How many VLANs question

[SF18C]

I have a large non-Cisco switch (Layer 3 capable) that is that operates at the core/distribution layer with about 55 other switches connected to it. I have about 3000 users spread across a Campus area network and currently we only have 6 VLANS. The VLANS are not created with any logical since and 2 of the VLANS hold about 80% of the users.

I have proposed to create about 20 VLANs and include membership by location (basically VLAN 1,2, or 3 of the access layer switches based on building location).

Also for your information, basically a M$oft network with a few Novell systems, most locations are on modems/4wire circuits, most of our traffic is web related and email. I got half a class B subnet (30000 IP address), we use DHCP and I was going to give each vlan its on sub net (i.e. VLAN 4, 10.10.4.1-254, VLAN 5 10.10.5.1-254) and create a DHCP scope for each VLAN (i.e. 10.10.4.1 as the gateway, 10.10.4.2-10 Servers, 10.10.4.11-30 printers, 10.10.4.31-54 static workstations, 55-254 DHCP address)

So now management wants to know how does the increased VLANs affect the switch (processors, latency, packet loss) they feel that to many VLANS will swamp the switch and slow data. I told them 20 VLANS should hardly affect the switch but I would look into it.

So can anyone give me a few points to back my plan up or tell me where I need to improve it?

 

[webnetwiz]

What kind of switch is it (make & model)?

 

[SF18C]

Foundry Big Iron 8000

 

[SF18C]

Foundry Bir Iron 8000, it is about a close to Cisco gear as legally possible. I have found very little difference in Foundry and Cisco switches.

 

[webnetwiz]

A switch like that would support over 4000 VLANs in a heartbeat. Even if you ran 100 VLANs, you wouldn't be able to see a performance dip. Plus, having VLANs means you're segmenting, thus decreasing your broadcast domain. You would see less broadcasts.

 

[SF18C]

thanks webnetwiz, I didn't think 20 VLANs would phase the Big Iron, but will it really run 4000 VLANS?

What do you think about my IP plan (1 subnet per vlan with a DHCP scope attached)? It sounds logical to me, I haven't been able to find any good implemention guides on scoping out a medium sized CAN network, so I'm kinda winging it.

 

[webnetwiz]

I'm currently implementing over 10 VLANs for a location with about 300 users. They're all separate legal entities, which requires me to have different IP subnets for them. I provide them with a separate scope on the DHCP server (it's an all Cisco shop, so I add the ip-helper address, which is the DHCP server, so that the clients can obtain proper IP addresses) and they'll just have a class A address with a class B subnet.

So basically, it sounds like you're on the right track. I'd recommend that you create a test VLAN, and you probably have your default VLAN 1, enable InterVLAN routing and put a PC in the test VLAN to test connectivity between the 2 VLANs.

Good luck!

 

[tdonoghue]

Switching capacity is only half the story. You just need to keep an eye on the layer 3 (routing) side of this monster. If all the users will now be located 'further' from there main services (hop count), then a larger portion of users will need to be routed to there destination putting a larger load on the layer 3 portion of the switch.

That said I don't think the loading you are looking at should cause any major network problems regarding latency, etc.

Regards,

Tony.

 

[SF18C]

Thanks Tony,

The Switch is layer 3 capable so I don't forsee any issues there (one big vlan hitting the switch to access the internet vs. 20 smaller vlans hitting the switch to go to the 'net).

I've pretty much created all the VLANs and DHCP scopes so tomorrow I will start bring the interfaces into the correct VLANs and do some end user reconfig of statics and networked printers. I let you know how it goes!!!