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New Member

Multiple IP helper destinations

What is the default behavior if for multipe ip helper destinations defined on an interface?

Example:

int VLAN 400

ip helper-address 1.1.1.1

ip helper-address 1.1.1.2

Does the router first attempt to forward the appropriate traffic to 1.1.1.1, failing that then send it to 1.1.1.2? Or does it send the traffic to both and use whichever offer it receives first? OR is there something else it does?

3 REPLIES
Cisco Employee

Re: Multiple IP helper destinations

Hi Friend,

Yes the router will send request to both the servers and will forward the reply from both the servers to the client.It depends upon client what address it chooses.

The whole process will go something like this

1) Router will send request to both the servers

2) The DHCP servers returns a DHCP Offer packet and router will forward the reply from both the servers to the client.

3) The PC chooses from the different servers that replied to its Discover message and sends a DHCP request to one of them. Remember, the PC still does not have an IP address at this point, so it broadcasts.

4) The DHCP relay agent again forwards the packet to the DHCP server. The relay agent does have an IP address, so it unicasts the packet to the server using UDP.

5) The DHCP server selects an address from an appropriate pool of IP addresses and returns a DHCP reply to the PC, which now has its own IP address and with it the default gateway address.

HTH

Ankur

*Pls rate all helpfull post

Hall of Fame Super Silver

Re: Multiple IP helper destinations

Jeremy

The router will forward the appropriate traffic to both addresses. If the device that originated the request receives two offers (which it probably will since both servers will receive copies of the request) it will choose one of the offers (typically the first one received).

HTH

Rick

New Member

Re: Multiple IP helper destinations

DAMN! When I asked this questions I was 99.9% sure that it worked this way, but I was hoping beyond hope that it didn't.

Thank you

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