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Webcast-Catalyst9k
New Member

STP problem

Hi!

We have problem with the ring of 802.1q trunk links. All the links are 1Gbps, and only one of them is 100Mbps. 100Mbps link connects root switch (3750) port with non-root (CE-500) one. Surprisingly, SPT blocks 1Gbps ports on that non-root switch. Link cost of 100Mbps is properly set to 19, while one for 1Gbps is set to 4. What's the reason?

6 REPLIES

Re: STP problem

Hie,

This will also depend on total cost to root from that perticular switch. Can you provide your network topology to put more light on this

New Member

Re: STP problem

The topology looks like the attachement

all links are 802.1q trunks

Re: STP problem

Well so SW1-4 are using their paths to root through SW1 but only SW5 is taking the direct path.

If the above understanding is correct, then STP is behaving normally

Selection of root path depends on the port cost. Gig port cost is 4 and FE cost is 19. For SW5, the cost through sw4-3-2-1-root becomes 20 but the cost of the path directly to root is 19. So the switch is choosing the direct path even though it is having less bandwidth.

New Member

Re: STP problem

thank you!

but can I artificially alter STP choice ?

Can I alter path cost and priority on root-switch port ?

Re: STP problem

Yes you can alter port cost on a perticular switch for a perticular VLAN or for all the traffic. Of course, if you HAVE to do this, please do it very carefully and after considering other techincal details of your network. Please check thefollowing link for this

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008007e71a.html#wp1020971

New Member

Re: STP problem

thanks a lot!

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