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Secondary DHCP server needed, can only broadcast one helper add's

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Last Modified: 2019-08-12
We have multiple sites all on different subnets, we have one central DHCP server which I want to change (so I can do maintenance).

I want to add a second DHCP server (for redundancy) but I can only broadcast one helper address from the site routers.  I have read up on virtual IP's but am unsure if this is the correct way to go.

Notes.
We have multiple DC's in different geographical locations
Routers are part of our MPLS so I cannot make changes (I can have the helper add changed)
All servers are in the hosted data center (VM's)

All help/advice gratefully received.
Steven
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Commented:
you probably should setup dhcp servers in each location. When the main site is down, you will not loose everything

Most dhcp servers can operate together with zeroconf because they will ping addresses before issuing a lease. Most dhcp server can work more reliably as pairs if you configure them so they know they have a sibling. In that case, you need both to be the same soft. Buf you can rely on the ping while transitionning from one setup to another
OllecoTechy

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Commented:
Its a MPLS with all servers in one location, so a DHCP at the sites is useless.  I want to run two DHCP servers in the datacentre for redundancy.

Problem is I can only broadcast one helper address.
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Commented:
What do you mean by "helper" ? If that is the gateway, yes carp, vrrp or whatever shared virtual address would be the way. Dhcp servers do not need to shared addresses
atlas_shudderedSr. Network Engineer
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Commented:
What kind of network equipment do you have sitting downstream from the MPLS routers?  Switches?  Are they layer 2?  Layer 3?
Shaun VermaakSenior Consultant
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Commented:
I want to add a second DHCP server (for redundancy) but I can only broadcast one helper address from the site routers.
Are you sure?

I have read up on virtual IP's but am unsure if this is the correct way to go.
If you cannot do multiple helpers your only option is VIP -OR- manual server IP change
atlas_shudderedSr. Network Engineer
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Commented:
Above is not necessarily a complete answer above.  Still waiting to hear what type of equipment is in use on the network....
OllecoTechy

Author

Commented:
Lowest denominator is Netgear smart switches on some sites.

Each site has a Cisco router but we do not have any access to the config, these connect to our MPLS.
atlas_shudderedSr. Network Engineer
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Commented:
where are your helper addresses?  if they are on the router, then (unless you are running a small soho) you should be able to run multiple helpers. you note you dont have access to them.  are they carrier routers?

otherwise you can go the route of trying to build a cluster but that is problematic as a general rule.
OllecoTechy

Author

Commented:
Hi Atlas,

The helper address is broadcast at each site from the routers (which we have no access to).  Each site is currently directed to one DHCP server in the DC.  I am being told by the WAN provider they can only broadcast one helped address to each site.

Thanks,
Steven
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Commented:
if the helper target is the regular unicast address of the dhcp server, a shared address will work.
atlas_shudderedSr. Network Engineer
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Commented:
Olleco - what kind of routers are being used by your provider?  make/model?
OllecoTechy

Author

Commented:
They are a mix of Cisco routers - most being 2900's
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Commented:
multiple "helpers " in cisco terms does not make much sense. Using a broadcast address as the helper would produce lots of useless traffic on the link.

So you are stuck with a shared ip. Most likely a multicast address would also work but i see little point in bothering witb the setup.

you may also consider multiple teaming dhcp servers. One or a pair per location. The servers would exchange the leases each of them provide. You can keep the helper for failover if you keep a single server per location.

You may also consider a dhcp proxy in each location. But that would be as painful to setup as an actual server and produce less resilience.
Sr. Network Engineer
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Commented:
On the cisco 1900's, if they are telling you that they can't configure more than one helper, they are;

1.  Ignorant of their own equipment
2.  Incompetent or
3.  Just plain lazy

Regarding cisco helper addresses - these are simply listener addresses on a given link which will listen for the DHCP client broadcast, convert it to a unicast and then forward it directly to the DHCP servers configured in the address.  In the instance of two helpers, a unicast is sent to both devices configured and the one that the client sees respond back first wins.

In short, contrary to some of the comments made above, you are not immediately limited in your solution options, you just need to put some pressure on your carrier(s) to step up.  Go back to the carrier and pound on them.  For those sites where they have not provided cisco equipment, tell them to get it figured out.
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Commented:
hmm : i figured the author wants to forward one single helper address to two different dhcp servers. which does not make much sense.

if he actually needs a single dhcp server to handle multiple separate lans, i concur. one "helper" per lan is trivial to setup.
OllecoTechy

Author

Commented:
Advice taken - will lean on provider for help.

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