Search Blog Content

Saturday, February 9, 2013

Root Bridge for CCNA and CCNP, Part 1



As a CCNA / CCNP candidate you are expected to understand the purpose of the Root Bridge and how it is elected.
The first role which the spanning-tree protocol needs to elect is which switch device on your LAN will act as the Root Bridge, or as I like to refer to it "The reference point."

Each spanning-tree protocol enabled switch will commence the election of the reference point by issuing out of all live ports a frame called a BPDU (Bridge Protocol Data Unit), within this frame are details regarding the transmitting switches spanning-tree credentials that are used to determine if it will be the reference point.
When a switch receives a Bridge Protocol Data Unit it will compare the contents with it's own values to determine if the received values are better than it's own. The first value which the switch will look at will be a value called the "Default Priority", by default this value is set to 32768, which ever switch advertises the lowest Default Priority will be elected the Reference Point (Root Bridge), if on the other hand the Default Priorities are the same the lowest switch MAC address is used to break the tie.
Note: The Default Priority and MAC values are carried within the BPDU.
Once the Reference Point (Root Bridge) has been determined the switches will now need to choose which of their ports will need to block and which can remain open.

The best way to understand how they choose which ports should stay open and which block is to picture two switches connected together via one link, port 1 on switch 1 to port 1 on switch 2. For our example we will imagine switch 1 being the Root Bridge.

As Switch 1 is the Root Bridge and as such will issue BPDU's claiming to be so out of the only port is has connected, switch 2 receives the BPDU on a single port and no other port, since switch 2 can only see the root bridge on a single port it will keep that on open and not block any other port since it cannot see the BPDU on any other port.

If you now connect the two switches together via their respective Port 2 switch 2 will start receiving BPDU's down both port 1 and port 2, in this scenario switch 2 has to block one of the ports but which one?
Joe Spoto is a senior lecturer at Commsupport networks in the United Kingdom. Joe teaches, CCNA courses CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNA courses Commsupport run free one day training sessions and free on-line webinars, CCNA training, we also run Live On-line CCNA courses as Virtual Classes
Article Source:

Article Source:

1 comment:

  1. Hi, Great.. Tutorial is just awesome..It is really helpful for a newbie like me.. I am a regular follower of your blog. Really very informative post you shared here. Kindly keep blogging. If anyone wants to become a Java developer learn from Java Training in Chennai. or learn thru Java Online Training from India . Nowadays Java has tons of job opportunities on various vertical industry.