Interconnectivity between Switches

[Farooq Razzaque]

Attached is the diagram of the existing setup (left side diagram) and new setup (Rite side diagram)

Existing setup has connectivity of Server farm switch-1 with Core-1 & Core-2 (Core switch are in VSS) in port-channel smilarly server farm switch-2 has connectivity with Core-1 & Core-2 (VSS). In this setup switch-1 & switch-2 don't have inter-connectivity.

In the new setup we also want inter-connectivity between server farm switch-1 & switch-2 along with the connectivity with Core-1 & Core-2 (like in existing setup).

I want to know if i make a inter-connectivity (port-channel) between server farm switch-1 & switch-2 then will there be any downtime or flunction for the servers connected with server farm switch-1 & switch-2 or will there be any disturbance in the connectivity of server farm switch-1 & switch-2 with core switch

[Rajeev Sharma]

Hey,

STP will kick in with link between two server switches and you may see some connectivity issues.

HTH.

Regards,

RS.

[Paraidomat]

Hi!

Shouldn't it be possible to avoid a possible downtime bei configuring appropriate Bride-IDs / priorities, so that the loop-free network that will be formed is predetermined?

It should be something like "make sure that the core-layer-switches get a very low Bridge-ID". With that it could be possible to force STP to "cut" the loop between the Server-Fabric switches.

Even though I'm not sure if this will make it free of downtime or not.

@OP: If there is any chance i would highly recommend to do a test on your own with 4 seperate switches that are not in your productional environment.

Best Regards,

Florian

[Farooq Razzaque]

Dear Rajeev

My server farm switches are cisco 4948 switches.

Server farm switches are connected with Core-1 & Core-2 (VSS) with Tengig interface.

If i make a interconnection between server farm switch-1 & switch-2 using giga interface then i believe STP convergence will happend.

You are referring some connectivity issue ... Will this connectivity issue means the connectivity issue of server farm switch-1 & switch-2 with core switch or the flunction for the servers connected with server farm switch-1 & switch-2 

After the interconnection between server farm switch-1 & switch-2, will the link between these switches be in the blocking mode and will the link from server farm switch-1 & 2 to core swtich be in forwarding mode.

[Rajeev Sharma]

Hey Farooq,

The connectivity issue may occur during STP reconvergence moreover the data path may change depending upon new Layer 2 topology.

Please check the STP root priority for all the vlans before connecting the link. Lets say the VSS core is root bridge so by connecting the two server farm boxes will not change the layer 2 path between two switches however if one of the server farm switch is root then traffic will not be forwarded through VSS pair.

HTH.

Regards,

RS.

[Farooq Razzaque]

Dear Rajeev

Thanks for the prompt response.

Please check the STP root priority for all the vlans (do u mean root bridge priority)

My VSS core is the root bridge.

Can you please answer the followings

If i make a interconnection between server farm switch-1 & switch-2 using giga interface then i believe STP convergence will happend.

You are referring some connectivity issue ... Will this connectivity issue means the connectivity issue of server farm switch-1 & switch-2 with core switch or the flunction for the servers connected with server farm switch-1 & switch-2 

After the interconnection between server farm switch-1 & switch-2, will the link between these switches be in the blocking mode and will the link from server farm switch-1 & 2 to core swtich be in forwarding mode.

[Rajeev Sharma]

Hey Farooq,

Regarding your queries:

If i make a interconnection between server farm switch-1 & switch-2 using giga interface then i believe STP convergence will happend. - Yes

You are referring some connectivity issue ... Will this connectivity issue means the connectivity issue of server farm switch-1 & switch-2 with core switch or the flunction for the servers connected with server farm switch-1 & switch-2. - It will be between the server farm switches only on the direct link.

After the interconnection between server farm switch-1 & switch-2, will the link between these switches be in the blocking mode and will the link from server farm switch-1 & 2 to core swtich be in forwarding mode. - Yes, as you mentioned your STP root is VSS core to this link between server farm switches will be in blocking mode and links to the core will be forwarding.

HTH.

Regards,

RS.

[Farooq Razzaque]

Dear Rajeev

Much appreciated your reponse.

Please find below my reponse, appreciate if you can reply

If i make a interconnection between server farm switch-1 & switch-2 using giga interface then i believe STP convergence will happend. - Yes

My Reponse. Will this STP convergence cause any fluctuation /downtime for servers connected to server farm switches or will it cause connectivity issue between server switches (1 & 2) with Core switch.

Will STP convergence also take place between the link of server farm swtiches with Core switch, if yes then will it cause any fluctuation.

After the interconnection between server farm switch-1 & switch-2, will the link between these switches be in the blocking mode and will the link from server farm switch-1 & 2 to core swtich be in forwarding mode. - Yes, as you mentioned your STP root is VSS core to this link between server farm switches will be in blocking mode and links to the core will be forwarding.

My Reponse : I think there is some type error in the reponse ( VSS core to this link between server farm) ?

I have re-checked and we have set 'spanning-tree vkab 1-1005 priority 0' on the core switches.

My Reponse: As the server farm switches are connected with Core switches with Tengig interface thats why it will be in forwarding state b/c of lesser root path cost ?

Interconnection between server farm switches will be with Gig interface thats why it will be in blocking state b/c of higher root path cost ?.

I also want to know for the Tengig interface the path cost is showing as 1000 and for Gig interface the path cost is showing as 5000. Can you please let me know how these costs are calculated.

[Farooq Razzaque]

Dear Rajeev

I would to know since we already have server farm switch-1 connectivity with Core-1 & Core-2 (VSS) with port channels and smilarly the same connectivity for switch-2. So do the interconnectivity between switch-1 & switch-2 is required , i thnk this link will only be in the forwarding state if the port-channel between switch-1 and core switch is down or link between switch-2 and core switch is down

[Farooq Razzaque]

Dear Rajeev

Appreciate if you can provide your feedback on my last reply

[Rajeev Sharma]

Hi Farooq,

Sorry was little occupied with some work.

Regarding your questiones:

Will this STP convergence cause any fluctuation /downtime for servers connected to server farm switches or will it cause connectivity issue between server switches (1 & 2) with Core switch. - No there will not be any issues.

Will STP convergence also take place between the link of server farm swtiches with Core switch, if yes then will it cause any fluctuation. - None.

I think there is some type error in the reponse ( VSS core to this link between server farm) ? - Indeed, its actually STP root is VSS core so the link between server farm switches will be in blocking mode and links to the core will be forwarding.

I have re-checked and we have set 'spanning-tree vlab 1-1005 priority 0' on the core switches. - its a good practice making sure one device remains the root of topology.

As the server farm switches are connected with Core switches with Tengig interface thats why it will be in forwarding state b/c of lesser root path cost ? - Yes.

Interconnection between server farm switches will be with Gig interface thats why it will be in blocking state b/c of higher root path cost ? - Yes.

I also want to know for the Tengig interface the path cost is showing as 1000 and for Gig interface the path cost is showing as 5000. Can you please let me know how these costs are calculated. - To understand the spanning tree cost calculation I am adding a link below, have a look and let me know if you have any queries:http://en.wikipedia.org/wiki/Spanning_Tree_Protocol#Data_rate_and_STP_path_cost

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html

HTH.

Regards,

RS.