When to Subnet

BRUNO WOLLMANN · ‎04-06-2004

I was hoping someone could point to a white paper or a best practices document that would give some guidelines as to when it is a good time to break-up a broadcast domain.

I have about 3000 IP nodes on my network and all are in the same subnet. All nodes are plugged into Fast Ethernet switches. 90% of my nodes are Windows NT/2000/XP/NT Server/2000 Server. The other 10% are printers and Unix servers.

My average broadcast traffic is about 43,000 bits/sec (or .043% on a fast ethernet port). 56% of that is ARP, 22% is SMB and the remaining 22% is a variety of different protocols. Everything is IP so I know I won't have to worry about breaking communication between devices that are broadcasting other protocols. These other protocols are active because some systems admins have left network devices at defaults rather than deactivating these protocols. On some systems (mainly print servers) it's impossible to turn off these other protocols.

I already have a good DHCP/DNS system in place to make this transition easier.

All thoughts, advice and stories of past experiences are welcome.

thanks

Bruno

thisisshanky · ‎04-06-2004

Even though its all IP, 3000 nodes in one flat subnet is really huge and should be segmented for better performance. Even though IP doesnt talk a lot unlike IPX, ARP broadcasts and SMB traffic involving broadcast requests will still be seen and processed by all the nodes. I believe you are using a Class B subnet space.

I would recommend not more than 500 hosts in one segment (vlan) in a TCP/IP environment. You might even want to reduce the number in case you are using Netbios over TCP/IP. If you use a class C subnet per vlan you are left with 254 valid hosts in each vlan. You could double the number to 500 hosts by using two class C subnets on the same vlan with secondary addresses on the routing device.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd20e.pdf

The link mentioned above is a good whitepaper on Broadcasts in networks.

Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus