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AR Router Troubleshooting Guide

This Product Documentation provides guidance for maintaining AR Enterprise Router, covering common information collection and fault diagnostic commands, typical fault troubleshooting guide, and troubleshooting.
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A Client Cannot Obtain an IP Address (the device Functions as the DHCP Relay Agent)

A Client Cannot Obtain an IP Address (the device Functions as the DHCP Relay Agent)

A DHCP client and the DHCP server are located on different network segment. The device functions as a DHCP relay agent. The DHCP server uses the DHCP relay agent to allocate IP addresses to DHCP clients.

Common Causes

This fault is commonly caused by one of the following:

  • The link between the client and the DHCP server is faulty.
    • The link between the client and the DHCP relay agent is faulty.
    • The link between the DHCP relay agent and the DHCP server is faulty.
  • DHCP is disabled on the device globally. As a result, the DHCP function does not take effect.
  • The DHCP relay function is disabled on the device. As a result, the DHCP relay function does not take effect.
  • The DHCP relay agent is not bound to the DHCP server.
    • The DHCP server IP address is not configured on the DHCP relay agent.
    • The interface on the DHCP relay agent is not bound to a DHCP server group or it is bound to a server group that contains no DHCP server.
  • The configurations of other devices along the link are incorrect.

Troubleshooting Flowchart

Figure 18-2 shows the troubleshooting flowchart.

Figure 18-2 Troubleshooting flowchart for the failure to allocate IP addresses using the DHCP relay agent

Troubleshooting Procedure

NOTE:

Saving the results of each troubleshooting step is recommended. If your troubleshooting fails to correct the fault, you will have a record of your actions to provide technical support personnel.

Procedure

  1. Check whether a fault occurs between the DHCP client and the DHCP server.

    1. Check whether DHCP snooping is enabled on devices between the client and server. If DHCP snooping is enabled on a device, run the display dhcp snooping configuration command to check DHCP snooping configuration. Ensure that the interface connected to the DHCP server is a trusted interface.
    2. Check whether a fault occurs between the DHCP client and the DHCP relay agent.

      Manually configure an IP address on the DHCP client to be on the same network segment as the user-side interface of the DHCP relay agent. This IP address must be different from allocated IP addresses. Ping the peer device from the IP address to check whether the link works properly.

      • If the ping operation fails, refer to A Ping Operation Fails to rectify the link fault.
      • If the ping operation succeeds, go to step b.
    3. Check whether a fault occurs between the DHCP relay agent and the DHCP server.

      Run the ping -a source-ip-address destination-ip-address command on the DHCP relay agent. source-ip-address specifies the user-side interface of the DHCP relay agent and destination-ip-address specifies the IP address of the DHCP server.

      • If the ping operation fails, refer to A Ping Operation Fails to rectify the link fault.
      • If the ping operation succeeds, go to step 2.

  2. Check whether DHCP is enabled globally on the DHCP relay agent.

    NOTE:

    If DHCP is not enabled globally, the device does not process DHCP messages sent by DHCP clients.

    Run the display current-configuration | include dhcp enable command to check whether DHCP is enabled. By default, DHCP is disabled.

    • If no information is displayed, DHCP is disabled. Run the dhcp enable command to enable DHCP.
    • If dhcp enable is displayed, DHCP is enabled. Go to step 3.

  3. Verify that the DHCP relay function is enabled.

    NOTE:
    • If the DHCP relay function is disabled, the DHCP client cannot obtain an address on another network segment.
    • If the address allocation mode global/interface and relay are both configured on the device, the device will function as a DHCP server. If the DHCP server is unable to allocate IP addresses, the device will function as a DHCP relay agent.

    In the interface view on the device, run the display this command to check whether the DHCP relay function is enabled.

    • If dhcp select relay is displayed, the DHCP relay function is enabled. Go to step 4.
    • If no information is displayed, the DHCP relay function is disabled. Run the dhcp select relay command to enable the DHCP relay function.

  4. Check that the DHCP relay agent is bound to the DHCP server.

    NOTE:

    If the DHCP relay agent is not bound to the DHCP server, no DHCP server can allocate IP addresses to DHCP clients connected to the DHCP relay agent.

    In the interface view on the device, run the display this command to check whether the DHCP relay agent is bound to the DHCP server.

    • If dhcp relay server-ip ip-address is displayed, the DHCP server IP address is configured on the DHCP relay agent. Go to step 6.
    • If dhcp relay server-select group-name is displayed, the interface on the DHCP relay agent is bound to a DHCP server group. Go to step 5.
    • If no information is displayed, the DHCP server IP address is not configured on the DHCP relay agent. Use either of the following methods to configure the DHCP server:
      • Run the dhcp relay server-ip ip-address command to configure the DHCP server IP address on the DHCP relay agent.
      • Run the dhcp relay server-select group-name command to bind the interface to a DHCP server group and run the dhcp-server command to add a DHCP server to the DHCP server group.

  5. Verify that DHCP servers have been added to the DHCP server group.

    NOTE:

    If the interface of the DHCP relay agent is bound to a DHCP server group but no DHCP server has been added to the DHCP server group, no DHCP server can allocate IP addresses to DHCP clients connected to the DHCP relay agent.

    Run the display dhcp server group group-name command to check whether DHCP servers have been added to the DHCP server group.

    • If the Server-IP field is displayed, DHCP servers have been added to the DHCP server group. Go to step 6.
    • If the Server-IP field is not displayed, no DHCP server has been added to the DHCP server group. Run the dhcp-server command to add DHCP servers to the DHCP server group.

  6. Verify that the configurations of other devices along the link between the DHCP client and the DHCP server are correct, including DSLAMs, LAN switches, and other clients.

    Check whether the configurations of other devices along the link are correct and modify configurations as needed. If the client still cannot obtain an IP address after the above steps are complete, go to step 7.

    NOTE:
    For details on how to check the configurations of the DHCP server, see A Client Cannot Obtain an IP Address (the device Functions as the DHCP Server).

  7. Contact technical support personnel and provide them with the following information.

    • Results of the preceding troubleshooting procedure
    • Configuration file, log file, and alarm file of the device

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Updated: 2019-08-09

Document ID: EDOC1000079719

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