Will STP work as expected with switches on orphan ports of Nexus

[ciantwohig]

Hi, 

I am adding VPC to a nexus deployment this weekend,unfortunately some new 2k switches haven't arrived so I will not be able to create a FEX , portchannel etc for the equipment on the other floor of the data centre.  

So I'm stuck with two standalone switches that are trunked together,  so for redundancy and to connect them to both Nexus's I would have to connect them as shown below

My question since these connections will be orpan ports ,is that if 5k-1 is the root ,  with a priority of 4k, 5k-2 is the secondary with priority 8k.  

Would I be correct in assuming all traffic from both standalone switches will use nexus 5k-1 with the uplink to 5k-2 in a blocked state.  I obviously want to prevent a loop 

thanks

[Mark Malone]

Hi Cian

instead i would create VPC on Nexus switches and standard PO on 3750 side and link each 3750 to each 5k that will work , 3750 will treat it as a standard PO back to the Nexus switches, concept is the same as a fex in design

[ciantwohig]

Hi Mark, 

Thanks for your response,

Just so I know I'm understanding your suggestion correctly would it be possible for you to elaborate a bit more regarding please.  Am I correct in thinking you mean

1) Create two po's (one for each switch and keep the trunk between them

2) create a VPC + PO on the nexus's for both the switchs , each switch has one connection with is part of the VPC PO and remain trunked. 

If I make each switch part of the VPC with its own PO, it will only have one connection to the each nexus which will inturn make it become regarded as an orphan port and will act as if it is not part of the VPC from a MAC point of view. 

If I do option2 I am not entirely sure how the STP will work and will it cause a loop

I know an obvious solution is to make a VPC + PO with two cables going to each switch , but due to cabling restrictions in the data centre I can only run one cable to each switch . 

thanks for your advise

Cian 

[Mark Malone]

Hi so like the 65s below same physical setup  , the 37s would be the same , VPC setup on Nexus side , each Nexus with same VPC and then on 37s side 1 link to each Nexus but using a standard PO

This way they don't become redundant the Nexus know its a VPC , the switches don't need to understand that they juts need standard PO and it will work as a normal PO would , both ports will be forwarding from switch perspective

Can you run cables for that ? if not then you will end up with blocking ports if you stick to the original design

[ciantwohig]

Hi Mark, 

Thanks for clarifying and I appreciate your suggestion. I was hoping I'd be able to do that, but as these are standalone switches and running cables is not an option.I will have to keep to the original design which will involve STP blocking,  

My question is, if in my diagram 5k1 is the root (4k) , 5k2 is secondary (8k)  would the uplink to 5k2 be blocked or the trunk between the two 3750 switches

thank you 

[Mark Malone]

I would say it would block between the switch's not the uplink , why not set the stp cost to be higher than the value of its link that way you can tell it where to block rather than guessing, if you set each side of the trunk to cost 5000 it should block there as the uplinks will be lower value

spanning-tree cost 5000