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Max devices in broadcast domain

Posted on 2013-06-06
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Last Modified: 2013-06-21
Hi

I know there is no hard fast figure for this but can any one give me some guidance on how many device I should have in a broadcast domain?

Currently I subnet everything as /24's but I've been asked to simplify things (there are no problems, just a nob that cant figure out routeing).  This would create a /21 wit over 500 devices in

Many thanks


Brasso
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Question by:brasso_42
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JoPeLo earned 250 total points
ID: 39225371
Well, there is not a single answer to your question and you will always get a "it depends"!

Large broadcast domains are sometimes needed in some cases. However, as you probably know, as you increase your broadcast domain, you increase your "problem area" in terms of layer 2 loops, for example. Also, your broadcast traffic will also be spread over that domain.

You can get some advantages with large broadcast domains:
- saves in the IP addressing
- switched faster. If your traffic is mostly local (within the same subnet), it does not need to be routed

However, there are lots of disadvantages. On top, if you are going to have that big broadcast domain over several switches, the STP: spanning tree is the protocol you must rely to keep that large broadcast domain loop free. Do you rely on it? I don't! :)
Besides, if you want to enforce network policies (security), it will be harder and many more hosts will receive that unwanted broadcast traffic that it is not target to it.
 
The great thing of keeping small broadcast domains is layer 2 fault-isolation and security: keep different things separated and protected.

It's up to you.

Hope it helps,
JPL
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by:DanJ
DanJ earned 250 total points
ID: 39225396
My suggestion is look at the devices and at the OS you will be running.
All the address tables are limited - HW and/or SW limited.
Ensure that your gear can hold all the MAC entries and the (e.g. router, firewall, load balancer) depending on what is your default next hop.
Also there are limits on the OS on the arp cache and you need to tune them for larger L2 domains .
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by:brasso_42
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Many t6hanks for your comments
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