set spantree portcost vs channelcost

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Apr 18th, 2008
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Hello,

Does anyone know the difference in using set spantree portcost vs set spantree channelcost??

I am converting my Data Center LAN from spanning-tree PVST+ to Rapid STP. When removing uplinkfast during this change, all of my Access Layer Switch uplinks (channeled) will default to the original bandwidth determined portcosts. I'd like to keep my blocked ports down at the Access Layer Switches and was trying to see the differences between these 2 commands. What is the difference outside of the obvious, one is used on ports and the other is used on channels??


When I enter set spantree channel cost, the console displays this:

gv2d-D1> (enable) set spantree channelcost 1666 25

Port(s) 5/1-8 port path cost are updated to 40.

Channel 1666 cost is set to 25.

Warning: channel cost may not be applicable if channel is broken.


If I loose my channel could this be a problem??


Thanks for your help,

Tony

Correct Answer by Francois Tallet about 9 years 6 days ago

I'm pretty confident that's the case;-)

I could found this in the CatOS configuration guide:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/8.x/configuration/guide/spantree.html#wp1195538

regards,

Francois


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Jon Marshall Fri, 04/18/2008 - 12:29
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Tony


Yes that is exactly what it means. If you set the STP cost on the channel this will also adjust the port costs that make up the channel.


If the channel is then broken for some reason the ports will fall back to their original costs and this may or may not be the cost you need for you to get your STP topology the way you want.


Jon

amaiale Fri, 04/18/2008 - 12:34
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Jon,


Thank you so much for replying.

Do you know which method is prefered? Or which method will provide more stability??


A colleague of mine has told me that Cisco TAC has recommended using channelcost and I guess I am having a hard time with it because of what can happen if a channel breaks down.


Thanks again,

Tony

Jon Marshall Fri, 04/18/2008 - 12:41
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Tony


To be honest i have never used the channel cost so it would be wrong of me to say which is more reliable. Perhaps someone else can comment.


What you could do is ensure that the port costs are low enough that if the channel fails they will still block. Just a suggestion, would need testing.


Sorry i can't be more help.


Jon

amaiale Fri, 04/18/2008 - 12:44
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Hey no problem. Thanks for your input!!


Tony

Francois Tallet Fri, 04/18/2008 - 13:50
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big disclaimer: It's been almost 10 years I've not worked on CatOS;-)


-1- If you set assign a cost to the member of a channel using portcost, all the members will assigned the same cost.

-2- If you assign a cost using the channelcost to a channel, then the channel as a whole (seen as a virtual interface) is assigned this cost.


So that's here I don't really remember how this works, but if you set the cost using method -1-, the whole channel will probably get a cost that is the cost of a member divided by the number of members (well, at least with the long path cost method, the result might be more difficult to predict with the short cost). While -2- would compute the individual cost of a member so that the channel gets the specified cost. Bottom line: play around with the CLI to find the cost that is appropriate to your design;-) Once a cost is set for a channel, I don't think there is any difference the way it was entered.


BTW, even in Rapid-PVST mode, the uplinkfast configuration as no impact on the way the protocol is behaving, it still tune the cost of the bridge and all its ports. We did that specially so that enabling rapid-pvst would not result in a change in the topology.


Regards,

Francois

amaiale Tue, 04/22/2008 - 09:26
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Francois,


Thanks for your reply.

I have been testing out different cost scenarios and I like your suggestion best, which is to enable the uplinkfast feature with Rapid STP.


Do you know if there is any Cisco literature identifying that uplinkfast will not affect Rapid STP??


Thanks again,

Tony


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