ip helper-address

ip helper-address

man, this is really ticking me off. I have set the ip helper-address on the router interface closest to the client, which should be recieved a dhcp address, but simply is not. i'm not having much luck with what i thought was going to be a very simple configuration...

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Netstudent

Do you have wireshark to confirm the that 1. the DHCP server is listening for DHCP discover and 2. the client is actually sending a discover? Can you ping the dhcp?

Is this dhcp superscoping or providing dhcp service to clients in its local subnet?

Any UDP based ACL's?

Try a "debug ip udp".

Try a "ip forward-protocol udp"

cisco_trooper

Netstudent wrote:

Do you have wireshark to confirm the that 1. the DHCP server is listening for DHCP discover and 2. the client is actually sending a discover? Can you ping the dhcp?

Is this dhcp superscoping or providing dhcp service to clients in its local subnet?

Any UDP based ACL's?

Try a "debug ip udp".

Try a "ip forward-protocol udp"

This DHCP Server is supposed to be providing ip addresses to 4 different subnets across two different routers. I have configure the ip helper-address on both of those devices for the client interfaces, so that those devices know to forward the BOOTP and DNS traffic received on those interfaces. This DHCP Server has successfully assigned address on it's own subnet, but for some reason is not assigning addresses across the two relay devices. There are no ACLs configured anywhere.

Please forgive my router names, I was using this for redistribution a few days ago. Don't worry, there are no remnant configs.

ISIS - DHCP Server
*Feb 27 22:09:18.563: UDP: rcvd src=0.0.0.0(6

, dst=10.1.0.1(67), length=584
*Feb 27 22:09:18.571: DHCPD: Sending notification of DISCOVER:
*Feb 27 22:09:18.571: DHCPD: htype 1 chaddr ca04.0e88.001c
*Feb 27 22:09:18.575: DHCPD: remote id 020a00000a01000110000000
*Feb 27 22:09:18.575: DHCPD: circuit id 00000000
*Feb 27 22:09:18.579: DHCPD: DHCPDISCOVER received from client 0063.6973.636f.2d63.6130.342e.3065.3838.2e30.3031.632d.4661.312f.30 on interface FastEthernet1/0.
*Feb 27 22:09:18.583: DHCPD: Seeing if there is an internally specified pool class:
*Feb 27 22:09:18.583: DHCPD: htype 1 chaddr ca04.0e88.001c
*Feb 27 22:09:18.587: DHCPD: remote id 020a00000a01000110000000
*Feb 27 22:09:18.587: DHCPD: circuit id 00000000
*Feb 27 22:09:18.591: DHCPD: there is no address pool for 10.1.0.1. <- See comments on Fa1/0
!
ip dhcp pool 10.1.0.8
network 10.1.0.8 255.255.255.248
default-router 10.100.100.3
dns-server 10.100.100.2
lease 10
!
ip dhcp pool 10.1.0.16
network 10.1.0.16 255.255.255.248
default-router 10.100.100.3
dns-server 10.100.100.2
lease 10
!
interface FastEthernet1/0
description ASBR1:Fa1/1
ip address 10.1.0.1 255.255.255.248 <- Why is DHCP looking for a pool for this address? The request is being relayed from another segment. I want it to look for THAT pool...

duplex auto
speed auto
!
ASBR1 - DHCP Relay
*Feb 27 22:09:21.587: UDP: rcvd src=0.0.0.0(6

, dst=255.255.255.255(67), length=584
*Feb 27 22:09:21.591: UDP: forwarding packet 255.255.255.255(6

to 10.1.0.1(67)
*Feb 27 22:09:21.595: UDP: forwarding packet 255.255.255.255(6

to 10.1.0.1(67)
*Feb 27 22:09:21.595: UDP: forwarded broadcast 67 from 0.0.0.0 to 10.1.0.1 on FastEthernet1/1
!
interface FastEthernet1/1
description ISIS:Fa1/0
ip address 10.1.0.2 255.255.255.248
duplex auto
speed auto
!
interface FastEthernet2/0
description OSPF:Fa1/0
ip address 10.1.0.9 255.255.255.248
ip helper-address 10.1.0.1
duplex auto
speed auto
!
interface FastEthernet2/1
description EIGRP:Fa1/0
ip address 10.1.0.17 255.255.255.248
ip helper-address 10.1.0.1
duplex auto
speed auto
!
OSPF - DHCP Client
!
interface FastEthernet1/0
description ASBR1:Fa2/0
ip address dhcp
duplex auto
speed auto
!

cisco_trooper

Ok. So I have posted the relevant configs. It looks like DHCP is attempting to fullfill the request from a DHCP pool that does not exist. It is looking for a pool in the same subnet as the interface that received the relayed DHCP request. This leaves me really confused...

I have different segments on the other side of the dhcp relay, which need IP addresses, that they are supposedly supposed to be able to get from a central location. What am I missing here? I really thought this was going to be a bonehead lab.....

Anyone else about to explode? this is ridiculously retarded brother....

cisco_trooper

Jesus tap dancing christ almighty. dhcp service needs to be running on the relay agent. i must have turned this off earlier today and forgot about it. after turning this back on for the relay agents i noticed the dhcp server didn't have a route to the remote segment where dhcp addresses are to be assigned, so once it sent the unicast offer back, it got black holed. problem solved fellas. See, I'm not ready for CCIE studies.

yet, that is.

Without going nuts in your lab environment, you've never really learned anything...no matter how simple it may seem....

kryolla

ping traceroute and show run are your best friends

dtlokee

You don't need the DHCP service on the router for it to relay UDP broadcasts. According to the debug output you posted the relay agent (helper-address) was doing it's job but there was no pool on the router that covered the subnet where the request came from, so it was ignored.

cisco_trooper

dtlokee wrote:

You don't need the DHCP service on the router for it to relay UDP broadcasts. According to the debug output you posted the relay agent (helper-address) was doing it's job but there was no pool on the router that covered the subnet where the request came from, so it was ignored.

You are right, gonna have to turn this back off and see what happened....

cisco_trooper

dtlokee wrote:

You don't need the DHCP service on the router for it to relay UDP broadcasts. According to the debug output you posted the relay agent (helper-address) was doing it's job but there was no pool on the router that covered the subnet where the request came from, so it was ignored.

You don't need the DHCP service on the router for it to relay UDP broadcasts, but something happens with the DHCPDISCOVER, because without the DHCP Service running on the DHCP Relay Agent it tries to fullfill the DHCP Requests with an address of the local subnet, and gives the error of not having a valid DHCP Pool. Once I turn the DHCP Service back on, it then begins filling requests for the far end subnet, the one for which I want an IP address. I'll upload full debugs somewhere if I can find somewhere to put them...I don't want to clutter things up here.

dtlokee

Well the only thing that makes sense would be if you're configuring one DHCP relay to forward the frame to a different DHCP relay then on to the DHCP server itself. The other thing would be look option 82 configuration on the relay and turn it on if it;s off with the "ip dhcp relay information option" command.

cisco_trooper

dtlokee wrote:

Well the only thing that makes sense would be if you're configuring one DHCP relay to forward the frame to a different DHCP relay then on to the DHCP server itself. The other thing would be look option 82 configuration on the relay and turn it on if it;s off with the "ip dhcp relay information option" command.

i'm gonna have to look that one up....

cisco_trooper

dtlokee wrote:

Well the only thing that makes sense would be if you're configuring one DHCP relay to forward the frame to a different DHCP relay then on to the DHCP server itself. The other thing would be look option 82 configuration on the relay and turn it on if it;s off with the "ip dhcp relay information option" command.

Enabling this option on the DHCP Relay Agent, with the DHCP Service now turned off, is leading to the same results as before. It's trying to assign addresses to the local subnet, which does not even have a dhcp address pool defined.....

cisco_trooper

http://www.megafileupload.com/en/file/46430/ASBR1-log.html

This shows the change that occurs when I reenable DHCP on the relay agent...this is what works.

cisco_trooper

For those interested I put these configs up http://www.ipnetworksllc.com/routerconfigs