DHCP Relap agent related query! Urgent

Unanswered Question
Mar 14th, 2009

We have 2 x cisco 6500 switches running as a core switches, recently one of the core failed and the other took over.. but this was not the issue.

The issue is we have a single DHCP server which was connected to the core which failed and hence there was an outage for all the end users, we don't have teaming configured on the DHCP server.

We are planning to add another DHCP server and connect it to the other core switch and add two IP helper-addresse under each SVI interfaces. This way we are not sure whether the client request would be sent to both the DHCP servers at the same? Ofcourse we don't want this...

If it works sequentially i.e, if the first DHCP is unavailable then it does go to the second.. this is fine but we are not sure whether it works in such a way.

Pls do forward any document or comments on this.

Appreciate your feedback.

Thanks & Regards,


I have this problem too.
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Giuseppe Larosa Sun, 03/15/2009 - 00:37

Hello Sayed,

the question has different solutions.

Some other collegues have reported to have configured the two DHCP servers with two not overlapping scopes dividing in two half each subnet.

In our case using two Cisco Network Registrar they work in a team and we don't have problems even if both have configured the same scopes because they communicate.

For some remote temporary branch offices we have configured two routers to be both DHCP servers and this work because they test an ip address with a ping before assigning it.

if your servers can do this icmp test the problem is minimized but not totally eliminated.

Hope to help


csco11049253 Sun, 03/15/2009 - 00:49

Thanks for your mail, well.. dividing each subnet in two scopes is certainely a good idea and we may do that as well.

One other thing I wanted to know was.. if two ip helper-addresses are configured, in this case whether a client request would be sent to both the DHCP servers? or it would be sent to the first DHCP server as per the configuration sequence? an ex:

interface vlan 10

ip address

ip helper-address

ip helper-address

In this case when a client sends a request how the request would be forwarded? is it squentially, I mean first to and if it is not available than to OR to both at the same time?

Thanks for you patience.



Giuseppe Larosa Sun, 03/15/2009 - 00:54

Hello Sayed,

>> a client request would be sent to both the DHCP servers? or it would be sent to the first DHCP server as per the configuration sequence?

to both the router makes its job relaying the same message to both servers.

the first one to answer is the DHCP server that actually assign the address.

In our case the two DHCP servers are in different towns at a distance of 700 km: depending on the network topology a client is likely served by one of the two.

Hope to help


csco11049253 Sun, 03/15/2009 - 01:08

Thanks a lot!!

In this case we will Install two DHCP servers with divided subnets.

So based on the understanding the DHCP server directly connected to the respective core may assign the ip address to the client and if one of the core fails the other assigns an ip address from a other part of the same scope or vice versa..

Thanks for quick response.


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