Cisco Switch Network Design

I was browsing the Cisco support forum the other day and found the following question post which is about designing a Cisco switched network. Some useful answers are given, so I thought I would repost here for my blog readers. You will find useful tips about switch port dimensioning, redundancy etc. I Hope you find it useful.

Initial Post Question

Hello all,

I am relatively new to networking and am currently doing some theory work with network design to gain some knowledge. I’m sure you have all been there!!

I have a specification which details the needs for the network across a large campus, including number of users, their bandwidth usage, their locations, overall budget etc. I have chosen what servers are need and their locations, and the number of routers needed and their connectivity to each other to build in redundancy. My next step is to calculate how many switches will be needed for each building to get all the users onto the network and this is where I am getting quite confused.

My understanding is that a host connects to one port on the switch and a standard switch has generally around 24 ports. Therefore one switch can theoretically connect only 24 users to the network (perhaps 23 if one of those ports are used to connect to the router). My problem is that is some buildings I have over 400 users (Ive added a small percentage to allow for growth of the company). Does this mean that I need one switch to connect to the router and then another switch on each of the 24 ports to have enough ports to connect this many users? If this or something similar is true how is redundancy built in because if the switch that connects to the router goes down all the users in that building lose their connection. This seems a very expensive solution and probably quite an ugly one.

If anybody could guide me through a solution it would be very much appreciated. I have spent a long time reading articles and such on the internet but nowhere seems to go though this so if you know of a resource that walks you through design a medium/large network that would be great.

Many thanks in advance
Darren

Reply 1

Hi Darren,

To answer a couple of your points, you can get access switches with 48 ports (Cisco 2960 or 3560 for example).
If you have 400 users in a building, will all 400 user locations be cabled back to a single network cabinet (closet)?
If they are then you may be better off looking at a chassis based switch such as the 4500 series for example.
If not, you might want to to distribute the access switches in different cabinets around the building and link them back to a central distribution switch or router.
The final design will depend on the physical size of the building because don’t forget that you are restricted to 100 meters of UTP cabling between the user outlet port and the switch.

Hope that gives you some pointers.

Pete

Reply 2

Hi Darren,

To have redundancy in network you should have redundant pair of devices in your network to support redundancy and to support 400 users you can have chassis based switches with ethernet modules to support more number of users.

with you setup redundancy will be the main question as one router and local lan so the switch which is connected to router goes down then network outage.

The better way is to have dual router and dual switch in bundling mode and connect a cris cross cable between router and switch to have full redundancy in your network.In this fashion you can achive full redundancy and no network outage if any one of the device goes down.

Hope that helps out your query !!

Regards

Ganesh.H

Reply 3

Hello Darren,

IMHO the design is mainly dependent on how the connectivity and the requirements per floor( if any). If there is a per floor cabinet then may be you can use C3750 stack for access layer solution per floor.To increase any capacity needs you just keep adding to the stacks. Then if there is fiber wiring between floors available you can chose to interconnect access layer and distribution using that. Say one closet/cabinet on any floor as the collapsed distribution model, the choice is vast here c3750/4500/6503. The access layer c3750 do come with 48 ports and 4 SFP based uplink ports. so you can use the sfp based uplink ports to connected these back to your collapsed distribution layer, which becomes your default gateway and may be the STP root port too. Now for redundancy, IMHO the access layer needs to be dual connected to a redundant gateway model (each acccess stack connected to both gateways) as Ganesh suggested earlier. You can either use HSRP/VRRP to provide L3 gateway redundancy. You can also use etherchannel for uplinks to provide more redundancy. Of course if one switch fails in the stack then we loose those ports, but still the other switches on the stack will work. this is just my thoughts on you requirements.

Hope that helps.

Deesh