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?

[Deepak Kumar]

No. Then you will go with Windows server 2012 or 2016. Cisco is not having this type of facility. 

Regards,

Deepak Kumar

[Poorya Naji]

Unfortunately using external server is out of scope, i must use available devices. No idea?

[Deepak Kumar]

Then you can do that take a DHCP scope and divide into two scopes. And configured one scope on each device. It's not a redundancy but it will decrease the load and will work as secondary if anyone goes down. 

Regards,

Deepak Kumar

[Karsten Iwen]

Make your DHCP-scope big enough (or your segments small enough) and split the scope in two parts. Each part is configured on one router. This is a "traditional" low-cost redundancy implementation.

[Poorya Naji]

Yes thats the thing i was working on it at first place, but theres some address sacrifice, and in Operator world, its not tolerable. i am realy confused what to do!!!!! 

[Poorya Naji]

one Q, what if we use HSRP + ip helper address and set the ip helper address for the HSRP IP address?

[Georg Pauwen]

Hello, 

are you talking about redundancy on the same router, or on two different routers ? That is, do you have one or two ASR1Ks ?

[Poorya Naji]

Hi,

Redundancy between 2 or even more routers and i have multiple ASR1k in my network.

[Georg Pauwen]

Hello,

just configure the same pools on two or more routers, if one is down, broadcast traffic will go to the next reachable router. 

[Joseph W. Doherty]

If I remember correctly, there should be a DHCP ping check for a duplicate IP before it's used, if so, even if two Cisco routers are working from the same address pool, all should function correctly. The only issue with overlapping IP pools, it may take longer for a host to obtain an "available" IP because the DHCP servers wouldn't really know whether an IP is available, or not, until they try to assign it. (Actually, believe this would be similar to having a single Cisco device providing DHCP IPs and it reloaded.)

[Richard Burts]

It seems to me that the original poster has some mutually contradictory requirements. To the suggestion that they use a more sophisticated server that could provide redundancy he says that it is out of scope and the solution must run on the router. To the suggestion of the low tech alternative of defining a separate pool on each router he says " in Operator world, its not tolerable". I can not think of an alternative that satisfies both requirements. I suggest that the solution is to relax one of those requirements.

HTH

Rick

[Poorya Naji]

Hi Richard,
Thanks for your reply, I am agree with your point that its best to chose External servers, but as i mentioned because of some silly mistake, we couldn't go for that external Server and we are left alone with Routers. the problem with spliting pools in not only loosing some IP address (we are talking about at least a dozen routers which must group 2 by 2 for redundancy so it means 12 pools which 6 of them are backup and will not be used unless main router goes down) another problem i have is with, what will happend to user which got their address from main router and the GW for them was that router so when that router goes down backup came up with the other half of pool and different GW.
So thats a nasty problem which i am wondering how to solve it without using external servers.
any idea?
is there any way for dhcp pool synchronization between routers?

Regards

[Joseph W. Doherty]

Perhaps I was unclear in my earlier post - i.e. I don't believe you need to split address pools. If I'm correct, address pools space will not be wasted.

As to the gateway issue, traditionally that's "solved" using a FHRP (i.e. HSRP, GLBP, VRRP).

[Poorya Naji]

2 Questions:
1. can i use the Virtual IP of HSRP for the ip helper-address?
2. r u sure about that ping u mentioned earlier?

Thanks