STP Convergence Mechanisms – Part 1

Portfast, Root Guard, BPDU Filter and BPDU Guard

  The are various mechanims used with STP to provide stable STP topology. In this lesson, we will focus on what are these STP Mechanims and what they do to provide a stable STP Topology.   So, what are these STP Convergence Mechanims? These STP Convergence Mechanims are given below:  

• PortFast
• BPDU Guard
• BPDU Filter
• Root Guard
• Loop Guard
• Uplink Fast
• Bachbone Fast
• UDLD

  Now, let’s talk about these STP Convergence Mechanims one by one detailly.

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PortFast

As we have discussed before, in STP (Spanning Tree Protocol) there are five port states. These Port States are :  

• Blocking State
• Listening State
• Learning State
• Forwarding State

In an STP Topology, this Layer 2 convergence time, accessing from blocking to forwarding state,is 50 seconds. These 50 seconds are spend in the states like below:  

• Blocking State : 20 seconds
• Listenning State : 15 seconds
• Learning State : 15 seconds

  Total 50 seconds From Blocking State to Forwarding State. To reduce this time, STP Portfast is used on the access ports connected to host devices like pc, laptops, printers, servers etc. With STP Portfast, Blocking State, Listenning State and Learning State are bypassed. So, the ports enters the Forwarding State immediately.   STP Portfast is used on only access ports. It is not recommended to use SP Port Fast On Trunk Ports. Also on the port that is connected to another switch is not recommended. Because, such a connection can cause an unaccepted Layer 2 Loop.   To configure STP Portfast Globally, you can use “spanning-tree portfast default” command globally. This will configure all switch ports with PortFast and put all the switch ports into Forwarding rapidly. To configure PortFast on a specific interface, you can use “spanning-tree portfast” under this specific interface.  

BPDU Guard

BPDU Guard is the mechanims that protect a port towards any BPDU. In other words, this mechanims avoid receiving BPDU packets. By doing this, BPDU Guard provide the stability of STP Topology. [sc name=”ContentRMessage”]