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Community Member

spanning tree

Hi all, quick question about spanning tree, I know it uses cost, I I had say 8 hops to the root bridge from a switch, each one at 1 gig, then 1 hop straight to the root bridge at 100 meg, would It choose the gig path pr the 100 meg direct path, Am I right it saying it would use the lowest overall cost ?

7 REPLIES

Re: spanning tree

Hello Carl,

Do not think STP as a routing protocol, end to end.. STPs run only on the bridged network, wherein the switch sees only its neighbouring switch and the BPDU info got from it... anyway, i guess you are talking about the path cost to the root bridge.. Yeah, according to the revised IEEE specification, the switch assigns path cost, based on the speed of the link.

10/1 Gbps - 1

100 Mbps - 10

10 Mbps - 100

and the lowest path cost is normally chosen as the forwarding port.. hence in ur case, the 1 gig link will precede over the 100 Mbps link, unless u forcefully give bridge priorities on the switch. Normally the scenario given by you will not exist, because no one designs the network this way.

Hope this helps.. all the best.. rate replies if found useful.

Raj

Re: spanning tree

Is that true even though the 100mb link is directly attached to the root bridge?

Re: spanning tree

yes it is true even if the root is directly connected and you have another route to the root via the gig links.

The lowest cost route to to the root switch wins and goes into forwarding

HTH, rate if it does

Narayan

Silver

Re: spanning tree

Here are the revised default STP port costs:

10Mb link - 100

100Mb link - 19

1Gb link - 4

10Gb link - 2

Re: spanning tree

agree with ahmed.. carl, take a note of teh new values.. thanks for the info ahmed.

Raj

Re: spanning tree

Hi

In addition u r network diameter should not increase by 7 switches as per cisco.In u r case it says 8 i think it is not recomended.

Thanks

Mahmood

Community Member

Re: spanning tree

what is recommended here, Do you mean if they are daisychained ?

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