Properly sizing your subnets (vlans) in relation to broadcast traffic

[cgandy]

Is there any type of document that provides a guideline for sizing your ip subnet on a Fast Ethernet, switched network? For instance I prefer to have fewer hosts per segment (vlan) to limit the number of devices that will issue broadcasts to minimize the chance for broadcast storms as well as to provide some resiliancy in the event we have a rogue nic or user on a particlar segment (vlan). In this environment all CSR's workstations are W2K and are browsing the web, running citrix, telnet sessions, exchange, etc. I do not want to have one, /23 to support 300 hosts and possibly have issues with broadcasts. On the other hand upper mgmt feels that having three, /25's or two, /24's is to difficult to manage for the staff at the site. Any input and / or formal documentation to support either case would be very much appreciated.

[konigl]

Cisco Press Book "Top-Down Network Design" by Priscilla Oppenheimer (ISBN 1-57870-069-8), page 103, has a table that shows the maximum recommended size of a broadcast domain (VLAN) based on desktop protocols in use:

IP - 500 workstations (reduce to 200 if users are running multimedia apps, have high-bandwidth plus low-delay requirements, and/or high level of broadcast or multicast packets)

NetWare (IPX) - 300 workstations

AppleTalk - 200 workstations

NetBIOS - 200 workstations

Mixed (any of the above) - 200 workstations

Per the same book (p.102), if more than 20% of the network traffic is broadcasts or multicasts, then the network needs to be segmented.

For what it's worth, I have set up IP-only VLANs with a /16 mask and over 1000 nodes that have worked fine. And I have seen mixed protocol VLANs with /16 IP masks and more than 500 nodes, also working fine. It really depends on the nature of your network's traffic. Keep in mind, though, that changes in the type of traffic or in the applications used may require you to redesign and re-number your network.

Some of the above-mentioned customers (school districts) that went with large flat networks because they wanted to get away from the "difficulty" of managing multiple subnets and routers/L3 switches, are finally seeing the downside to flat networks and certain older switching technologies now. When they use IP multicasting to get desktop images out to the classroom hard drives and computer labs, the main L2 switch in the MDF of a building treats the multicast as a broadcast and floods the entire VLAN, virtually paralyzing all other traffic until the image run is complete. To make matters worse, many of these VLANs span multiple buildings, so certain networks can get slammed by anyone imaging anywhere at any time! They are now in the process of limiting VLANs to a single building, and even defining separate VLANs per computer lab within each building, to contain the imaging traffic. (Newer switches with intelligent handling of IP multicasting would be better, but they have been told by the school board to make do with what they have.) And the staff are taking all the new subnet numbers and strange masks in stride.

Make your VLANs no bigger than a Class C IP netmask (255.255.255.0 or /24) and this should help control broadcast volume.

Hope this helps.

[cgandy]

Yes this does help. I appreciate your response.