STP Path Cost

Unanswered Question
May 30th, 2007

_____________ ___________

| Core 1 Sw |=============| ILM 1 Sw |

|___________| 1Gb |__________|---

||1Gb ||1Gb ||

|| || ||

|| || ||1Gb

|| || ||

__||_____||_ _____||____

| Core 2 Sw | 1Gb | ILM 2 Sw |

|___________|=============|__________|---


** All connections are 2 x 1Gb links port channelled together.


Core 1 = Root Bridge

Core 1 Priority = 8192


Core 2 Priority = 16384


Objective: All data from ILM2 should be forwarded to ILM1 before being sent across to Core1 to be routed. The link from ILM2 to Core2 should be the failover link.


Using default priorities (32768) when we connected ILM1 and ILM2, the links from ILM1 to Core1 and ILM2 to Core2 were forwarding and the interface from ILM2 to ILM1 blocking. This is not ideal for our situation. The default path cost for the link from ILM2 to ILM was 3 and the default path cost of the link from Core2 to ILM2 was 2. To force traffic the way we wanted it to go, we set the priority of the link from Core2 to ILM2 to 4. This then caused the link from Core2 to ILM2 to go into Blocking status and the link from ILM2 to ILM1 into Forwarding status (which is what we wanted). In keeping with our standard, we set the priority of both ILM1 & ILM2 to 49152. This is the priority used on other server switches in our environment. This however caused a problem. The interface from ILM2 to ILM1 changed to blocking status and the link from Core2 to ILM2 went into Forwarding status. We set the Priority on the link from Core2 to ILM2 to 5 to get it to block but at 4 it would forward.


Question: I read through the document 'Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches' from the Cisco website and near the end it states that 'The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.' Could someone please explain why then, in my situation mentioned above, did STP forward on the higher path cost (4) and block on the lower one (3)?? It was not until we raised the higher path cost to 5 that it started blocking. Are any other values taken into account??

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edsantos1 Wed, 05/30/2007 - 20:43

| Core 1 Sw |=============| ILM 1 Sw |

|___________| 1Gb |__________|

||1Gb ||1Gb ||

|| || ||

|| || ||1Gb

|| || ||

__||_____||_ _____||____

| Core 2 Sw | 1Gb | ILM 2 Sw |

|___________|=============|__________|



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pavansuraj Wed, 05/30/2007 - 21:55

Hi Eddie,


You have mentioned that you have set the priority on link from Core2 to ILM2 to 4.

It would be more helpful to evaluate if you could be more specific.

was it the port priority or the port cost? Also please mention on which device you have changed this value.

BR

Pavan

edsantos1 Wed, 05/30/2007 - 21:58

Hi Pavan,


Sorry, my mistake. It was the path cost that was set to 4 and it was set on the ILM2 interface that goes out to Core2. The ILM2 interface connecting to ILM1 had a path cost of 3 by default and we left this as is.

pavansuraj Wed, 05/30/2007 - 22:25

Hi Eddie,


Could you provide some more data.


Path cost of Core2 to Core1.

Path cost of ILM2 to Core 1.

Show spaanning-tree ouput from ILM1 & ILM2 after changing priority to 49152


Thanks

Pavan



edsantos1 Thu, 05/31/2007 - 20:58

Hi Pavan,


The path cost of Core2 to Core1 was 2. This is a 4 gigabit connections configured as 2 port channels.


The path cost of ILM2 to Core1 was 6.



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pavansuraj Thu, 05/31/2007 - 22:18

Hi Eddie,


The switch follows the below order for electing root port.


1. Root Bridge ID

2. Root Path Cost

3. Bridge ID

4. Port ID


When the priority of ILMs was 32768 and the path cost of ILM2 port to Core2 was 4, the ILM2 Selceted the port to ILM1 in forwarding mode going by the the 3rd option as a tie breaker as the root path cost on both ports has become 6.


After changing the bridge priorities of ILM1 and ILM2 to 49152, the port to Core2 is getting into forwarding mode.


in order to go by 2nd option you had increase the port cost from 4 to 5.


But I have a doubt on case why ILM2 opted ILM1 when ILM1 priority was 32768 and path cost to Core2 was 4. ILM2 has to opt Core2 as the bridge prioty of core2 is 16384(going by 3rd option as there is tie in root path cost).


Please do not hesitate for further discussion.

Thanks

Pavan

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