Problem With IP Helper Across 3750 Switch

jgimma · ‎08-12-2014

I am having problems getting DHCP across a 3570 switch.

The network that I need DHCP for is: 192.168.6.0 (VLAN 2)

DHCP Server: 192.168.6.2 (VMWare; vswitch configured as VLAN 2)

| connected to 2960

interface GigabitEthernet1/0/31
switchport mode trunk
switchport nonegotiate
spanning-tree portfast trunk

| 2960 to 3750 trunk port

interface GigabitEthernet1/0/1
description Trunk (3750X)
switchport mode trunk

| 3750 trunk port

interface GigabitEthernet1/0/4
description Trunk (VM Rack)
switchport trunk encapsulation dot1q
switchport mode trunk
mls qos trust dscp

| 3750 to 2960 LAN switch

interface GigabitEthernet1/0/12
description P1 LAN
switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport mode trunk
mls qos trust dscp

| 2960 trunk port

interface GigabitEthernet1/0/1
description Trunk (3750X)
switchport trunk native vlan 2
switchport mode trunk

All other ports on the P1 LAN 2960 are configured the same:

interface GigabitEthernet1/0/x
description P1 LAN
switchport access vlan 2
switchport mode access

The VTP server is the 3750:

interface Vlan2
ip address 192.168.6.1 255.255.255.0
ip helper-address 192.168.6.2

I can connect a PC with a static IP to the P1 LAN switch and that works, but I cannot get a DHCP assignment.

Any suggestions would be appreciated.

Thanks >> Joe

Rajeev Sharma · ‎08-12-2014

Hey Joe,

From this topology, I believe you have interface vlan 2 either on 2960 or 3750 (VTP server), so can you ping the 192.168.6.2 from that switch alternatively i want you to check the connectivity between DHCP server and relay agent.

HTH.

Regards.

RS.

jgimma · ‎08-12-2014

Yes, you are correct.

The 3750 is the VTP Server with the VLAN configs.

I can Ping from the DHCP server all the way across to a PC on the 2960 being used for the P1 LAN. The PC also works fine on the domain, when configured with a static address. It too can ping across to the DHCP server.

The only problem at this point is obtaining a DHCP lease.

I could not find instructions on how to test DHCP connectivity.

Also, we do not have a DHCP relay. I thought that the IP-HELPER setting would take care of this.

Thanks >> Joe

Rajeev Sharma · ‎08-12-2014

Hey Joe,

On the 3750 VTP server switch:

1. Try configuring service dhcp in global configuration mode and check if machine gets an IP address via DHCP,

2. If not, you may need to configure debug dhcp server packet on the 3750 VTP server switch.

HTH.

regards,
RS.

jgimma · ‎08-12-2014

Rajeevsh,

The 3750 is acting as a DHCP server for VLAN 3.

Are you suggesting to configure a DHCP scope for VLAN 2 as a test?

Thanks

Rajeev Sharma · ‎08-12-2014

If 3750 is already DHCP server for vlan 3 so i think creating a test scope for vlan 2 will help narrowing the issue. Try making the switch as DHCP server for vlan 2 as well and check if machine gets an IP address.

Regards,
RS.

jgimma · ‎08-13-2014

Rajeevsh,

Sorry for the false start.

I am remote and the onsite IT spun up the DHCP server. When I checked it, they had left the Allow Filter Policy enabled for the scope with no MAC addresses in the list. This was preventing everything from getting an IP.

DHCP is working now. The switch config was correct after all.

Thanks for the help.

Joe

Rajeev Sharma · ‎08-13-2014

:)