Design for DHCP redundancy.

[renzil]

Hi,

Can somebody advice , how can i have dhcp server redundancy in my network. I need to design a network that will have around 30 vlans spanned across 50 buildings.

[amit-singh]

Hi renzil,

DHCP server redundency in a network can be build by setting up multiple DHCP server. You have to define multiple scopes in the DHCP servers for each Vlan.On your Vlan interfaces you can define Ip helper-address using the DHCP server IP or the entire subnet and the client request will be forwarded to the DHCP server as a unicast request. For example

Int vlan 1

ip helper-address 1.1.1.1

ip helper-address 1.1.1.2

Or

Int vlan 1

ip helper-address 1.1.1.0

In the first example the Ip addresses for both the DHCP servers are defined and the request froma paticular vlan will be forwarded to the first server. In case if that is unavailable then the request will be forwarded to the second DHCP server.The DHCP server servers can reside any where on you network but I would recommend creating a separate vlan and putting up the DHCP server in that.

In second example, you can define the whole subnet where the DHCP server servers are located and the request will be forwarded to all the servers. Depending which server gets the information first, it will relays the IP's.

However, I have not tested the DHCP redundency in a network . I have been told by some one that you cannot have multiple DHCP servers in network with the same scopes defined on them. I dont know if that statement is true or what would be the impact of that. I would request some one to put some light on this.

HTH,

-amit singh

[paddyxdoyle]

In a previous life, the company used to have two DHCP servers, each with all scopes configured however only the even VLAN scopes were active on one server and the odd VLAN scopes were active on the other. Should a DHCP server fail then the scopes on the other server were activated.

This mean't configuring two helper addresses for each server on the VLAN interfaces.

HTH

PJD

[andrew.butterworth]

There are some design guidelines on Microsofts website regarding this with quite a few different scenarios.

It seems to fall down to either 2 DHCP Servers with all scopes on each server - each scope split so 80% of the usable IP addresses are on one server and 20% are on the other. Generally the closest server to the DHCP Clients has the 80% share and a remote one has the 20%. The other option is Clustered Servers with the DHCP Service running as a Clustered Application so you get redundancy of the original Scope. This is usually backed up with the 80/20 split as well.

http://www.microsoft.com/technet/itsolutions/wssra/raguide/NetworkServices/ignsbp_3.mspx

There are alternative DHCP Servers (Cisco Network Registrar being one of them) that work in a distributed environment and have knowlege of leased addresses between servers (something MS DHCP doesn't). This way you have 1 logical DHCP Scope but multiple Physical servers serving addresses from the scope.

Andy

[mcamocardi]

Is it possible that the second DHCP serverd duplicate a IP address?

DHCP1 - 10.0.0.10 -20

DHCP2 (Backup) - 10.0.0.10 - 20

If DHCP1 has clients from 10.0.0.10 - 15 and fails

DHCP2 assume, it does not have the bindings from DHCP1. If a clients asks for a IP will it give 10.0.0.10 and diplicate the address on the network?

[Richard Burts]

Marcelo

Putting the same addresses on both servers will cause problems and result in duplicate addresses on devices in your network. The reason for this is that when a client sends a DHCP request it is sent to both servers. Each server will send a DHCP offer using one of the addresses in its scope and the client will choose one offer (usually the first one received). Server A has no knowledge of what is being offered by Server B and so if they have the same addresses in the scope then they will be offering the same addresses and duplication will result.

HTH

Rick

[paulstone80]

Hi Renzil,

I'm actually working on a similar solution at work at the moment. There are a couple of options (some have mentioned already)

1. Have two dhcp servers in different locations, with the same scopes and settings configured. Add the IP helper address for both servers to any device that needs to relay dhcp broadcasts. Then choose either;

a) Active/Standby - Configure the dhcp servers to act as a Primary/Secondary, by disabling all scopes on the secondary server until a situation arises where you need to failover. When you failover, enable the scopes on the secondary dhcp server. Be careful to prevent the Primary from giving lease when it comes back online.

b) Active/Active - Split the scopes across both dhcp servers in a 20/80 or 30/70 configuration as mentioned above. You will need to ensure the same reservations are configured on each dhcp server.

c) Active/Active - Disable some scopes on one of the servers, and have the same scopes active on the other server. When failover occurs, enable the scopes that are disabled on the server that is still reachable. Again be careful of the failed server issuing leases when it comes back online.

2. If you have the option, Windows Server 2008 Enterprise now offers High Availability Clustering for DHCP servers. This is a good solution because the DHCP database is shared between the two DHCP servers, so you don't need to worry about duplicate addresses being issued. This also works across geopgraphically dispersed sites, which is great if you have multiple data centres in different locations. You need to have a storage solution in place thatcan replicate the DHCP database to the failover location.

HTH

Paul