DHCP IP HELPER CLARIFICATION

[Raja_D]

Hi,

We have a dedicated DHCP server from which we are trying to get ip addresses polled to the PC's dynamically for which on the router we have the below configuration. DHCP Lan pool scope defined in the dhcp server is 10.56.12.112/28, GW is set to 10.56.12.113 on DHCP server.

Below is the configuration that is set on the router which is acting as "DHCP RELAY AGENT"

int vlan10
ip add 10.56.12.113 255.255.255.240 --- connected to lan
ip helper-address 10.25.31.1

we have reachability to the DHCP server from router. But the ip's are not getting assigned to the PC's dynamically.

I notice that the "DHCP service"is disabled on router using "no service dhcp" on router. Is it recommended to enable the DHCP service on the router for the router to act as "DHCP relay agent" so as to have the PC's get ip's dynamically from router.

Please clarify.

James..

[Joseph W. Doherty]

You shouldn't need anything more on the router other than the IP helper statement.

Is the DHCP server configured to provide IPs to the LAN network being relayed?

[Julio E. Moisa]

Hi

Your configuration looks fine, now is the DHCP scope enabled on the server?

If you are able to ping  the server from the vlan 10, the computer should obtain IP address otherwise the issue could be on the server.

ping 10.25.31.1 source vlan 10

Just a question this IP 10.56.12.113 is the gateway for your computers and it is configured on the DHCP server?

[Raja_D]

Hi,

Yes the DHCP scope is enabled in the dedicated DHCP server and able to ping the DHCP server with source as vlan.

Also the gateway is configured in DHCP server, but the ip's are not getting detected by clients in network.

Infact there was an acl over vlan 10 and I have removed that acl as well and checked but still no luck.

Also with acl I have permitted below statement.

Ip access-group WAN in

10 permit udp host 0.0.0.0 eq bootpc host 255.255.255.255 eq bootps

With acl I could see matches over this statement but ip's not getting assigned still..

I felt like dhcp service need to be enabled but did not enabled it expecting that its not required if a router is just acting like a "DHCP relay agent" and not as a DHCP server.

Alternatively from pc's I have tried with ipconfig /renew followed by ipconfig /release but still no luck.

Any suggestions more , pls advise..

James...

[Rolf Fischer · Accepted solution]

James,

my understanding is that you need to enable service dhcp on the router in order to use the relay agent feature:

Cisco IOS IP Configuration Guide Chapter: Configuring DHCP

Router(config)# service dhcp

Enables the Cisco IOS DHCP server and relay features on your router.

IOS Command Reference: ip helper-address

"The service dhcp command must be configured on the router to enable IP helper statements to work with DHCP. If the command is not configured, the DHCP packets will not be relayed through the IP helper statements. The service dhcp command is configured by default."

HTH

Rolf