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We're on a /20 subnet (255.255.240.0), which means 4k addresses. A consultant told us that moving to a subnet with less addresses might increase performance. This network is heavy in file transfers and block-level SAN.

Does the creation of a new subnet, separate from the main one, to prioritize traffic for clients who use a lot of bandwidth make sense when using a switch?

It was my understanding that a switch would already direct traffic only to the proper destination. Performance decrease would then be limited to broadcast packets, such as ARP requests? How frequent are those?

Zopiro
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  • `We're on a /20 subnet (255.255.240.0), which means 4k addresses` - No, that means 4K "potential" hosts. Do you actually have 4K hosts connected? `A consultant told us that moving to a subnet with less addresses might increase performance` - The consultant sounds like he/she really doesn't know what they're talking about. If you have 100 hosts then you have 100 hosts worth of traffic, regardless of the subnet mask/size. Changing the subnet mask is not going to improve performance. – joeqwerty Feb 02 '17 at 17:29

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Yes some systems can be quite chatty, but is it a problem in your situation?

A minimum live verification is quite quick and painless. Choose one random workstation/server and execute tcpdump/wireshark/tshark for a minute or so. Count non-IP packets including ARP. Count IP packets if the src!=server and dst!=server, these include IP broadcast/multicast.

You will know the rate of packets, so you will be able to estimate the maximum possible financial benefit of the change. Chances are it will be less than a cost of few hours of admin's time. Most modern devices would handle many thousands broadcast-like packets per second with 1% or 2% CPU.

Edit Actually, you don't really need to gather/count packets. Just take an idle system (laptop for example) and look at its CPU stats when unconnected. Then connect it, see if the CPU usage is noticeably higher. Repeat with a system software firewall enabled (results may vary).

kubanczyk
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