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DHCP Relay - ip helper-address

Posted on 2008-10-28
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Last Modified: 2012-05-05
Hello,

If your Router is working as a DHCP Relay and you have more than one ip helper-address command what actually happens:

1. The Router forwards the DHCP Discover to the first DHCP Server and if this one does not respond then the Router works sequentially down the list until a Server makes a DHCP Offer to the Client?

2. The Router forwards the DHCP Discover to all DHCP Servers defined by the multiple ip helper-address commands and the first Server to respond with a DHCP Offer Message?

Thanks in advance :)
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Question by:MarcBorst
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by:Darksied9
ID: 22821544

1. The Router forwards the DHCP Discover to the first DHCP Server and if this one does not respond then the Router works sequentially down the list until a Server makes a DHCP Offer to the Client?

This is accurate, I would suggest configuring the DHCP servers with the 80/20 Rule.  Put 80% of each subnets available addresses on the primary with 20% on the backup.
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btassure earned 50 total points
ID: 22821553
It passes the broadcast to all the listed ip-helpers. After that the router doesn't care about the traffic other than passing it back and forth. So 2) is the way it works.

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by:btassure
ID: 22821567
Are you sure Darksled? Our implementation works the same as point 2...
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by:JFrederick29
ID: 22821838
The router handles it via option 2:

2. The Router forwards the DHCP Discover to all DHCP Servers defined by the multiple ip helper-address commands and the first Server to respond with a DHCP Offer Message?
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by:Darksied9
ID: 22821893
Ha!  Sorry, I miss-read that.  Basically you are right option 2 is the way it works, however...  When you deploy a DHCP service scenario you should remember that the client will accept the DHCPOffer from the first server that responds to the DHCP request.  Keeping this in mind, the 80/20 rule is good for redundancy especially if you have more than 1 subnet (which you do if you are talking about helper addresses).

So, place your two DHCP servers in two different subnets (site differentiated if you can) and Set one to server 80% of the leases for the subnets nearest it (in terms of latency) and the other for 20%.  Do the same with the other.  Be sure not to overlap the address spaces at all.

Sorry for the error, I will be sure to double check the questions next time.  :)
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