VLan - Broadcast vs Routing

[leepeiwai]

Hi,

One of our regional office has grown with more sub sites.

All sub sites are connected through microwave to the regional main office.

Currently, these sub sites and the regional office is sharing a subnet.

We are discussing is it better to VLAN each sub site to limit the broadcast vs the cost of routing between VLAN - when a request is made from the client to the servers (in the regional main office).

Cheers,

pei

[terrygwazdosky · Accepted solution]

Ideally I'd break them up into seperate subnets, but the answer will depend somewhat on what you have in place currently.  Are there routers and/or multi-layer switches at each hub location?

[leepeiwai]

We only have a Catalyst 3750 at the main office, and L2 switches at each hub location.

Yeah, VLAN the subnet will limit the broadcast boundary.

But other sorts of broadcast traffic do we have to worry about, besides the first time DHCP request from client PCs?

The servers are located in the main office, the site requests have to be directed to the main anyway. So routing to be performed by 3750 (at main) shouldn't a high cost - compare to without VLAN.

cheers

[ajay chauhan]

Big subnet cause higher broadcast traffic within LAN/VLAN and that cause higher resource utalization. DHCP over the wan treated like unicast so nothing major in that case. Small subnets are recommanded within LAN to avoid higher level of broadcast.

For example suppose if you have single vlan for every request there will be broadcast since L2 works on broadcast basis and higher hardware resources will be used. DHCP over wan for that we use ip helper-address.

Thanks

Ajay