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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 DHCP Client Takes a Long Time to Obtain an IP Address from a DHCP Server

A DHCP Client Takes a Long Time to Obtain an IP Address from a DHCP Server

Table 3-14  Troubleshooting roadmap and solution
Possible Cause Troubleshooting Procedure Solution

Multiple DHCP servers are deployed on the network.

If multiple DHCP servers are deployed on the same network segment as the DHCP client, the client accepts the first DHCP Offer message.

Configure DHCP snooping on the access device of the client so that the client receives DHCP messages only from the trusted DHCP server. When a device functions as the DHCP snooping device, for details on how to perform the configuration, see Inserting the Option 82 Field in a DHCP Message in the S1720, S2700, S5700, and S6720 V200R012(C00&C20) Configuration Guide - Security.

Broadcast traffic suppression is configured.

Check whether broadcast traffic suppression is configured between the DHCP server and client. If the number of DHCP Discover messages broadcast on the network exceeds the threshold, the DHCP Discover messages will be discarded. Set the broadcast traffic suppression threshold based on the service volume.

A malicious attack occurs.

Run the display cpu-defend statistics command to check statistics about packets sent to the CPU of the DHCP server. The command output shows that a large number of DHCP messages are discarded and these messages are simultaneously sent from one MAC address. This MAC address is the source of a DHCP flood attack. Add this MAC address to the blacklist. For details, see Configuring CPU Attack Defense in "Local Attack Defense Configuration" in the S1720, S2700, S5700, and S6720 V200R012(C00&C20) Configuration Guide - Security.

The STP function is enabled on the DHCP server or DHCP relay agent.

By default, the STP function is enabled. If the STP function is enabled on the DHCP server or DHCP relay agent, address allocation may be slow.

If the STP function does not need to be enabled, run the undo stp enable command to disable it.

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Updated: 2018-12-24

Document ID: EDOC1100038342

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