DHCP relay fault: IP address in the same network segment

The complexity of the network structure also makes it difficult for us to maintain some of its services. Sometimes, some faults may inevitably occur. Here we will mainly explain the problem that the client in the DHCP relay fault cannot obtain the IP address of the same network segment as the IP address through the relay proxy.

DHCP relay troubleshooting ◆ Network Environment

Shows the networking environment. During use, it is found that the client cannot obtain the IP address of the same network segment from the relay proxy.

Figure DHCP relay typical networking applications

DHCP relay troubleshooting ◆ Fault Analysis

1. Run the "display dhcp server tree all" command to check whether the DHCP server is configured with an address pool in the network segment of the DHCP client.

2. Run the display ip interface brief command to view the configuration information of the relay proxy and DHCP server interfaces. Check whether the primary IP address and slave IP address are correctly configured for each interface.

3. run the display dhcp relay address all command to check whether the DHCP server address of the interface that enables the relay proxy function is configured, and whether the slave IP address that uses this interface is configured as the relay proxy address. That is, whether there are two configurations, ip relay address between 110.1.1 and ip relay giaddr 11.110.1.1.

4. Run the display fib 11.110.1.1 command to view the route information.

Check the route information on the server side and find that the route does not reach the Relay.

Run the ping pai110.1.1 command on the relay proxy to ping the proxy. Run the ping 11.110.1.1 command on the DHCP server. The routing is abnormal.

5. enable DHCP debugging on the DHCP server and run the debugging dhcp server packet command.

DHCPOFFER packets returned by the DHCP server fail to be sent.

Because the route does not reach the slave address of the relay proxy, the client cannot normally obtain the IP address of the same network segment as the slave IP address. You can use the routing protocol to publish a route or configure a static route to solve the problem.

DHCP relay troubleshooting ◆ handling steps

1. Check whether DHCP is enabled for the relay proxy and DHCP server.

2. Check whether the DHCP server is configured with an address pool in the same network segment as the IP address required by the DHCP client.

3. Check whether the IP address of the proxy DHCP server is correctly configured on the interface connecting to the IP address of the DHCP client; whether the IP address of the proxy is conflicted due to the configuration of Multiple DHCP server addresses.

4. Check whether the DHCP relay proxy and DHCP server are configured with mutually accessible routes, that is, the primary IP address that arrives at the relay proxy interface and the route from the IP address.

5. Check the configuration of the DHCP server. A maximum of four relay proxies can be configured between the DHCP server and the client.

6. Check whether commands for DHCP relay proxy are enabled on the relay proxy interface. If no enable is available, go to the interface view of GigabitEthernet1/0/0 and run the dhcp select relay command.

7. Check whether the relay proxy address on the relay proxy interface is the master or slave IP address of the interface. If the relay proxy address is not configured, the primary IP address of the relay proxy interface is selected as the relay proxy address by default. The relay proxy address must be the primary IP address or slave IP address of the interface. Otherwise, the route cannot be published, and the client cannot obtain the IP address normally.

DHCP relay troubleshooting ◆ case summary

Although the relay configuration is not complex, it is easy to encounter mismatching commands, missing commands, and other situations. By default, the server assigns an IP address from the global address pool. To assign an IP address from the interface address pool, you must run the dhcp select interface command. If you want to assign an IP address from the relay, the dhcp select relay command must be executed. In addition, the ip relay address and dhcp select relay commands must be executed on the Interface connected to the client, GigabitEthernet1/0/0), and cannot be configured on the GigabitEthernet1/0/1 interface.