Unanswered Question
Sep 22nd, 2009


I have a hub and spoke network in which small offices are connected to regional offices. Rightnow static IPs are configured in branch offices and no dhcp mechanism is enabled.

I am planning to enable dhcp in branch offices. Can anybody tell me what are the options that I could explore. The options that I can look into is:

1) Enable dhcp on the branch router.

2) Put a PC based dhcp server on the Branch LAN.

Anyother option that can be implemented? If I enable dhcp on the router than is their any overhead on the router?

Please help me out.



I have this problem too.
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Jerry Ye Tue, 09/22/2009 - 22:36

How big is your branch office?

Putting DHCP on the router will definitely add some CPU cycle on the router.

Another option is adding ip helper-address at the branch and set up the scope at the regional office. The draw back of this is when you lose the WAN, and if someone reboot their PC, they will not able to obtain an IP address.

If the branch is not that big, it should be fine with option 1.



Giuseppe Larosa Tue, 09/22/2009 - 22:40

Hello Asim,

there is a third option commonly deployed:

use centralized DHCP servers and on every branch router use

under client vlan interface

int fas0/0

ip helper-address dhcp-serv1-ipaddr

ip helper-address dhcp-serv2-ipaddr

this enables the so called DHCP relay function:

the router modifies the client DHCP request by putting a unicast routable destination address = dhcp server ip address (or subnet directed broadcast)

modifies the source to one ip address of its interfaces and put the GW field inside the packet = ip address of interface that received the original request.

In this way the DHCP server can:

answer to the request

understand from what scope to take the ip address to give to the requesting client.

The router receives the answer and forwards it to the requesting client.

We use this and it is effective.

We have two DHCP servers in two different towns and they serve also our sites abroad.

Hope to help


waridtel.com Wed, 09/23/2009 - 01:48

Thanks for your reply Giuseppe and Jerry.

Let me describe some more points which I did not mentioned previously.

1) I have around 20users per branch and 2 IP Phones working per branch.

2) I am using ce500 and 3com switches in the branches.

3) No vlans are configured in branches.

Now keeping the above mentioned points what option is suitable.



Jerry Ye Wed, 09/23/2009 - 09:35

You didn't mention how many WAN link do you have. If you have a single WAN link from the remote to the hub site, I will suggest you to configure it at the remote's router. If you have redundant WAN link, using ip helper-address and centralized DHCP can eliminate some administration overhead.



waridtel.com Wed, 09/23/2009 - 19:14


I have 2 WAN Links per branch which connects to regional offices.

Please suggest.



Jerry Ye Thu, 09/24/2009 - 05:48

It is really your decision on the business requirement

1) ip helper-address will give you centralized architecture and single point of admin.

2) Configure it on branch router will give you decentralized architecture and you have to do configuration on multiple places.



waridtel.com Thu, 09/24/2009 - 19:14


I think better option would be to configure two commands of ip helper-address at branch so that if one dhcp server at regional office goes down than other would take over.

What do you think?



iyde Sat, 09/26/2009 - 00:32

Absolutely. If you don't do that the DHCP server at the regional office will be single point of failure.


Giuseppe Larosa Sat, 09/26/2009 - 03:56

Hello Asim,

I agree.

one point of attention :

actually router sends out one modified copy for each helper-address and the first DHCP server to answer will serve the client.

It doesn't wait a timeout to send second modified copy.

We have a cluster formed by two Cisco Network Registrar and they communicate, for each scope one is master and one is backup.

They exchange information about current leases on each scope.

So they are able to work together without conflicts.

Other collegues using indipendent DHCP servers have reported to have divided each subnets in two parts one served by DHCP1 and one served by DHCP2.

Hope to help



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