Rules of Thumb

Unanswered Question
Mar 20th, 2007

Are there any resources on LAN "Rules of Thumb" i.e maximum number of hosts on a 100 M-bps Switched network broadcast domain is XXXX. Maximum sustained CPU on a Layer 3e switch should no exceed XX%. I assume there is nothing hard and fast, just guestimates and expeariance

I have this problem too.
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There are tons of references. The Cisco CCNA reference material is a good start, as well as getting familiar with the various RFCs of the business.

CPU loads are very particular to specific implementations. A sight with a ton of QoS and Security requirements might tolerate 20%-50% CPU usage rates on some key Routers, the same holds for switches. The bottom line is always "are you meeting your performance expectations?" If you don't know, then you've found a place to start base-lining, testing, measuring, evaluating your system.

And lastly, to grab one of your questions, 802.3 standard says 1024 hosts can be on 1 boradcast domain.... but I would not hesitate saying that I would not consider a design like that.

nyr.hakeem-habeeb Thu, 03/22/2007 - 15:36

In addition, over the cause my career i have found 150 hosts to be a safe line to draw, however some technical publ. say 200.


cratejockey Wed, 04/04/2007 - 10:34

I'm going to throw in my 10 cents. I have alot of Education clients. Some all Apple some all Microsoft and some mixed. All of them started flat with between 380 to 2500 users on the same flat address space (same broadcast domain). Honestly I don't even know how some of them were functioning. As we reconfigured these sites to routed VLANs we stuck our VLANs to Class C networks. We use Address 1-10 as reserved and unused. 11-40 for Printers (if no Printer VLAN is used) and static devices such as scanners. 41-240 as DHCP for clients and 241-254 as reserved space. With that said at most sites we have set a VLAN per 48 port switch which limits the VLAN to no more than 192 users (we have set MAC limits on all ports to 4 due to heavy usage of unmanaged user switches for port expansion) In a 100M Enviro with 1G uplinks and 1G server connections this setup has worked very well. As part of this we have eliminated all non-IP protocols with the exception of HVAC controllers that tend to use BACNET and are super chatty. As for PROC utilization thats a toss up. I'm with the other guys who responded. Keep it low. If you are slamming you switches then there is going to be a performace hit. However I see alot of older 3500XL and 3550 L3 switches that with a properly configured and designed network don't even breath heavy under pretty solid loads that include file shares, printing, Multicast A/V streaming and HVAC system calls.

Hope this helps.


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