Core Switch not forward DHCPOFFER packet

[kokkiong77]

Hi,

I am configuring a Catalyst 4000 layer 3 switch. I have 6 VLAN and subinterfaces for inter-VLAN routing.

I plug a PC to one of the VLAN but couldn't get an IP address from DHCP server. This only happens when PC and DHCP server are on different VLAN. I have ip helper-address configured. ip directed-broadcast is enable on DHCP server VLAN. Using Static IP address is working.

I did a debug and found that the Core Switch received PC's DHCP DISCOVERY packet and forward it to DHCP server. DHCP responded with DHCPOFFER but the Core Switch did not forward it to the PC.

Since PC not receiving DHCPOFFER, it keeps generate DHCPDISCOVERY packet.

Kindly advice. Thank you very much.

[sourabhagarwal]

Hi,

I understand from your post that you have configured helper address and ip directed broadcast on DHCP server VLAN.

you have to configure helper address command on the VLANs configured for PCs to which users will be connceted to.

cisco recommends not to enable directed broadcast command on your VLAN interfaces.

all you need to do is just to configure ip helper-address command on all the VLANs which has to get IP from DHCP.

hope it helps ... rate if it does ...

[royalblues]

In addition to the above post if you are having any access-list configured on the VLANs, make sure it is permitting both UDP 67 & 68

HTH

Narayan

[kokkiong77]

Hi Royalblues,

Thanks for your reply. When i turned on the debugging on the core switch, i can see that the relay agent is receiving DHCP packets (UDP 67 & 68). Then the relay agent actually forward these packets to DHCP server. The DHCP server responded with DHCPOFFER. Again, the relay agent did received the DHCPOFFER from DHCP server. The funny thing is that the relay agent did not forward the DHCPOFFER back to the PC.

I have checked if any access-list is configured to drop UDP traffic but can't find any.

Thanks for your help.

CHeers !

[ankbhasi]

Hi Friend,

Can you confirm that the IP which DHCP pool may be returning in its DHCPOFFER is not assigned to any other clients or netwpork device anywhere?

Can you attach the complete sniffer trace here?

Ankur

[kokkiong77]

Hi Ankur,

The IP in DHCPOFFER is not assigned.

Please find the debug LOG below. The DHCP relay agent don't seems to forward the DHCPOFFER back to PC. The PC did not received the DHCPOFFER and keeps generate DHCPDISCOVERY. This pattern repeat again and again. The DHCP relay agent just stop after it got the DHCPOFFER from DHCP server.

PC is in VLAN 50 (Gateway is 172.16.50.1)

DHCP server (IP 172.16.10.30) is in VLAN 10

Dec 8 12:20:08.395 SG: UDP: rcvd src=0.0.0.0(68), dst=255.255.255.255(67), length=325

Dec 8 12:20:08.395 SG: BOOTP: opcode 1 from host 0.0.0.0 on Port-channel1.50, 3328 secs, 0 hops

Dec 8 12:20:08.395 SG: UDP: forwarded broadcast 67 from 172.16.50.1 to 172.16.10.30 on Port-channel1.10

Dec 8 12:20:08.395 SG: UDP: rcvd src=172.16.10.30(67), dst=172.16.50.1(67), length=314

Dec 8 12:20:08.399 SG: BOOTP: opcode 2 from host 172.16.10.30 on Port-channel1.10, 0 secs, 0 hops

[kokkiong77]

Hi Sourabhagarwal,

Thanks for your reply. ip helper-address is already configured on the rest of the VLAN except the VLAN where the DHCP is located.

i have to enable directed broadcast on the DHCP server VLAN otherwise the DHCPDISCOVERY packets will not be "broadcast" into DHCP server VLAN - according to CISCO DHCP troubleshooting procedures.

Thanks anyway. Cheers

[royalblues]

Hi Friend,

Are you facing this issue across a particular vlan or across all the VLANs?

Narayan

[kokkiong77]

Hi,

Its across all the VLANs.

i have additional info for your reference. The only situation it will work beside configure static IP address for that particular VLAN is as below:-

Let say currently my laptop is in VLAN 2 and i would like to go to VLAN 4. If i unplug cable from VLAN 2 and plug it to VLAN 4 it will not work.

i need to do a release IP on VLAN 2 and plug it to VLAN 4 then it will grab IP from DHCP server.

Thanks.

[mahmoodmkl]

Hi

In addition check weather u have configured portfast under u r interfaces.

Mahmood