'Good form' rules in designing networks

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Apr 24th, 2007
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Are there some simple rules (as best practice) witch we must follow in designing LAN?

I debated with a customer’s specialist how to build LAN in his new edifice.

There is a building with the next planned desktop distribution per floor: 50, 40, 130, 50 and about 10 various servers. Customer is a bank.

Client is sure the next structure will satisfy his requirements: as core will be used 3750G-12S + 3750G-48TS (two for each for resilience). On floors there will be groups of two 3560G-48. Each group will be connected with 2 ports Etherchannel to “core”. 3560G in group will be interconnected with CAB-SFP-50CM (Catalyst 3560 SFP Interconnect Cable).

I am sure this is wrong way. But I have only intuition and theoretical arguments.

At first I think the 48:1 oversubscription on wiring closet is too much. I saw somewhere the recommended value is up to 20:1.

Next: for LAN with 250-500 workplaces Cisco recommend using Catalyst 4500 as core. 4500 Supervisors had at least 64Gbps and 48Mpps (Sup-II) and I think it is may be not enough for him. Client solution has 32GB bandwidth (3750 StackWise bus). But may be he’s right? For his 8GB all uplinks speed this is enough?

Has anybody any ideas? May be offer me any useful links, where such questions are discussed.

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mark.j.hodge Tue, 04/24/2007 - 02:40
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Does the client require Gb speed to the desktop? If so will this be a sustained throughput, or just temporary peaks? If sustained then you are probably correct in that the uplinks are under sized, however I would suspect he/she just want Gb capability.

In the situation you have described, only one of the edge 3560 devices can be connected to the core, the second will be cascaded from the first, presumably by an second etherchannel. This is because an etherchannel cannot span two physical devices with seperate IOS.

For resiliance the two connections from the edge 3560 should be connected into different 3750 devices in the same stack. You can do this because even though they are seperate devices, they are running the same IOS.

pavelalex1 Tue, 04/24/2007 - 03:09
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I am sure client isn’t yet aware about his requirement. At first they plan to change substantial banking system shortly. And new network infrastructure isn’t builder for current needs. May be at moment we might be satisfied with 100Mb to desktop, but how about development?

Only one from two 3560G will be connected to core, the second will be connected to first by CAB-SFP-50CM. These 2 links from Etherchannel will be connected to different 3750G-12S.

mark.j.hodge Tue, 04/24/2007 - 05:04
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Unless there is a large ammount of streaming video or other on-demand data, Gb to the desktop seems over kill. You could limit the majority of access layer ports to 100 Mb.

If you want to futureproof the environment, and provide Gb capabilities to the desktop then I would say the environment you describe needs to be tweaked. Take a look at the "E" type switches, which provide 10Gb uplink capabilities.

The 3560-E comes in a number of variations, including a 48 port device that can provide ppower simultaniously to all ports. With the TwinGig converter, it is backwards compatable with standard SFP converters.


The 3750-E can support 68Gb backplane and provides 2 10Gb ports per device.


pavelalex1 Tue, 04/24/2007 - 05:35
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Am not sure streaming video is unique motive for 1Gb for desktop access. On large number of PC OS image, new software or patches deploying became an often task. Sometimes this procedure may take a long time. MS likes to produce Service Pack with volume comparable to initial distributive.

From my experience in bank, there are lot of PC with special software, not trivial in installation and configurations. Thus necessity to backup desktops became a ordinary task.

I looked already to this direction (10G Ethernet), but it seems to be too expensive at this moment.


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