Sunday, November 11, 2012

Spanning Tree protocol

Span Tree - nothing to do with coconuts...

 1. What does the Spanning Tree Protocol achieve?

A) Determines a loop free topology
B) Sets up a meshed switched topology
C) Creates bridge loops and broadcast storms
D) Disseminates VLAN topology

2. What does the following statement do?Switch(config-if)#spanning-tree portfast disable

A) Disables spanning-tree on the switch port
B) Disable spanning tree on the switch
C) Changes the spanning-tree switch to portfast mode


1.  Answer: A

2. Answer: A

Portfast is a method of disabling STP on a switch port.


STP (Spanning Tree protocol) is used at layer 2 to prevent routing loops for frames on switches with multiple connections. Without STP, imagine 2 switches connected together with 2 separate Ethernet cables, a frame passes from one switch to the other over cable number 1. Switch 2 by default sends the frame out all ports except the one it came from so then this same frame is routed back via the second cable to the first switch, first switch then looks at the address and routes the frame back out the first cable to switch 2 and around it goes again. 

This is a routing loop, also known as "race tracking", and the frame will eventually cause the interface buffers on the switches to overflow, causing traffic degradation, increased CPU and ultimately switch crash. 

STP prevents this from happening by blocking ports between switches, allowing traffic to pass over one link only.  It has to be remembered that STP works vlan by vlan, so you can balance traffic across multiple links by the use of weighting, to allow some vlans on one trunk and others on the other.  Very useful protocol.