Spanning Tree Protocol

Question 1

What is a loop, and why is it bad?

Answer 1

If two switches are connected to each other with two cables, a loop is created. It is bad because it will send traffic back and forth forever; there is no “counting” mechanism at the MAC layer. This loop will overwhelm the switches and bring down a network.

Question 2

What is spanning tree protocol, and what does it do?

Answer 2

A protocol (IEEE standard 802.1D) that prevents loops in bridged (switched) networks. Created by Radia Perlman.

Question 3

What are the Spanning Tree Protocol states and what do they do?

Answer 3

Blocking - not forwarding traffic to prevent a loop
Listening - not forwarding traffic and cleaning the MAC table
Learning - not forwarding traffic and adding to the MAC table
Forwarding - data passes through and is fully operational
Disabled - administrator has turned off the port, nothing can pass through the port.

Question 4

What are the three types of ports that are configured automatically through STP?

Answer 4

• Root Port: Only one on the Spanning Tree Network. It is closest to the root switch.
• Designated Port: The other interfaces on this root port switch where traffic is allowed to traverse.
• Blocked Port: Any port on any of these switches that have been disabled by STP to prevent a loop.

Question 5

What is RSTP (802.1w)?

Answer 5

Rapid Spanning Tree Protocol (802.1w)

• an updated version of STP, this is the latest standard
• much faster convergence, from 30-50 sec to 6 sec
• backwards-compatible with 802.1D STP