vlan & subnet

Unanswered Question

hi! I have some doubts about vlan and subnet.

Is it the best pratices to have one vlan for one subnet? What's the advantage of this? Will there be any problem if i've multiple subnets in one vlan?

In term of network performance, will it decrease the performance of the network if i do variable subnetting in a network? or will it be better if i use the same subnet mask for all the subnet?

Please advise. Thanks.

I have this problem too.
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husycisco Tue, 04/22/2008 - 03:33

Hi David,

Only VLANing will have an impact on performance. More VLANs, more amount of smaller Broadcast Domains instead a one big domain. Other implementions you mention wont have a decreasing or increasing effect on performance.

There wont be any problems if you have multiple subnets in one VLAN if you already handled inter-vlan connectivity with a L3 device, yet, there are many implementions out there which have more than 2 or 3 subnets in one VLAN especially the Primary VLAN.


VictorAKur Tue, 04/22/2008 - 03:44

Agree. As VLANs create a layer 2 segregation of the network you can in theory have a number of subnets within one VLAN. However I think it will create a rather complex solution and will in my opinion defeat the purpose.


Jon Marshall Tue, 04/22/2008 - 03:44

Generally speaking yes it is best practice to use one subnet per vlan. You can use multiple subnets in one vlan and have sedcondary IP address on the vlan interface but in most situations this should be seen as a stop-gap until you can migrate to one vlan per subnet. Using one subnet per vlan allows a more logical configuration for example applying access-lists, QOS etc.

No it really won't decrease the performance by doing VLSM.



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