DHCP Redundancy on cisco Routers

[Poorya Naji]

Hello All,

I am trying to find a way to have DHCP redundancy on cisco routers and i couldn't use any external server for DHCP server, so i must use my routers (ASR1K) as DHCP Server. So now i must add redundancy for DHCP too. I know we can use EEM, but with EEM theres no way (as far as i know) for the main router to know about IP assignment status and it may assign duplicate address after it came onlineagain.

Is there any solution to make this fix?

[Joseph W. Doherty]

#1 Interesting question - sorry, don't know for sure the answer (although it might work)

Of course, if what I'm saying is true, you would could just have helpers for all the DHCP servers.

#2 Not 100% certain, as it has been years since I've dug into DHCP operations at such a level of understanding, but I would expect there to be some like feature, if for no other reason, to deal with the case of a reloaded Cisco device (which I believe would lose all current DHCP lease information) - also it precludes a DHCP host using a IP used by another device that might be using it statically, unknown to the DHCP server.

[Richard Burts]

I have several comments:

- initially the discussion seemed to be about redundancy between 2 routers. Now we are told that there are a dozen or so routers involved. It would help us give better advice if we had a better understanding of the environment and the topology of the network.

- I would worry about depending on ping to prevent assignment of an IP address to two different hosts.

- There have been some mentions of HSRP. I believe that it would be advantageous to configure the default gateway in the pools to use the virtual address as the gateway. Using helper addresses pointing to the virtual address would make the choice of router/pool somewhat deterministic but you are likely to wind up with some addresses assigned by one router and other addresses assigned by the other router, especially if HSRP is configured using track.

- if using helper addresses and the helper address is to individual hosts, then redundancy would have a helper pointing to each router, therefore each router would receive a copy of all requests and would respond to each request that it saw So clients would generally receive offers of two addresses and would choose one.

I still think that we have a situation with mutually contradictory requirements and that means that any solution is going to have some disadvantages.

HTH

Rick

[Poorya Naji]

i will post the topology and more explanation for it, but i am 100% agree with you each solution will have its advantage and disadvantages.