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spanning tree cost

does the root have a cost of zero, and how can i check this ?

2 REPLIES

Re: spanning tree cost

Hi Carl,

When the root bridge sends a BPDU is has a cost of zero.When this BPDU reaches the downstream switches they add their root path cost and relays the BPDU's downstream. Its the root bridge who always relays the BPDU's downstream, BPDU's doesnot travel upstream. Its only the TCN ( topolgy change notification ) BPDU.

If you want to check the root path cost, you have to sniff the link between the root bridge and the downstream switch.

HTH, Please rate if it does.

-amit singh

Re: spanning tree cost

Hi Carl.

Yes the port cost on root bridge is 0.

Take it this way when the BPDU leaves the root switch it keeps adding the port cost as it keep passing the donstream switches and this way the downstream switches eg: 2-3 hops away from root bridge learns the path cost to reach the root bridge so the port cost on the root bridge should be 0 because that is the place from where BPDUs start.

Sweet example of real world is when you start from home on your car to reach your office you are starting with 0 km and when you reach the office you will say you travelled 10 kms.

To check that if you know which switch is a root switch issue a command

show spanning-tree active

show spanning-tree interface

and check what value you see in "cost" feild

HTH, if yes please rate the post.

Ankur

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