DHCP Server Not Found

[ling20125]

Hi All,

  I'm having a very weird problem with the DHCP server on the network. I have a bunch of devices from different vendors that connects at a L2 switch whose uplink connects to a router. I'm using router-on-a-stick to carry all the VLAN information between the switch (3560) and router (2960). And I have a Windows 2008 as my DHCP server somewhere in the production network (I'm not in charge of that, but I have access). Each scope has the default gateway, DNS server, and DNS domain name configured.

  It couldn't be anymore simplier, I need to provide address for the devices from the DHCP server. I have doubled checked the DHCP server, and I can ping it from both the router and the switch. And on the switch, I can also ping the router's IP for each of its subinterfaces. The router also has set its 'ip helper-address' command on each of its sub-interfaces (Router-on-a-stick).

  Here is the dilemma: If I plug in a device, it's not going to get an IP address automatically (resulting in APIPA addresses). However, if I manually configure its IP address, subnet mask, and default gateway, I can ping everything.

  Now, here is what I'm thinking:

- The local network has a rogue DHCP server that I don't know about, and it doesn't have any scopes configured so no IP addresses can be assigned.

Result: I practically have checked every single port and I can't find a rogue DHCP server, at least not through its cables, anyway.

- The local network shuts down TCP port 69 (I believe the port for DHCP), therefore, ping works but not DHCP messages.

Result: I can't find a single ACL on the router or switch that does this.

  So, I ran out of options. And I want to ask, if anyone here knows such a situation and if so, what is the cause?

Regards,

Angela

PS: not sure if this is revelant, but I'm also experiencing MAC address appearing and disappearing. The end devices that are connected to the switch sometimes show up on the MAC address table, and sometimes they don't. And during this time, the port remains physically up.

[ling20125]

Anyone? I'm having difficulty grasp this, but I have some new information about the network. I have a machine, among the machines, that has an OS. I can verify that it didn't get an IP from the DHCP (even though the server can't ping this IP address, I wonder why). Another thing is that, I can ping the default gateway and every other server from the device, but on the servers and other equipments, I can't ping it back. In other words, right now, I have a one way ping.

  I thought it was the gateway problem at the router, so I configured static routes pointing to every single subnet that I have in the local testing network, but still no luck. What could be the cause of this?

Regards,

Angela

PS: the DHCP server is about 3 hops from the testing network.

[cadet alain]

Hi,

angela zou a écrit:
 I can verify that it didn't get an IP from the DHCP (even though the server can't ping this IP address, I wonder why).

The DHCP server is pinging the address he is offering to get sure there isn't a duplicate IP in the network but if this ping fails he considers this address is not duplicated but that doesn't guarantee the client will get it.

The gateway is the router doing the inter vlan routing? If it had been  a routing problem  the ping from the device wouldn't have worked. Did you verify if there was any ACL or firewall config on this router.

Can you remove these static routes and do the ping tests again but this time putting an ACL on all interfaces permitting ip any any log inbound and one outbound to see if traffic traverses the router.

Regards.

Alain.

[ling20125]

Hi Alain,

  Thanks for your answers, it turns out, these non-Cisco devices, all need some sort of pre-configuration to make sure they use DHCP. And sometimes, they ended up outside their binding address, but with some time, I finally figured it out. Thank you all for walking me through this patiently.

Regards,

Angela