CCNP - Segmenting with VLANs

Author

Segmenting with VLANs

firechicken

2001-10-18, 10:00 am

Hi everybody.

Would there be any disadvantages with segmenting a network with VLANs, instead of routers?

We have a department meeting soon, and I'd like to bring some ideas to the table. Currently, we have a 7000 w/ an RSM and the rest is a flat network. Becau$e of managerial i$$sue$ and deci$ion$, we mu$t u$e the exi$ting hardware to accompli$h $egmenting the campus.

Any thoughts welcome. TIA.

BlueBaron

2001-10-18, 1:49 pm

Disadvantages? Sure, it's unnecessary. To be perfectly honest, I have YET to find an instance where segmenting with VLANs was a more attractive alternative to either segmnenting with routers, or just leaving the network flat.

VLANs add an unnecessary complication to the network that really outweighs any benefits. Unless you have a very large network, or an abundance of broadcast traffic, I see really no reason not to leave the network flat.

I ran a network that was around 1000 nodes in size - flat as a board . . . no problems. . .

How big is the network, and what are the reasons for segmentation?

Retired-Mod

2001-10-19, 3:45 am

There are very good reasons to segment a flat network with vlans. For instance if you have more then 500 users using only IP, that's the suggested upper limit for a broadcast domain. Add other protocols (IPX and AppleTalk) and the limit drops rapidly.

VLANs are a good thing if designed and implemented properly.

Yankee

depamo

2001-10-19, 7:55 am

There is a little bit of confusion here for me. You say that you are segmenting with the 7K router right now. If you segment with the VLAN, you will logicaly be doing the same thing. If your 7k is capable of NetFlow switching, that is a different story all together, or if you have a switch that you are using the 7k as a RSM.

The performance jump that you are looking for with your Campus network is to have the majority of the traffic that is moving from segment to segment be processing in the switching fabric and touching the routing process as little as possible. Defined, this is what L3 switching will do for you and the benefits are staggering. Depending on how your 7k is configured, this can or can not work to your advantage.

Your build will determine how much luck you will have with this.

firechicken

2001-10-19, 9:05 am

quote:
How big is the network, and what are the reasons for segmentation?

We have approximately 650 98/95 machines, and a server farm of around fifteen NT, 2000, Linux & Novell machines. The reason we are looking into segmenting is for performance reasons. I looked at the sniffer logs the other day and saw that roughly 25 percent of the bandwidth usage was from broadcasts.

quote:
For instance if you have more then 500 users using only IP, that's the suggested upper limit for a broadcast domain. Add other protocols (IPX and AppleTalk) and the limit drops rapidly.

I remember reading this in one of my CCNA books from Cisco...but at Cisco's site they say 800 (there was a topic about this many-many months ago at this forum, but I can't seem to locate it). Either way, it sure seems like it's too many

quote:
There is a little bit of confusion here for me.

That's because I'm confused myself

I'm only a lowley network tech, trying to help out our overburdened administrator and learn some real world skills. Our current administrator is so overburdened with other non-admin crap (handed down by the upper ranks), that it's impossible for her to do the job she was hired to do.

To be totally honest, I don't know what a 7000 is capable of, but I do know it is our core, which is connected to our skinny DS-1 pipe.

quote:
Your build will determine how much luck you will have with this.

By "build", do you mean the network topology, or the configuration of the IOS? Tell me what you need to know and I'll get the definitive answers

Thanks again for all your help.

BTW, hi Yankee! What's with this retired mod stuff?

strikeattack

2001-10-19, 10:53 am

Depamo is on the right track with this one... I think we are missing an important point here.

Inter-vlan switching and routing while logically equivelant, are not physically equivelant. Intervlan routing is much faster than routing because the switching is handled in ASICs rather than in software, so the performance is much higher.

quote:
VLANs add an unnecessary complication to the network that really outweighs any benefits.

I'm afraid that I cannot agree with you on this one. Just because a network does not have any problems, does not mean that it is not susceptible to them. I agree with Yankee in that you should try and keep your IP devices per broadcast domain to under 500.

depamo

2001-10-21, 6:47 pm

Since you have a 7K router with an RSM on it, you have the internal capability to execute Net Flow switching (depending) but you need to have a few more things first.

First you need an up-to-date IOS that supports Net Flow switching. When you say that you have an RSM, you need one that supports this type of switching. If you meet all this criteria, you will see a massive increase in your ability to move traffic.

Your router will no longer need to move information up to the RSM over the CxBus (biggest bottleneck) since you can switch in the switching engine along.

I can be more specific but I need to dig into some books. Just letting you know the options you have with this setup.

It is a good idea that will pay dividends in the long run with performance of existing equipment even if you have to buy additinoal software and switching components.

Look up Net Flow switching under 7K routers, some pretty cool stuff under there with reference to the RSP7000

http://www.cisco.com/univercd/cc/td...tm#xtocid270865

Good Luck!!

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