Does Cisco have oversubscription guidelines/standards for all three switching layers? I heard they do but cannot find a link.
For eg: access should not be oversubscribed over 1:125 and so on...
If there's no link, can you guys tell me what do you typically oversubscribe at? This is no fancy environment but they may get VOIP in the future
There is a white paper which outlines some of this stuff, its called design guidelines for campus networks or something similar, I dont have the link at the moment but it is there.
I cant recall any of the current Cisco edge switches which are oversubscribed within themselves so you would normally not need to worry about stack backplanes.
The rule of thumb I work to on the edge/distribution is 20:1 at the uplinks.
So if you have a gig uplink from your edge to the core/distribution then you can allow for a stack with 192 100meg ports, which gives 19.2gig of aggragate bandwidth on the uplink. Which is a stack of 4 x 48 10/100 ports (assuming you are not using the spare gig ports for hosts).
Naturally, if you have two up links, and are doing some load balancing over them for example HSRP groups or by using multiple vlans and HSRP. In this scenario, you would probably want any vlans to be configured across all switches in the stack for resilliancy. Then you can use more switches per stack because you effectevley have twice the uplink bandwidth (2 x 20:1), but there are a number of problems with large stacks..
First, it is easy to put too many ports in a subnet and you may run into broadcast issues.
This is the last thing you want in a voip environment.
A further problem can arrise if you are using non-stackwise switches and are limited to using gig ports for stack interconnects. In this case there is a danger that you can oversubscribe the inter-switch links (normally gig links). The aggragate bandwidth arriving at the uplink from all switches in the stack may increase the oversubscription ratio too much. If you are using 3750's with the stackwise then the same issue applies, only you have 32gig (assuming non 'E' variant) in the stack interconect, so you wont have to worry about interstack bandwidth.
You will be ok with 5 in a stack giving 24:1 if you are expecting light/medium traffic types, but I wouldnt go any higher than that. You may even get away with 6,7 even 8 switches in a stack up to a point if using load balancing across uplinks from top and bottom switches, but an uplink failure may lead to insufficent remaining uplink bandwidth being available for all traffic. Also think of the broadcasts over 384 ports ! even if they are spread over a number of vlans, the vlan may not suffer from broadcast issues but the effect will be felt on the uplink, its all forwarded on the trunk, just tagged..along with BPDU's..and we all know what happens if we start loosing a few of them from time to time.
I find Its best to stick to 192 10/100 ports per stack. If you need multiple 1000 host ports then factor that into the 20:1 ratio, then again is the host really capable of forwarding 1000 ? probably not. You could consider reducing the stack to 3 x 48 (144) and using 1000 ports for the remaining 4 or 5 gig, thus maintaing the ratio. If I had to provide a large number of gig ports to hosts, then I would either consider 10gig uplinks, which could prove expensive, or use gigabit etherchannell (flowbased concerns accepted) to maintain the 20:1 ratio. Either way, gig or 10 gig I think the ratios are the same.
As for distribution to core links then I would work on as close to 1:1 as possible. We all know that in some circumstances it is not possible or practical, so work on something in the area of 4:1, 8:1 max but it depends on traffic flows/patterns. I cant recall the cisco advice on that but I think it says 1:1.
I always work on 4:1 for server farms, server to edge connectivity.
Hope this helps.