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Webcast-Catalyst9k
New Member

VLAN "ip address dhcp" issue

Overall, I'm having issues with a VLAN interface (let's say, vlan100) receiving an IP address through DHCP. The network topology is set up similar to this-

[DHCPserver] - - - -((network cloud))- - - - [3650] -g1/0/1- - - - - - g1/1/1- [3650]

Both interfaces are configured as a trunk link using 100 as native vlan. Troubleshooting steps- test hosts were connected to g1/0/1 and were able to get IP addresses just fine. Also, a static IP was placed on vlan100 and was able to ping out. A sniffer was placed on a spanned port of g1/0/1 and found that it was receiving requests from the client switch but no replies.

dhcp was debugged on the client switch with this output:

*Aug 29 14:17:02.823: DHCP: DHCP client process started: 10
*Aug 29 14:17:02.824: RAC: Starting DHCP discover on Vlan100
*Aug 29 14:17:02.824: DHCP: Try 1 to acquire address for Vlan100
*Aug 29 14:17:02.826: DHCP: allocate request
*Aug 29 14:17:02.826: DHCP: new entry. add to queue
*Aug 29 14:17:02.826: DHCP: MAC address specified as  0000.0000.0000 (0 0). Xid is EFD
*Aug 29 14:17:02.830: DHCP: SDiscover attempt # 1 for entry:
*Aug 29 14:17:02.831: DHCP: SDiscover: sending 287 byte length DHCP packet
*Aug 29 14:17:02.831: DHCP: SDiscover 287 bytes 
*Aug 29 14:17:02.831: B'cast on Vlan100 interface from 0.0.0.0
*Aug 29 14:17:06.337: DHCP: SDiscover attempt # 2 for entry:
*Aug 29 14:17:06.337: DHCP: SDiscover: sending 287 byte length DHCP packet
*Aug 29 14:17:06.337: DHCP: SDiscover 287 bytes 
*Aug 29 14:17:06.337: B'cast on Vlan100 interface from 0.0.0.0
*Aug 29 14:17:10.334: DHCP: SDiscover attempt # 3 for entry:
*Aug 29 14:17:10.334: DHCP: SDiscover: sending 287 byte length DHCP packet
*Aug 29 14:17:10.334: DHCP: SDiscover 287 bytes 
*Aug 29 14:17:10.334: B'cast on Vlan100 interface from 0.0.0.0%Unknown DHCP problem.. No allocation possible
*Aug 29 14:17:23.642: DHCP: Waiting for 5 seconds on interface Vlan100

2 REPLIES
Cisco Employee

Hi,A couple of questions:It

Hi,

A couple of questions:

  1. It it possible to run a debug on the DHCP server or capture packets on the DHCP server to see if the requests from the client switch truly arrive and perhaps why they are not being replied to?
  2. Would it be possible to post the DHCP communication dump captured during the SPAN session on the gi1/0/1 port? Ideally, a PCAP file for Wireshark analysis would be very helpful.
  3. Is there any other configuration on the interface Vlan100 beyond ip address dhcp?
  4. I am surprised to see that the debug on the client switch says "MAC address specified as 0000.0000.0000". Do you have any idea where this comes from? What is the MAC address of the interface Vlan100?

Best regards,
Peter

New Member

Thank you for the response,1.

Thank you for the response,

1. From what I can see in the logs of the dhcp server, there is no receipt of a dhcp request from the switch. I may be overlooking something or looking in the wrong spot so I'll continue to look into this.

2. My wireshark crashed over the weekend so I'll do a recapture and post it for you when I get a chance.

3. There is no other configuration on vlan100 other than that command.

4. I thought this looked funny as well. Here is the output of 'sh int vlan 100' (I shut down the interface for the time being). Is this the information you're looking for?

Vlan100 is administratively down, line protocol is down
   Hardware is Ethernet SVI, address is f40f.1b47.6c4e (bia f40f.1b47.6c4e)

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