DHCP Relay Question

[udo.konstantin]

Hello,

in a given environment with nexus 55xx switches connected with fabricpath and vPC+ there are also Catalyst Switches 2960 connected via port-channel to the Nexus vPC Switches

On a Wireshark trace we found DHCP Offer packets on the 2960X Port-channel with the source from the DHCP server and destination to the Nexus SVI (Relay Agent).

Normally this packets should be here because the "DHCP Offer" Packet should have the IP Address of the Relay Agent (SVI)?!

Can anyone confirm this?

Thanks

[Philip D'Ath]

A DHCP relay takes a locally broadcast DHCP request from a client and unicasts it to the DHCP server.  The DHCP server sends a unicast reply back to the DHCP relay, which then passes it back to the host on the local subnet.

What you are seeing sounds correct to me.

[udo.konstantin]

I agree with you. The DHCP Server sends the packet back to the Client. But when we measure in the Access Switch Port-Channel I expect the unicast back to the Client with the source IP from the relay agent and not from the server direct!? Maybe I'm wrong and cisco relay agent on nexus works different!?

[Philip D'Ath]

It is trying to send to the client the IP address it should use.  How can it unicast to the client the IP address it should use - when the client doesn't know which IP address it should use because it has not received it?  A chicken and egg scenario.