Unanswered Question
Mar 3rd, 2007

If I was told that 3 network devices needed to be in the same subnet. Does this necessarily mean the same vlan?

I have this problem too.
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scottmac Sat, 03/03/2007 - 23:07

Yes. A VLAN equates to a broadcast domain: You would typically have only one subnet per VLAN / broadcast domain.

Good Luck


smashtov82 Wed, 04/18/2007 - 14:24

Hi All

Say that you have a VLAN across several switches. And you want to use a 20 bit subnet mask. What are the cons? The broadcast domain is so big that your network is going to collapse? That will happen?

Thanks for any comments...

scottmac Wed, 04/18/2007 - 14:57

It's the same, regardless of the nature of the broadcast domain.

If the broadcast domain (VLAN, Subnet)has too many hosts, you end up eating a lot of bandwidth (too many broadcasts) ... and if the broadcast domain spans one or more WAN links, you've wastd a lot of expensive bandwidth.

The other downside to too many broadcasts is that the nature of the broadcast means that all hosts that hear it (everything in teh broadcast domain) must accept the traffic, and process it up the stack, at least to a point where it can decide that it doesn't apply to that host and it can throw the traffic away (like an ARP, DHCP request, multicasts ....)

So, if you decide you want to have a single flat network with a couple thousand hosts on it, and it's going to span several physical locations, plan on running at about 20% efficiency, and for Spanning-tree convergence to take a loooooooooong time.


Good Luck


smashtov82 Thu, 04/19/2007 - 05:52

Thanks Scott

There is documentation about what are the limits on a broadcast domain? I meant, How many host will make a broadcast domain unusable...


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