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New Member

STP port cost

HI all

I have the following topology

6513a --- 3550a---stack of three 2950 switches ---3550b--6513b

The two 6513's are connected through a portchannel of 4 gig links.6513a is my root and 6513b is my secondary root. All the links are gig ports. I want to pass all the traffic from the stack[ 2950 are conneted by gig links like g2 of switch 1 goes to g1 of second and g2 of second conneceted to g1 of third 2950] of 2950 through 3550a. But now the top two switches traffic is passing through 3550a and the last switch traffic is passing through my 3550b and 6513b and going to the root. The port cost is 3.

NOw i am thing to change this portchannel cost to higher values so that all the traffic passes through 3550a. Couls you please suggest the value of the portchannel to waht i can change. All the remaining port costs are set to default.

If there any side effect by doing so. My network is production. I have similar type od 3550 and 2950 combination in 20 locations. Thanks

4 REPLIES

Re: STP port cost

Hie Bhanu,

The STP is working as expected in your design. For a gig port port cost is 4. So for 1st 2950 cost to root is 8 thr 3550a. For 2nd 2950 cost to root is 12 thr 3550a and 16 thr 3550b so first one in preferred and later one is blocked. But for 3rd 2950 cost to root is 16 thr 3550a but thr 3550 b it is 12. Hence 3rd 2950 forwards traffic thr 3550b-6513b-6513a. This operation seems to be ok unless you have some specific requirements to change.

Changing port channel cost is not a good idea. both the 6500s are using this channel for communication between them. If you block this link, the 6500s will have to communicate thr all the access switches which is not a good design.

New Member

Re: STP port cost

Hi Anand

Thanks for your reply. If i change the cost of portchannel going from 6513b to 6513a to 8 will ther be any probelm. Then the 6513b will communicate with root directly.

One more thing id i have switch 1 and switch2 connected can the link cost from s1 to s2 be different from s2 to s1. By doing so will there be any problem.

Re: STP port cost

If you configure cost of 8, it should work. But it is not a very good idea to change default values of STP unless we have a very strong reason :)

Re: STP port cost

For your second question, the cost will work only in one way. You can keep the configure cost on both the ports to keep it identical.

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