 

Loop Guard is the STP mechanims that is used to protect the network against the layer 2 loops.

 

For example a switch can send BPDU but can not receive any BPDU from the other switch. And this is detected by UDLD. If the Loop Guard is configured, Loop Guard puts the port in “Loop Inconsistence State” instead of going through Forwarding state in this situation. But if there is no Loop Guard, then, the port continue to sends BPDU and goes through Forwarding State. In such a situation, a Layer 2 Loop occurs. Loop Guard is mainly used to avoid such cases.

To configure Loop Guard globally on a Cisco switch, you can use “spanning-tree loopguard default” command. Beside, to enable Loop Guard on a specific interface, you can use “spanning-tree guard loop” command.

Uplink Fast

 

Uplink Fast is the convergence accelerator mechanims that can be configured on only Access switches and on the uplink ports. With Uplink Fast mechanims, if a port fails, then the second way on the uplink become active imediately. This immediate access, reduces the convergence time.