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Configuration Guide - IP Service

S2720, S5700, and S6720 V200R013C00

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

A Client Fails to Obtain an IP Address

Possible Cause 1

DHCP is disabled.

Verification

Check whether DHCP is enabled. Enter the user view and run:
display current-configuration | include dhcp enable

If the command output is empty, DHCP is disabled.

Solution

Enable DHCP. Enter the system view and run:
dhcp enable

By default, DHCP is disabled in the system.

Possible Cause 2

The configuration is incorrect.

Verification

  • On the DHCP server:
    • Check whether the address pool and client are on the same network segment and whether the configuration is correct. Run:
      display ip pool
    • If a DHCP relay agent is deployed, check whether a route to the network segment of the client is configured on the DHCP server.
  • On the DHCP relay agent:
    • Check whether the DHCP relay agent is correctly configured. Run:
      display dhcp relay
    • In the view of the interface connected to the client, check whether DHCP relay is enabled on the interface. Run:
      display this

Solution

Modify the configurations on the DHCP server and, if deployed, DHCP relay agents. For details, see Configuring a DHCP Server and Configuring a DHCP Relay Agent.

Possible Cause 3

The address pool has no available IP addresses.

Verification

Check whether there are IP addresses available in the address pool. Run:
display ip pool

The Idle(Expired) field displays the number of idle IP addresses in the address pool.

Solution

  • If the number of DHCP clients is greater than the number of available IP addresses in the address pool, increase the range of IP addresses in the address pool.
    • For an interface address pool, run:
      ip address ip-address { mask | mask-length }
    • For a global address pool, run:
      network ip-address [ mask { mask | mask-length } ]
  • If the number of DHCP clients is less than the number of available IP addresses in the address pool, reduce the lease period and ensure that the DHCP server reclaims unused IP addresses. For details, see (Optional) Configuring Automatic Reclamation of Conflicting IP Addresses.

Possible Cause 4

The Spanning Tree Protocol (STP) is enabled on a diskless workstation functioning as a DHCP client.

Verification

Check whether STP is enabled. Run:
display stp

Solution

Disable STP on the diskless workstations. The timeout period of DHCP Discover messages sent from clients is shorter than the STP convergence time, and therefore the DHCP server cannot receive DHCP Discover messages or allocate IP addresses to the diskless workstations. Run:
undo stp enable

Possible Cause 5

The IP address is manually configured for another host on the network. This causes an IP address conflict because the DHCP server does not exclude manually configured IP addresses from the address pool.

Verification

Disable the network adapter of the client or disconnect the network cable. Then, from another connected host, ping the conflicting IP address. If you can ping the IP address, it is manually configured.

Solution

  • Exclude the conflicting IP address from the address pool.
    • For an interface address pool, run:
      dhcp server excluded-ip-address
    • For a global address pool, run:
      excluded-ip-address

    Then, on the client, release the IP address and apply for a new one.

  • Change the host's manually configured IP address.
NOTE:

IP address conflict detection can be configured on the DHCP server to prevent conflicts. For details, see (Optional) Configuring IP Address Conflict Detection.

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Updated: 2019-04-20

Document ID: EDOC1100065678

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