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Community Member

Recommended hosts per vlan.

Hi all,

I am presenting the benefit of vlan to my customer and I need some numbers/white paper that might help to convince him in implementing vlan. I could not find the recommend no. of users/vlan or the no. of workstations that will degrade a network broadcast traffic. Currently, there are IP subnets in the network for 1000 users.

Tks for any help.

9 REPLIES
Community Member

Re: Recommended hosts per vlan.

IP only network upto 500 hosts

mixed environment upto 250 hosts

generally most people go for 200 rule and use 24 bit

masks.

Community Member

Re: Recommended hosts per vlan.

Thank, r-godden.

Sorry for asking more, is there any white paper or design articles from cisco that recommended on the figures?

Community Member

Re: Recommended hosts per vlan.

is the network having problems? is the precentage utilization above 60%? There are general rules of thumb that most of us live by: multiprotocol network segments range from 30-100 hosts. single protocol networks 100-200. try and show the customer that performance is a problem. it is a luxury to have small subnets.

Community Member

Re: Recommended hosts per vlan.

I have around 500 users on one VLAN using a /21 mask. Since the number grew to that, I have been experiencing very high CPU utilization on that interface of our RSM, plus multiple throttles and dropped packets. Would you say this was related?

Daniel,

Community Member

Re: Recommended hosts per vlan.

Did you find any Cisco documentation regarding your question? I could not find any but would agree with the statements of the 200 rule and the 24 mask.

Community Member

Re: Recommended hosts per vlan.

Community Member

Re: Recommended hosts per vlan.

The Cisco Press CCDA book has this documented in the design section. Generally these are the guidelines.

IP - 500

IPX - 300

AT -200

Hybrid (IP/IPX/AT) - 200

If you can, verify traffic loads with a Sniffer. That would show the client their traffic patterns and backup your recommendation of VLANing the network.

Hope this helps.

JK

Community Member

Re: Recommended hosts per vlan.

I tend to use the following guidelines as max:

IP 512 hosts

other 256 hosts

mixed 128 hosts

Additionally, given sufficient L3 performance, I use the rule of two VLANs per access switch (to do pVST load-sharing). So, if it's a 48-port switch, that makes for only 24 hosts per VLAN...

Small is beautifull.

-A

Community Member

Re: Recommended hosts per vlan.

There aren't any "hard" rules about how many devices should be on a subnet or VLAN. The main reason is that there aren't two networks just alike. Each one is different and should be evaluated on it's own merits.

Here are some steps to take:

1. Gather information

a)protocols - which ones are being used

b)applications - how many?; how "chatty" are they; what protocols does they use?

c)devices - how many?; which apps on what devices

d)network - shared?; switched?; operating limits of the switch itself?; protocols supported by the switch; and so on.

You will need some sort of protocol analyzer to examine traffic patterns and determine what protocols are running. This will also help you in determining what percentage of your traffic is broadcast and the current utilization.

The newer switches can handle an amazing amount of traffic (forwarding rate) while offering features like the ability to limit the amount of broadcast traffic. In my network most of our VLANs are either /24's or /25's but we do have one /23 (510 devices). The /23 is a VLAN implemented on a Nortel Passport 8600 and has absolutly no problems. There are currently only approx. 325 devices but with past performance, it should eaisly handle 500. We run only IP (a BIG factor-single protocol) and a lot of different apps. Any good switch (we also have Cisco and Extreme) should be able to handle the traffic.

From a design standpoint, I try to seperate my networks by purpose, with a general rule of thumb as to the number of devices at /24.

One of the main advantages of a VLAN lies "hidden". In my opinion, one of the biggest benefits is the lower administrative cost associated with AMC, (adds,moves,changes). Very seldom do we have to change any cable, just add the port to the VLAN, or remove if that is the case, and your done.

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