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Broadcast Segmentation using L3 Switching

I was recently hired to work at a new company and one of the first things I realized was that the network was using a single logical network for all machines. IE. All 500 servers/clients/printers were using the same network ID ( So I did a packet capture on that network and realized that broadcast traffic was hovering around 11 packets per second (16 packets per second if you include multi-cast).

1) That seems to high to me.

2) It seems like a poor design to have that many clients in the the same logical network.

First question for you is this...Do you agree with me so far? If you do, do you agree for the same reasons? different ones? If you don't, then why not??

Second, let's just say that we do believe that it's bad and it needs to be "fixed". My idea is to use a Cisco switch and implement L3 switching to segment my broadcast domains. This seems like a no-brainer to me. But I wanted to put it up for discussion if anyone else had any thoughts about how to solve it.

And that's about it, any thoughts or requests for additonal info will be appreciated and honored asap. Thanks a ton.

VIP Purple

Re: Broadcast Segmentation using L3 Switching


I agree, 500 clients on the same subnet is a lot. Although it doesn´t necessarily have to be a problem, and my experience is that most people would think about dividing it up only when users start complaining about network performance. I guess it depends on if your company has money for a Layer 3 switch (like the 3550) and if you predict problems in the future. But your ideas make sense.



Re: Broadcast Segmentation using L3 Switching


I agree. But there might some problems occure when moving a network from flat (all clients/servers in one subnet) to a routed one.


Microsoft Windows "networking" may get into trouble, e.g.

See for details.

You will have to change IP addressing plan first.

There also might be some applications in your network which might be configured for flat network and be delay sensitive (routing is supposed to increase the client/server response delay).

Another possible problem: some application licenses can be derived from the machine IP address and require relicensing when changing the IP address, some applications might even require reinstallation.

If you are using DHCP in your network it would also have to be reconfigured to provide IP addresses to routed clients.

Etc., etc.

So moving a big flat network to a routed one can really be a challenge.

I'd recommend to start with a pilot project first, i.e. moving a little group of users to a routed subnet and "observe what happens".

Good luck,


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