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New Member

Switched LAN subnet size

I've been doing this for almost 20 years but I deal with dumb management and out-of-control user groups. I'm interested in CISCO DOCUMENTATION specifying LAN/Broadcast domain size guidelines. Please, no "opinions" I have enough of those.

Thanks!

6 REPLIES
Silver

Re: Switched LAN subnet size

Jeff,

Unfortunately you're going to get little but opinions on this question, since there are no hard fast numbers for BCAST domain size. You'll see certain docs like "how many OSPF routers can be in an area" are gone as well.. this is due to the fact that BCAST domain size or "why to implement VLANs" is entirely based on your network requirements (e.g. Security, etc) and traffic patterns. I've seen folks have 600 - 1600 users in one VLAN and work okay.. but I've seen others that have less than 100 have a faulty node (BCAST flooding) bring the entire BCAST domain to a stand still... So, I hope that this "opinion" does not sit to bad with you but the numbers you are looking for are not there... and if you find some they (numbers) are opinions as well.

Hope this helps,

Don

New Member

Re: Switched LAN subnet size

Don,

Thanks for the reply. Unfortunately, we can take all of these considerations into our planning but when dealing with others, some pretty crazy suggestions get way too much consideration. Cisco has some great "Best Practices" design document that we use for things like ISP and AVVID. There should be something as basic to reference.

Regards,

Jeff

Silver

Re: Switched LAN subnet size

Jeff,

Sizing best practices are to hard to come up with due to there being to many moving parts that can make a "1000 users is the limit for a BCAST Domain" recommendation bogus. Many of these, as mentioned with the OSPF item, recommendations have been pulled due to these problems. There are tons of great best practices docs for switching that go over configuration and mangement best practices but there is little in the way of sizing info.. (I'll send it to you unicast). There are also some good design docs that show best practices as well but again, don't touch on sizing to much. This is entirely unique to your requirements and traffic patterns.

Regards,

Don

New Member

Re: Switched LAN subnet size

Don,

I agree. I am the lead engineer at a university and we have some student segments with 1800 in a BCast domain, and they are fine. But I have another with 600 and they constantly complain. I think I am going to break them all up next summer.

Brad

Re: Switched LAN subnet size

If you look to Cisco Press: Designing Cisco Networks by Diane Teare (CCDA preparation official coursebook) it says:

"Protocol - Maximum number of workstations

IP – 500

IPX – 300

AppleTalk – 200

NetBIOS – 200

Mixed – 200

The number shown are provided as a guide. Actual ststion limits depend on factors such as broadcast/multicast loads, IP addressing structure constraints, inter-VLAN routing requirements, management and fault isolation constraints, and traffic flow characteristics."

Regards,

Milan

New Member

Re: Switched LAN subnet size

Milan,

Thanks for the info. I recall a few years back, the Cisco TAC web site did have such recommendations which I referenced at that time.

Regards,

Jeff

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