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New Member

2 different ISP,routers both configured as DHCP server

Good Day to all,

My customer has a requirement having 2 2851 with different ISP located at the same place, for redundancy purposes, im going to implement HSRP and make the 2 routers with different ISP as DHCP servers

May i know if this one is possible?

Can anybody help me and give a hint how to configure it,

Thanks you very much for your support.


Hall of Fame Super Bronze

Re: 2 different ISP,routers both configured as DHCP server

If I understand correctly, both routers will be configured as DHCP server and you want only the active HSRP router to reply to DHCP requests ?

I'm afraid it's not possible. When a workstation boots up, it sends a DHCP requests and any DHCP server will reply. HSRP won't prevent that.

HSRP's only controls which router will respond to traffic send to its Virtual IP Address and DHCP requests aren't sent to a specific address, it's a broadcast.

New Member

Re: 2 different ISP,routers both configured as DHCP server

what is the solution to this problem? Since if the primary failed then DHCP also failed, then rest of new workstation will not able to get IP address.

Re: 2 different ISP,routers both configured as DHCP server

it is correct that when ip helper us used to forward DHCP requests both DHCP server ( Primary ISP and backup ISP) receive a REQ, and hosts will accept 1st offer.

so the crunch here, is to make sure that Backup ISP never gets a REQ, unless Primary ISP is its a matter of routing and it's down to IGP.

If u are getting a default route from ISPs, or specific dynamic routes for DHCP-A and DHCP-B make sure route to DHCP_B is always least prefered and u might also want to filter it with An ACL.

alternatively, u can override routes to DHCP by using static routes, so both DHCP are pointing to Primary ISP, when it fails the static route should not longer be valid (if using Next hop on link to ISP) route to DHCP-B will appear from dynamic.



Hall of Fame Super Silver

Re: 2 different ISP,routers both configured as DHCP server

Perhaps I am understanding the requirements and the plan of Lester's post a bit differently. I certainly did not understand him to say that he was going to have the ISP act as the DHCP server. And I am not sure that I understood him to say that only the primary DHCP server should respond, and certainly both DHCP/routers will respond.

What I did understand is that the customer has 2 ISPs, the customer wants 2 routers, and that Lester was planning to run HSRP between the routers which is quite possible. And that he wanted these routers to function as DHCP servers which is also possible as long as he is willing to have both of them respond (each with its own pool of addresses) and that clients may get addresses from either of the rotuer/DHCP servers.

One implication of this is that internal traffic going out to the Internet will go through the router that is the active HSRP router and that the connection to the other ISP will only be used if the primary connection or the primary HSRP router fail. If Lester and the customer are comfortable with this then I believe that it is quite possible.

If I have misunderstood something in Lester's post then perhaps he can clarify it.



New Member

Re: 2 different ISP,routers both configured as DHCP server


I believe you are correct. What I have done before is divide the C class from /24 to /25 and have both router provide IP address to the client.

New Member

Re: 2 different ISP,routers both configured as DHCP server

Further to this thread ...

I agree it certainly is possible to have both routers providing DHCP service as long as the scope is split, each router having a different DHCP scope (pool) to the other.

What if the pool has to be identical because the number of devices far outweigh the available IP space if the scope was split in half?

Theoretically speaking if you set up a remote database agent, then both routers will write their bindings to that database. That way both routers can be configured with identical pools, and conflict resolution can be determined via the common database.

The question I have is what happens if connectoivity to the database agent is lost or the database agent is down. Will the routers still respond to DHCP requests and allocate IP addresses, but not write to the database agent?

What happens when the database agent comes back?

Does anyone have experience here?

Hall of Fame Super Gold

Re: 2 different ISP,routers both configured as DHCP server


yes the dhcp service in the router will detect loss of connectivity to db agent and provide addresses from a local pool. I don't have the config details, but it can be done.

New Member

Re: 2 different ISP,routers both configured as DHCP server

Can someone please provide more information on this configuration can be accomplish?

New Member

Re: 2 different ISP,routers both configured as DHCP server

The command is: ip dhcp database url [timeout seconds | write-delay seconds]

and the following are the acceptable URL file formats:

- tftp://host/filename

- ftp://user:password@host/filename

- rcp://user@host/filename

Make sure you have enabled "ip dhcp conflict logging".

You can find out more at this URL (but the detail is kind of light):

You apply this command to both routers.

I'm not sure (because I've not done this before) what happens when the database agent is lost and comes back but in between that, the router continues to hand out IP addresses.

Do the routers update the database agent of changes during the outage? If so, how are conflicts then handled?

It would be neat if someone who've done this can advise.

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