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New Member

spanning tree

when spanning tree works, does it add up the cost of all the switches to get to the destination switch ? and would it use the lowest cost ?

5 REPLIES

Re: spanning tree

hi carl,

you need to implement stp when you are having multiple or you can say redundent connectivity with other switch.

spanning tree is used to prvent the loop in the swithcing network. It will elect the root bridge from the whole switch network. Root bridge is elected using lower priority and lowest MAC address..which is known as the Bridge ID. It means switch with the lowest bridgeID will become a root bridge. and then it will elect the root port as well as designeted port and put some port in blocking state so that loop will not occur.

please rate this post if it helps

regards

Devang

Re: spanning tree

hi carl,

http://www.cisco.com/en/US/tech/tk389/tsd_technology_support_category_home.html

this link will help you to clear the STP.

remember to rate the post if it helps

regards

Devang

New Member

Re: spanning tree

hey devang

your link given is almost provide me all the information thanks a lot for help.

dporter

New Member

Re: spanning tree

I assume you are talking about cost of the path to the root bridge. the simple answer is Yes.

if you run "show spanning" or "show spantree" .. the root cost is the sum of the port cost of all the paths leading to the root bridge.

Say this the path from Switch3 to the ROOT:

ROOT ---cost=4--- Switch1 --cost=4-- Switch2 --cost=4-- Switch3

At Switch3, the root cost will be 12.

HTH.

Eric

New Member

Re: spanning tree

Carl,

Paths through a network all relate back to the Message Priority Vetor [Bytes 6 - 27 of the actual BPDU, so starting with the 23rd bytes of the ethernet frame].

The Root Path cost [Bytes 14 to 17 of the actual BPDU] shows the "rolled-up" cost back to the Root Bridge and a switch will select the lowest cost back to the root Bridge when selecting it's root port.

Regards

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