DHCP Servers, Double routers etc

[synaesthesia]

Probably over-complicating this massively but I'll try and explain why the headache

 

Two physical sites. New network infrastructure built on SCCM going in August. Each site has 2 servers each, a DC and the SCCM/other bits server.

DC will do DHCP and DNS.

So we can manage collections and boundaries properly in SCCM (2012) we need to separate the two sites via IP.

Subnets created in AD to do this

 

(rough outline : if we have 192.168.48.0/22 allocated, 192.168.48.0/24 and 49.0/24 are bound to site A, 50.0/24 and 51.0/24 to site B)

That makes the SCCM side of things a piece of cake to manage. AD is simple enough.

 

However, I'd like to make sure the right machines get the right address from DHCP - from the right DHCP server, mostly!

So Site A PC's pick up IP addresses in the scope from the Site A DC, and Site B from Site B DC etc. A client will usually pick up its DHCP stuff from the nearest server which *should* in practice work OK, especially if we lengthen lease time a little. But that won't always be the case, and I'd prefer to keep stuff that deploys from boundaries rather than collections to each site rather than traversing the link.

 

There's a decent core switch at each site - a HP5406ZL. Currently it's set up entirely flat, and we'd rather not go into too much bother with vlans etc.

I'm vaguely estimating that involves:

 

Giving each switch a reasonable IP within the two subnets (i.e. site A router would be 192.168.48.2 and site B would be 192.168.50.2)

Enable routing on each switch with default route being the router supplied by ISP for internet access

Set dhcp-helper on each switch to the relevant site DHCP server

 

That seems reasonable simple to me - however a few concerns.

The routers would be on separate subnets but being within within the /22 would still be able to communicate - do I need to therefore split into 4 x /24s and have a superscope on each DHCP server to cope with the 2 x /24s?

Failover - this, I believe means we'd lose failover (i.e. if site A DHCP server keeled over, B wouldn't be contactable for the clients)

 

Would anyone have any suggestions on keeping failover/resiliency whilst still being able to differentiate between the two sites in SCCM/AD?

[gaiacraig]

I think I understand what you're asking...

 

You should create a routed link between the sites. This would mean creating a new subnet purely between the switches/routers which link the sites, or use routed ports on your HP switch (whichever is most suitable) with a small /30 subnet.

 

If you configure IP helpers on each site, they won't really be a problem as the local DHCP server should respond first and offer an IP address. The client should only accept an offer from the remote DHCP server if it doesn't receive one from the local subnet.

 

You won't need to create Superscopes either, as the DHCP server will know the netmask based on its own NIC.

 

Also, you'll need to configure some kind of routing between the two site routers, or configure static routes so each LAN knows the way to the other one (vital for DHCP to be able to service hosts on the remote sites).

 

Hope that makes sense