DHCPCCNA 200-301

DHCP Server Unreachable Due to Routing Issue

Presenting Symptom

Clients are unable to obtain IP addresses from the DHCP server, receiving 'No DHCPOFFER' messages.

Network Context

A small branch office with a router (Cisco 4321, IOS XE 16.9) acting as the default gateway and DHCP relay agent. The DHCP server is located at the headquarters (10.1.1.10) across a WAN link. Clients are in VLAN 10 (192.168.10.0/24) and the router's GigabitEthernet0/0/0 connects to the WAN (10.1.1.0/30). The router has an IP helper-address configured for DHCP.

Diagnostic Steps

1. Verify DHCP Client Request

show ip dhcp binding

No output or empty list

If no bindings exist, the router has not received or relayed any DHCP requests successfully.

2. Check DHCP Relay Configuration

show running-config interface GigabitEthernet0/0/1

interface GigabitEthernet0/0/1
 ip address 192.168.10.1 255.255.255.0
 ip helper-address 10.1.1.10

Ensure the helper-address points to the correct DHCP server IP. If missing or incorrect, DHCP relay is not configured.

3. Verify Reachability to DHCP Server

ping 10.1.1.10 source 10.1.1.1

Success rate is 0 percent (0/5)

Ping fails, indicating a routing issue between the router and the DHCP server.

4. Examine Routing Table

show ip route 10.1.1.10

Routing entry for 10.1.1.10/32
  Known via "connected", distance 0, metric 0 (connected)
  * directly connected via GigabitEthernet0/0/0

If the route shows as directly connected or via a next-hop that is unreachable, there may be a mismatch in subnet masks or a missing route. In this scenario, the route might show as 'directly connected' but the interface is down or misconfigured.

Root Cause

The router's WAN interface (GigabitEthernet0/0/0) is configured with an incorrect IP address (10.1.1.2/24) instead of the correct /30 subnet, causing the router to believe the DHCP server is on a different subnet and not routing traffic correctly.

Resolution

1. Enter global configuration mode: configure terminal 2. Configure the correct IP address on the WAN interface: interface GigabitEthernet0/0/0 ip address 10.1.1.1 255.255.255.252 no shutdown 3. Exit and verify: end

Verification

1. Ping the DHCP server: ping 10.1.1.10 source 10.1.1.1 Expected: Success rate is 100 percent (5/5) 2. Check DHCP bindings: show ip dhcp binding Expected: List of assigned IP addresses

Prevention

1. Use consistent subnet masks across WAN links, preferably /30 for point-to-point. 2. Implement IP address management (IPAM) to avoid misconfigurations. 3. Use routing protocols like OSPF to dynamically learn routes instead of relying on static or connected routes.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of DHCP relay (ip helper-address) and basic IP routing. Questions may present a troubleshooting scenario where clients cannot get DHCP addresses, and you must identify the root cause as a routing issue (e.g., incorrect subnet mask on the router interface). The exam expects you to know that DHCP relay requires IP reachability between the relay agent and the DHCP server.

Exam Tips

Remember that 'ip helper-address' forwards DHCP broadcasts as unicasts to the specified server; the relay agent must have a route to that server.

In troubleshooting, always verify the routing table first when DHCP fails over a relay; a common mistake is to assume the helper-address is misconfigured when the real issue is no route.

Know that 'show ip dhcp binding' shows only locally served addresses; for relayed DHCP, check the server logs or use 'debug ip dhcp server packet' on the server.

Commands Used in This Scenario

ping [ip]

The ping command sends ICMP echo requests to a destination IP address to test network connectivity and measure round-trip time.

show ip route

Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.

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