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I have a scenario like this topology

Customer End device --> Customer Switch --> My Cisco Router --> My network --> My Cisco Router --> Customer Switch --> Customer End device

The only part of the network where I can view the config is on

My Cisco Router && My network

The customer's end device needs to get a DHCP address assigned to them, and their DHCP server lives at a Data Center, across my network.

The customer states that they are sending DHCP requests but they are not being received.

I am making the 'assumption' that the switch is using a DHCP helper address, I say 'assumption', because nobody will tell me YES or NO.

The same customer has other working scenarios on my network.

So, I want to prove/disprove what is happening with the DHCP requests that are being sent from the customer end device.

I want to enable debugging on My Cisco Router, but what debugging do I enable?

  • debug ip dhcp server packet - don't get any hits
  • debug ip udp port 67 - don't get any hits
  • debug ip udp port 68 - don't get any hits

So if the customer has IP helper, I think this will send a replicated unicast to My Cisco Router, I think this is TCP rather than UDP.

I don't think I can debug a particular port, because I think when My Cisco Router receives the packet, it will only look up the OSI model until Layer 3, and then realize this is not for me, and send it on its way.

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  • If the customer switch is a layer-2 switch, you need to put the IP helper on your router.
    – Ron Maupin ♦
    Dec 17, 2015 at 5:43

1 Answer 1

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DHCP is a link-local protocol. (broadcast) For it to span networks (broadcast domains, the internet, etc.) there has to be a relay-agent. (or more than one) That agent listens for requests (unicast and broadcast) and transmits them on to some other server. In theory that server can be anywhere, but some (many?) DHCP servers aren't happy getting requests from non-local agents. (like the DHCP server in windows 2000) Also note, there is a maximum size for DHCP requests; each relay adds to the packet.

I would recommend packet captures. (debug ip packet, with an ACL defined to catch dhcp) As your router(s) aren't a DHCP server or relay-agent, debug ip dhcp isn't going to get anywhere.

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  • Sure enough, the access-list trick worked, thanks Ip access-list extended 234 permit udp any any eq bootps log permit any any debug ip packet 102 detail
    – psniffer
    Dec 17, 2015 at 6:15

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