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New Member

A doubt about DHCP forwarding

Hi all,

im cofinguring my Layer 3 switch to forwarding DHCP broadcasts to 2 DHCP servers, 192.168.0.1 and 192.168.0.2. I put the command "ip helper-address 192.168.0.1" "ip helper-adress 192.168.0.2" on the interface vlan.

The question is: For wich DHCP server will the Layer 3 switch forward the packets? to just one? Both...?

Thanks!

12 REPLIES
Blue

Re: A doubt about DHCP forwarding

Both. It will forward the DHCP request to both servers simultaneously, and the first DHCP server to answer will be the one whose offer is advanced to the client.

HTH

Victor

Hall of Fame Super Silver

Re: A doubt about DHCP forwarding

Tauer

Victor is correct that the L3 switch will forward the request to both servers. Both servers will respond and responses from both servers are sent to the client. The client chooses which DHCP offer to accept (usually the first one received).

HTH

Rick

Blue

Re: A doubt about DHCP forwarding

Hey, Rick:

Good catch. Thanks for straightening me out on that one. Its the client that makes the decision, not the switch. The client will get both offers. Got it.

Thanks

Victor

Re: A doubt about DHCP forwarding

I'm in the process of setting up a new network that must pass thru a firewall for DHCP. I have two helpers configured. In reviewing my firewall logs, only the first server is queried (maybe b/c it gets an address it never asks the second server). I'll put a rule in to block the first and I'll see if the second is then contacted.

New Member

Re: A doubt about DHCP forwarding

Thank you guys.

I'll test soon...

Any news... a I post here again...

I agree the test of firewall.. its a good idea.

Hall of Fame Super Silver

Re: A doubt about DHCP forwarding

Collin

That is quite interesting. Forwarding to the second server is quite independent of whether the client receives an offer from the first server. I believe that there must be some other explanation of the behavior that you describe. Is it possible that there is some flaw in the configuration of the second helper or that the path to the second helper avoids the firewall?

HTH

Rick

Re: A doubt about DHCP forwarding

DHCP was not working on one of the servers, and now that it is working, I now see requests coming through to both servers. Any ideas? Any precursor test to verify the server is available before sending a DHCP request?

Hall of Fame Super Silver

Re: A doubt about DHCP forwarding

Collin

The only precursor test that I can think of is a check of IP reachability (can you ping or traceroute to the server) and that only says that you can get to the server and not whether DHCP is running on the server.

HTH

Rick

New Member

Re: A doubt about DHCP forwarding

Both, it is TOP DOWN logic first it will go to 192.168.0.1 because it is on the top you have configured fist and 192.168.0.2 in the last because it is in down.

New Member

Re: A doubt about DHCP forwarding

Shambhu_k,

and if the first DHCP server (192.168.0.1) has no more IP to distribute? I think the second DHCP server will do that...am I right?

Hall of Fame Super Silver

Re: A doubt about DHCP forwarding

Tauer

If the first DHCP server has no more IP addresses to distribute then it will stop responding. As long as the second DHCP server has IP addresses to distribute it will continue to respond.

HTH

Rick

New Member

Re: A doubt about DHCP forwarding

Thats right Rick...

Thanks once more...

I didn't do this yet, but the answers here have are been so clarifyed to me...

Many thanks

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