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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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Common Configuration Errors

Common Configuration Errors

This section describes common faults caused by incorrect DHCP snooping configurations and provides the troubleshooting procedure.

DHCP Clients Cannot Go Online Due to DHCP Snooping

Fault Description
The possible causes are as follows:
  • The network-side interface connected to the DHCP server is not configured as the trusted interface.
  • The number of DHCP clients connected to the user-side interface reaches the maximum value.
  • The transmission rate of DHCP messages exceeds the upper rate limit, and the DHCP messages from new DHCP clients are discarded.

Procedure

  1. Check whether the trusted interface is correctly configured.
    1. Run the display dhcp snooping command to check in which VLANs and on which interfaces is DHCP snooping enabled and whether "Trusted interface" is displayed on the network-side interface.

      NOTE:

      After DHCP snooping is enabled on an interface, the interface is an untrusted interface by default. When receiving messages from the network-side interfaces, the device processes only the DHCP Reply messages received on the trusted interface and discards those on untrusted interfaces. When receiving messages from user-side interfaces, the device forwards the messages only to the trusted interface.

    2. Check whether the network-side interface on the DHCP server is the trusted interface. If it is not the trusted interface, run the dhcp snooping trusted command in the VLAN view or interface view to configure the interface as the trusted interface.
  2. If the interface is correctly configured, check whether the number of access DHCP clients reaches the threshold.
    1. Run the display dhcp snooping command to check whether "DHCP user max number: XX" is displayed globally, in the VLAN or on the user-side interface.

      NOTE:

      If "DHCP user max number: XX" is not displayed, the default maximum number of DHCP clients is 32768. The configured value takes preference.

      If you set the maximum value in the system view, VLAN view, and the interface view, the smallest value takes effect.

    2. Run the display user-bind dhcp snooping all command to view the number of dynamic DHCP snooping entries on the DHCP snooping-enabled interface. If the number of entries on the interface reaches the maximum value, new DHCP clients cannot access the network.
  3. If the number of access DHCP clients does not reach the limit, check whether the transmission rate of DHCP packets exceeds the limit.
    1. Run the display dhcp snooping command to check whether "Rate-limit value(pps): xx" is displayed globally, in the VLAN or on the user-side interface.

      NOTE:

      If "Rate-limit value(pps): xx" is not displayed, the default rate limit is 100 pps. The configured value takes preference.

    2. If DHCP clients cannot access the network because the DHCP snooping rate limit is small, run the dhcp snooping rate-limit command in the system view, interface view, and VLAN view to increase the rate limit values.
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Updated: 2019-08-09

Document ID: EDOC1000041694

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