DHCP Relay Problem

[thestagman]

I have a problem passing DHCP requests via L3 Switch (3750G) running 3750-ipbase-mz.122-25.SEB4.

To keep it simple I have 1 layer 3 switch, with 2 Vlans on different Class C networks and I'm using RIP ver 1 to route between both networks.

On one network I have a DHCP Server on the other I have a DHCP Client. On the client side router interface I have "IP Helper-Address 192.168.200.255" I have also tried the actual DHCP server address 192.168.200.247.

I cannot seem to get the client to pick up an IP address from the server..

I have performed a debug and get the following output:-

18:16:15: IP: s=0.0.0.0 (Vlan100), d=255.255.255.255, len 328, rcvd 2

18:16:15: IP: s=0.0.0.0 (Vlan100), d=255.255.255.255, len 328, stop process pak for forus packet

18:16:15: IP: s=192.168.100.1 (local), d=192.168.200.247 (Vlan200), len 328, sending

18:16:15: IP: s=192.168.100.1 (local), d=192.168.200.247 (Vlan200), len 328, sending full packet

18:16:15: IP: s=192.168.200.247 (Vlan200), d=192.168.100.1, len 176, rcvd 2

18:16:15: IP: s=192.168.200.247 (Vlan200), d=192.168.100.1, len 176, stop process pak for forus packet

The above output shows the broadcast leaving the client network and the relay 192.168.100.1 (Client Router I/F) passing the packet onto the DHCP Server and the packet coming back... But the client does not pickup the address.

I have used IP helper address numerous times in the past with no problems. ALthough this is the 1st time I have used it on a 3750g.

Does anyone have any advice ?

Thanks

Mike

[sourabhagarwal]

Hi Mike,

can you post configs of VLAN on 3750 ?

also check for service dhcp command on your switch

[thestagman]

Hi

Here is the config as requested.....

Not sure I understand what you mean by service dhcp commands?

interface GigabitEthernet1/0/1

description Client Network

switchport access vlan 100

switchport mode access

spanning-tree portfast

interface GigabitEthernet1/0/48

description DHCP Server

switchport access vlan 200

switchport mode access

spanning-tree portfast

interface Vlan100

ip address 192.168.100.1 255.255.255.0

ip helper-address 192.168.200.247

!

interface Vlan200

ip address 192.168.200.1 255.255.255.0

!

router rip

network 192.168.100.0

network 192.168.200.0

[sourabhagarwal]

config seems to be fine.

I mean to check if "no service dhcp" command is set on your 3750 ?

also try below:

1. Configure static IP to PC from VLAN 100, say 192.168.100.250/24 and gateway 192.168.100.1 and see if PC can ping gateway, DHCP server and other resources. If PC is able to ping, then there is a problem during DHCP process/DHCP scopes.

2. make switchport connected to PC a member of VLAN 200, same in which DHCP server resides and see if PC is getting IP from server. Just to isolate if inter-vlan routing is a problem.

[thestagman]

Hello , Sorry for the delay

There is layer 3 connectivity between the client and the DHCP server. I have also tested the DHCP server on the same Vlan as the client and it all works well.

With the DHCP Server and Client PC on two seperate networks it does not work.

I can tell you that the DHCP server recieves the request for an IP Address and allocates an address. But that address never reaches the client PC.

All interfaces are up and running and there is still layer 3 routing between the 2 vlans.

The "no service dhcp" has not been set on the 3750..

Cheers

Mike

[glen.grant]

Does the nic show an address . Like you said it looks to be allocating the address then stops . It almost sounds like the server is not allocating the default gateway to the nic so it can not finish the allocation process.

[thestagman]

I have just ran an analyser on both devices. I can see the DHCP Discover packets leaving the client Lan and can see the DHCP Relay, the Clients Local Router interface collecting the "DHCP Discover" packet and passsing them on to the DHCP Server.

The DHCP Server responds with a "DHCP Offer" to the DHCP Relay Router Interface...

But nothing arrives at the client..

Immediatley after the DHCP offer Packet, there is a ICMP destination unreachable (Port Unreachable) between 192.168.200.1 (The DHCP Vlan Gateway) and the 192.168.200.247 (DHCP Server). I can't explain this as there is layer 3 connectivity.

The mac address of the Client machines appears in with lease display along with the assigned mac address....is this what you mean ?

[thestagman]

I think the ICMP destination unreachable maybe a red herring..

[mahmoodmkl]

Hi

can u verify that have u configured proper scopes for the different subnets.Can u try configuring different scopes instead of a superscope.

Thanks

Mahmood

[thestagman]

Hi

I have the scope set up for 10 addresses, starting at 192.168.100.10/24. The gateway address is 192.168.100.1. These are located on a DHCP server located on the 192.168.200.0 network

[gavin.reid]

Hi Mike,

Did you ever fix this? I have the exact same problem.

Thanks.