DHCP Clients Not Receiving IP Addresses

I was setting up a small branch network using a Cisco ISR router as the DHCP server. Everything seemed fine—DHCP was configured, interfaces were up, and the router was connected to a switch distributing access to multiple clients. But none of the devices were getting IP addresses. They just sat there with APIPA addresses (169.254.x.x), unable to reach the network.

🔍 Diagnosis

I started with:

• show ip dhcp binding — no bindings were listed.
• debug ip dhcp server packet — showed DHCPDISCOVER packets arriving, but no DHCPOFFER responses.
• show run | include dhcp — confirmed the DHCP pool was defined correctly.

Then I checked the interface configuration and found the issue: the router’s LAN interface was missing the ip helper-address command, which is essential when DHCP clients are on a different subnet or VLAN.

✅ Solution

Here’s how I resolved it:

1. Verified the DHCP pool:

ip dhcp pool BRANCH_POOL network 192.168.10.0 255.255.255.0 default-router 192.168.10.1 dns-server 8.8.8.8

2. Added the helper address to the interface:

interface GigabitEthernet0/1 ip helper-address 192.168.10.1

3. Checked for ACLs blocking DHCP: I ensured there were no access lists denying UDP ports 67 and 68.
4. Restarted DHCP service:

clear ip dhcp binding *

Within seconds, clients began receiving IP addresses from the router. The show ip dhcp binding command showed active leases, and connectivity was restored.

📚 Reference That Helped

The Cisco DHCP Configuration Guide was incredibly helpful. It explained the role of ip helper-address in forwarding DHCP requests across subnets, and clarified how DHCP relay works in routed environments.

This issue is common in multi-VLAN setups or when DHCP is centralized. It’s a great reminder that DHCP isn’t just about defining pools—it’s about ensuring the requests reach the server, especially when routing is involved.