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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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Users Fail to Go Online After DHCP Snooping Is Configured

Users Fail to Go Online After DHCP Snooping Is Configured

Common Causes

This fault is commonly caused by one of the following:

  • The network-side interface connected to the DHCP server is not configured as a trusted interface.
  • The number of DHCP users connected to the user-side interface reaches the upper limit.
  • The transmission rate of DHCP packets exceeds the upper limit, so the DHCP packets of new users are discarded.

Troubleshooting Flowchart

Figure 22-13  DHCP snooping troubleshooting flowchart

Troubleshooting Procedure

NOTE:

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

Procedure

  1. Check that the trusted interface is correctly configured.

    Run the display dhcp snooping configuration command to check the VLANs and interfaces enabled with DHCP snooping.

    • Run the display dhcp snooping interface interface-type interface-number command to check whether "dhcp snooping trusted" is displayed under the network-side interface.

    • Run the display this command in the VLAN view to check whether "dhcp snooping trusted interface xxx" is displayed.

    After DHCP snooping is enabled on an interface, by default, the interface is an untrusted interface. When receiving packets from network-side interfaces, the AR100&AR120&AR150&AR160&AR200&AR1200&AR2200&AR3200&AR3600 processes only the DHCP Reply packets received by the trusted interface and discards DHCP Reply packets received by untrusted interfaces. When receiving packets from user-side interfaces, the AR100&AR120&AR150&AR160&AR200&AR1200&AR2200&AR3200&AR3600 forwards the packets only to the trusted interface.

    • The network-side interface of the DHCP server must be configured as a trusted interface. If the network-side interface is not configured as a trusted interface, run the dhcp snooping trusted command in the interface view or run the dhcp snooping trusted interface interface-type interface-number command in the VLAN view to configure the interface as a trusted interface.

    • If the trusted interface is correctly configured, go to step 2.

  2. Check whether the number of DHCP online users has reached the upper limit.

    • Run the display dhcp snooping interface interface-type interface-number command to check whether "dhcp snooping max-user-number xxx" is displayed under the user-side interface.

    • Run the display this command in the VLAN view to check whether "dhcp snooping max-user-number xxx" is displayed.

    In the command outputs, max-user-number indicates the maximum number of DHCP users. If this field is not displayed, the default value 1024 is used. If max-user-number is displayed in both the preceding command outputs, the smallest one among the displayed values is used.

    Run the display dhcp snooping user-bind all command to view the number of DHCP users on the DHCP snooping-enabled interface. If the number of DHCP users on the interface has reached the upper limit, new users cannot go online.

    If the number of DHCP users on the interface is lower than the upper limit, go to step 3.

  3. Check whether the transmission rate of DHCP packets exceeds the upper limit.

    Run the display this command in the interface view, VLAN view, and system view to check whether a limit is set for DHCP packet rate. If the output information does not contain dhcp check dhcp-rate xx, the DHCP packet rate is the default value 100.

    The DHCP rate limit can be set in the system view, interface view, and VLAN view. After a rate limit is set, the number of packets sent to the protocol stack within a certain period of time cannot exceed the limit; otherwise, the excess packets are discarded. The smallest value among the rate limits set in the system view, interface view, and VLAN view takes effect. If users cannot go online because the DHCP rate limit is small, run the dhcp check dhcp-rate command in the system view, interface view, and VLAN view to increase the rate limit values.

    If the fault persists after the rate limit values are increased, go to step 4.

  4. Collect the following information and contact technical support personnel:

    • Results of the preceding troubleshooting procedure
    • Configuration file, log file, and alarm file of the AR100&AR120&AR150&AR160&AR200&AR1200&AR2200&AR3200&AR3600

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Updated: 2019-05-10

Document ID: EDOC1000079719

Views: 456786

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