Root Bridge Election process

[stillnumber1]

I am having a hard time figuring out how this whole process works. I have the sybex CCNA 2.0 and it doesnt give a through explantion on this process. It basically just states that the Designated bridge is the root bridge and that bridge is the forwarding bridge. All other bridges are non-designated bridges are in a blocking state. I didnt get very detailed information about how this process works.

 

[Jsteve]

OK let say you have this switch network:

Switch A
__________
- -
- -

Switch B| | Switch C
- -
- -
__________
Switch D

If each router boots up, an election takes place to see who will be the King (root Bridge)switch, the lowest bridge ID.

If Switch A becomes the root bridge, the links from switch B and Switch C that connects to thr root bridge (SwitchA), are the root ports. One of the final two links coming from switch D must be elected a desginated port and the other must be blocked. The total path cost is used to determined which one is blocked and which is designated. So if you have the cost path from D->B->A = 20 and the path cost from D->C->A = 15, than the link from D->B becomes Blocked.


Hope this helps some

 

[CCIEcandidate]

http://www.cisco.com/warp/public/473/spanning_tree1.swf

Nick