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Configuration Guide - IP Service

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DHCP policy VLAN, DNS, mDNS gateway, mDNS relay, UDP Helper, IP performance optimization, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel, and IPv4 over IPv6 tunnel.
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A Client Fails to Obtain an IP Address from a DHCP Server

A Client Fails to Obtain an IP Address from a DHCP Server

Table 3-13  Troubleshooting roadmap and solution
Possible Cause Troubleshooting Roadmap Solution
DHCP is disabled.

Run the display current-configuration | include dhcp enable command in the user view to check whether DHCP is enabled. If the command output is empty, DHCP is disabled.

Run the dhcp enable command in the system view to enable DHCP.

By default, DHCP is disabled in the system.

The configuration is incorrect.

On the DHCP server:
  • Run the display ip pool command to check whether an address pool on the same network segment as the client is configured, and check whether the configuration is correct.
  • Run the display this command in the view of the interface connected to the client to check whether the DHCP server function is enabled on the interface.
  • If a DHCP relay agent is deployed between the DHCP server and client, check whether a route to the network segment of the client is configured on the DHCP server.

On the DHCP relay agent:

  • Run the display dhcp relay command to check whether the DHCP relay agent is correctly configured.
  • In the view of the interface connected to the client, run the display this command to check whether DHCP relay is enabled on the interface.

On the DHCP server:

  • Modify the configuration based on the network plan. For details, see Configuring a DHCP Server.
  • Run the dhcp select global command in the view of the interface connected to the client to enable the interface to use the global address pool to provide the DHCP server function; alternatively, run the dhcp select interface command to enable the interface to use the interface address pool to provide the DHCP server function.

On the DHCP relay agent:

  • Modify the configuration based on the network plan. For details, see Configuring a DHCP Relay Agent.
  • In the view of the interface connected to the client, run the dhcp select relay command to enable DHCP relay on the interface.

The address pool has no available IP addresses.

Run the display ip pool command to check whether there are available IP addresses in the address pool. The Idle(Expired) field displays the number of idle IP addresses in the address pool. If the value of this field is 0, there are no available IP addresses in the address pool.

Determine the number of DHCP clients on the network.

  • If the number of DHCP clients is larger than the number of available IP addresses in the address pool, increase the range of IP addresses in the address pool by using either of the following methods: Run the ip address ip-address { mask | mask-length } command in the interface view to reduce the mask length. Or run the network ip-address [ mask { mask | mask-length } ] command in the global address pool view to reduce the mask length.

  • If the number of DHCP clients is less than the number of available IP addresses in the address pool, check whether the lease is too long and whether the DHCP server reclaims IP addresses of offline or disconnected clients. If the lease is too long, shorten the lease of IP addresses in the address pool.

    In locations where clients often move (cafes, for example), clients' online time generally does not exceed one day. In this situation, reduce the default lease (one day) of IP addresses assigned by a device functioning as a DHCP server.

STP is enabled on access devices of some diskless workstations.

The timeout period of DHCP Discover messages sent from clients is shorter than the STP convergence time. The DHCP server cannot receive DHCP Discover messages and therefore cannot allocate IP addresses to the diskless workstations.

Disable STP on access devices of the diskless workstations.

The IP address is manually configured for another client on the network. Since the DHCP server does not exclude this IP address from the address pool, an IP address conflict may occur.
  1. Disable the network adapter of the client or disconnect the network cable.
  2. Perform a ping operation on another host to check whether the host with this IP address exists. If the ping operation is successful, the IP address has been manually configured.
  • Change the manually configured IP address of the host.
  • Exclude the conflicting IP address from the address pool on the DHCP server by running either of the following commands:dhcp server excluded-ip-address for an interface address pool or excluded-ip-address for a global address pool.
  • Release the conflicting IP address of the client and apply for a new IP address. Using a PC running on Windows 7 as an example: Run the cmd command to enter the DOS environment. Run the ipconfig/release command to release the IP address. Run the ipconfig/renew command to apply for a new IP address.

To prevent clients from obtaining conflicting IP addresses, configure IP address conflict detection on the DHCP server. When an IP address conflict is detected, the DHCP server allocates another available IP address. For details, see (Optional) Configuring IP Address Conflict Detection Before a DHCP Server Allocates IP Addresses.

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Updated: 2018-12-24

Document ID: EDOC1100038342

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