Network Design ?

Unanswered Question
Jan 12th, 2009

I'm looking at forklifting a network that I've inherited and am looking for help in determining the best design for my needs. The network currently suffers from extreme slowness, and excessive outages. There has been relatively little documented about this network so I'm still in discovery process myself. The network hardware is all layer2 Allied Telesyn, which most if not all are extremely outdated. There are 7 buildings connected with a mix of single and multi-mode fiber backbones. There are about 600 nodes attached to this flat network which are a mix of printers, Windows, and Mac machines. My current thoughts are to replace all Allieds with Cisco gear. I will be working with a relatively small budget so I may need to work on this project in a several stage approach. I'm thinking that I would like to implement between 10-12 routable vlans for some network segmentation and security. My current thoughts are to use possibly something like 3750's to handle all the vlan routing in key locations throughout the organization and then use something a little less expensive like 2960's in each building for client access. Hopefully the experts will chime in and be able to help with a successful deployment.

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Overall Rating: 4 (3 ratings)
Jon Marshall Mon, 01/12/2009 - 12:58


"The network currently suffers from extreme slowness, and excessive outages."

In a way you can't really go wrong with whatever you do :-). I'm not familiar with the Telesyn switches but i suspect part of the problem is that it is a flat layer 2 setup.

What you propose sounds perfectly acceptable. Key thing is, as you propose, to segment the network with vlans. If possible you should look to ioslate vlans to buildings. What i mean by that is if possible don't have the same vlan across all 7 buildings unless you need. If you can isolate vlans to buildings then you can configure your trunk links to only allow the relevant vlans. This limits the range of STP. An additional plus is if some time in the future you decide a L3 routed access-layer is the way to go the isolated vlans will fit this very well - altho obviously 2960's are not L3 switches.

There are other factors which are not covered here. Even though i said at the start that you can't really go wrong we probably need more information.

Do you know where the outages are coming from ?

What amount of data do you need to move between the buildings ?

Do you have any VOIP requirements ?

Servers ?

Internet access ?


Collin Clark Mon, 01/12/2009 - 13:03

I agree with Jon and I'll add a couple of things; add layer 3 as close to the access layer as you can afford and now is probably the best time to address any issue with the IP scheme. Try and create contiguous subnets so you can summarize at each building or floor. Mon, 01/12/2009 - 15:46

I have been considering very similar designs to this recently and have been wondering what hardware would be suitable.

Another thought would be, is this the kind of scenario that would suit a collapsed core or should there be a dedicated distribution layer, assuming perhaps a small server room and dual WAN links? Would 3750's be suitable for a collapsed core in this scenario?

Joseph W. Doherty Mon, 01/12/2009 - 19:38

If you too are looking at a network of about 600 nodes, there's a good chance a collapsed core would be fine using 3750s.

Joseph W. Doherty Mon, 01/12/2009 - 19:32

From what you describe, spliting the one flat L2 domain into multiple subnets, by reducing broadcast domains, will likely improve the network for 600 nodes. Generally, a /24 or /25 subnet usually works well, so you might not need to define 10 to 12 subnets, but more wouldn't hurt with a typical L3 switch. (Collin makes a good suggestion about IP address allocation.)

For a tight budget, the 3750 is often a good L3 LAN selection. For your "core", a 3750 stack of two or more will allow you to provide reduncancy if you can Etherchannel downstream devices to different 3750 stack members; it may also increase bandwidth to/from "core".

Another choice for your L3 switch, when you will not need the stacking capability of the 3750, would be the 3560. It's more or less identical with the 3750, but without a stacking capability.

For more than 2 or 4 fiber ports within a single device, you'll need to use the 3750G-12S. This 3750 might be mixed in a stack with copper port 3750s providing a "core" that hosts both downstream network devices and corporate servers.

For you downstream devices, again for minimum cost, you could use L2 switches VLAN trunked to your L3 "core". 2960s could do this, as could even less expensive Cisco switches that support VLAN trunks, e.g. Cisco Express 5x0 series, or more expensive Cisco switches, e.g. the new 2975 stackables. Similar to what Jon notes, best L2 selection would depend on additional factors.


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