path cost

Unanswered Question
izackvail Wed, 07/23/2008 - 09:43

I assume you are talking about the command "spanning-tree pathcost"? This is a command would only affect spanning-tree and its decision as to which port is closest to the root.

hi! Yes, you are right i'm referring to the spanning-tree pathcost....In that case, for normal traffic load.....can i have a preference for certain link over the other? The environment are multiple stacks of switches, with each stack having 2 links to the 2 core switches (with 4 sup engines).

what will actually determine which link the edge switches will use to reach the core? Is it something like if the resources(eg. server) is in core 1, the traffic will automatically have higher preference to use the link that's connected directly to core 1?


If the certain vlan interface is active in the sup engine at core 2 it will then use the link connected to core 2 instead of the one connected to core1?

Please advise.

Jon Marshall Wed, 07/23/2008 - 10:01

Yes, you can manipulate the path cost and this will affect the path the traffic takes.

The other way to do this is if you want a stack to use the link to core switch 1 for its traffic then make core switch 1 spanning-tree root for the relevant vlans.


hi! Just to confirm, so setting the spanning-tree path cost will determine the path for the traffic flow (server resources)?

So, you are saying the root bridge will have higher priority over the location of the active router (vlan interface with hsrp)? Are the vlan/router interface relate to the root bridge in this case? i meant in term of the traffic flow?

how's the traffic flow like if the root brige is core2 but the server resources is in core1? Must the traffic go to the root bridge first then to the server resources even if the edge sw has direct connection to core1? a bit confused here.


Jon Marshall Wed, 07/23/2008 - 10:25

It's a good idea to match up th STP root with the HSRP active gateway.

As for the flow it really depends on whether traffic is contained within the same vlan or not ie.

If the source and destination IP's are in the same vlan then only STP path is relevant. If the source and destination are in separate vlans then the HSRP gateway and STP path comes into play.


hi! Thanks for the explanation,

Can you explain why teh STP path will be relevant if the source and dest IPs are in the same vlan? I thought all the Mac tables are all stored locally at the edge switch?

If both source and dest are in different vlan, how does the HSRP gateway and stp path come into play? can you briefly explain how's the flow like in this and how this 2 interelated?

For our case, can't match the stp root with the active gateway, coz different vlans are active in different sup engine :)


Jon Marshall Wed, 07/23/2008 - 10:41

If the source and destination are on the same switch then STP doesn't come into it. But assuming the source and destination are on different switches then the path the traffic takes will have been determine by STP.

If source and destination are on different vlans then source will send packet to it's default gateway which is HSRP active gateway for source vlan. To get to this gateway the traffic will take the path determined by STP. So STP is always involved, unless you have everything connected on one switch. HSRP is involved when traffic needs routing.



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