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New Member

Multiple IP Helpder-Address

Hi,

Can someone tell me what the effect of having multiple ip helper-address commands on the same interface is ?

Do the broadcasts get "load balanced" to each of the IP's in turn, do they get send to the second IP if the first is down or do they just get sent to both IP's all of the time?

Thanks,

Ellis

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Re: Multiple IP Helpder-Address

Hi Edixon,

The broadcast will only reach till the layer 3 interface where multiple helper address is configured bur after that it will be a unicast to multiple DHCP addresses whatever is configured in ip helper command and as Narayan updated the DHCP request will go to each and every DHCP server what is configured in helper address command and all the servers will reply and it depends on client which one to take.

HTH

Ankur

*Pls rate all helpfull post

Hall of Fame Super Silver

Re: Multiple IP Helpder-Address

Ellis

Yes, if you have configured two helper addresses and a broadcast packet comes in the interface then two copies will be transmitted (one to each of the addresses configured in the helper address). There is not any alternation or anything like that - a copy sent to each helper destination every time.

HTH

Rick

9 REPLIES
New Member

Re: Multiple IP Helpder-Address

Hi,

I beleive that the effect will be the same as when you have multiple DHCP servers on your LAN segment. I don't think that there will be any real "load balancing".

//Mikhail Galiulin

Re: Multiple IP Helpder-Address

Hi Friend,

Having multiple helper address doesn't really load balance.

When a user sends a DHCP broadcast it is relayed to both the DHCP server.

Both the servers then send the response which is forwarded to the host

Now the client choses the DHCP server and sends a DHCP request.

HTH, rate if it does

Narayan

New Member

Re: Multiple IP Helpder-Address

So to clarify, the effect will be all broadcasts will get directed to both addresses all of the time?

Re: Multiple IP Helpder-Address

HI

In addition i would like to put it like this.The reason to have multiple DHCP servers is redundancy.one server acting as primary and the other secondry.so if one server is out the other can serve client requests for address.

Thanks

Mahmood

New Member

Re: Multiple IP Helpder-Address

Hi,

Nobody has answered my original question !!!!!

What is the effect of having multiple ip helper-address commands on a single interface?

Is it that broadcasts get sent as directed unicasts to ALL of the IP's listed on the ip helper lines ALL of the time?

Cisco Employee

Re: Multiple IP Helpder-Address

Hi Edixon,

The broadcast will only reach till the layer 3 interface where multiple helper address is configured bur after that it will be a unicast to multiple DHCP addresses whatever is configured in ip helper command and as Narayan updated the DHCP request will go to each and every DHCP server what is configured in helper address command and all the servers will reply and it depends on client which one to take.

HTH

Ankur

*Pls rate all helpfull post

Purple

Re: Multiple IP Helpder-Address

It is not a broadcast once it hits the router , it gets modified into a directed unicast in the case of a specific helper address or a directed broadcast in which case the router at the far end will turn it back into a broadcast if the helper address is like 10.2.3.255 then everyone on the far end subnet will hear and the dhcp server on that subnet will reply .

New Member

Re: Multiple IP Helpder-Address

Glen,

Thanks for the clarification.

To clarify again : The original broadcast will get sent as a directed unicast to both of the addresses specificed by the IP Helper-Address commands?

Thanks,

Ellis

Hall of Fame Super Silver

Re: Multiple IP Helpder-Address

Ellis

Yes, if you have configured two helper addresses and a broadcast packet comes in the interface then two copies will be transmitted (one to each of the addresses configured in the helper address). There is not any alternation or anything like that - a copy sent to each helper destination every time.

HTH

Rick

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