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Configuration Guide - IP Service

CloudEngine 12800 and 12800E V200R005C00

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
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DHCP Relay Implementation

DHCP Relay Implementation

The DHCP relay function enables message exchanges between a DHCP server and a client on different network segments. When DHCP clients and a server are on different network segments, the DHCP relay agent transparently transmits DHCP messages to the destination DHCP server. In this way, DHCP clients on different network segments can communicate with one DHCP server.

Figure 3-7 shows how a DHCP client uses a DHCP relay agent to apply for an IP address for the first time.
Figure 3-7 Working process of a DHCP relay agent

Figure 3-7 shows the working process of a DHCP relay agent. The DHCP client sends a Request message to the DHCP server. When receiving the message, the DHCP relay agent processes and unicasts the message to the specified DHCP server on the other network segment. The DHCP server sends requested configurations to the client through the DHCP relay agent based on information in the Request message.

  1. After receiving DHCP Discover messages or Request messages, the DHCP relay agent performs the following operations:
    • Discards DHCP Request messages whose number of hops is larger than the hop limit to prevent loops. Alternatively, increases the value of the hop by 1, indicating that the message passes through a DHCP relay agent.
    • Checks the giaddr field. If the value is 0, set the value of the giaddr field to the IP address of the interface which receives the Request message. Selects one IP address if the interface has multiple IP addresses. All the Request messages received by the interface later use this IP address to fill the giaddr field. If the value is not 0, do not change the value.
    • Sets the TTL in the request packets to the default value 255 in the DHCP relay device, not the value calculated by decreasing the original TTL by 1. You can change the value of the hops field to prevent loops and limit hops.
    • Changes the destination IP address of the DHCP Request message to the IP address of the DHCP server or the IP address of the next DHCP relay agent. In this way, the DHCP Request message can be forwarded to the DHCP server or the next DHCP relay agent.
  2. The DHCP server assigns IP addresses to the client based on the Relay Agent IP Address field and sends the DHCP Reply message to the DHCP relay agent specified in the Relay Agent IP Address field. After receiving the DHCP Reply message, the DHCP relay agent performs the following operations:
    • The DHCP relay agent assumes that all the Reply messages are sent to the directly-connected DHCP clients. The Relay Agent IP Address field identifies the interface directly connected to the client. If the value of the Relay Agent IP Address field is not the IP address of a local interface, the DHCP relay agent discards the Reply message.
    • The DHCP relay agent checks the broadcast flag bit of the message. If the broadcast flag bit is 1, the DHCP relay agent broadcasts the DHCP Reply message to the DHCP client; otherwise, the DHCP relay agent unicasts the DHCP Reply message to the DHCP client. The destination IP address is the value in the Your (Client) IP Address field, and the MAC address is the value in the Client Hardware Address field.

Figure 3-8 shows how a DHCP client extends the IP address lease time through a DHCP relay agent.

Figure 3-8 Extending the IP address lease time through a DHCP relay agent

  1. After accessing the network for the first time, the DHCP client only needs to unicast a DHCP Request message to the DHCP server that assigned its currently-used IP address.
  2. The DHCP server then directly unicasts a DHCP ACK message or a DHCP NAK message to the client.

DHCP Releasing

The DHCP relay agent, instead of the client, can send a Release message to the DHCP server to release the IP addresses that assigned to the DHCP client. You can configure a command on the DHCP relay agent to release the IP addresses that the DHCP server has assigned to the DHCP client.

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Updated: 2019-04-18

Document ID: EDOC1100039535

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