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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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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Principles

Principles

This section describes the implementation of DHCPv6.

DHCPv6 Overview

DHCPv6 runs between a client and a server. Similar to DHCP for IPv4, DHCPv6 clients and DHCPv6 servers exchange DHCPv6 packets using the User Datagram Protocol (UDP). In IPv6, packets cannot be broadcast; therefore, DHCPv6 uses multicast packets. In this case, DHCPv6 clients do not need to be configured with IPv6 addresses of DHCPv6 servers.

IPv6 Address Allocation Methods

The IPv6 protocol provides huge address space formed by 128-bit IPv6 addresses that require proper and efficient assignment and management policies.

Currently, the following methods are available to allocate IPv6 addresses:
  • Manual configuration: You can manually configure IPv6 addresses, prefixes, and other network configuration parameter, such as addresses of the Domain Name System (DNS), Network Information Service (NIS), and Simple Network Time Protocol (SNTP) servers.
  • Stateless address autoconfiguration: Hosts generate a link-local address based on the interface ID and automatically configure IPv6 addresses based on prefixes carried in Router Advertisement (RA) packets.
  • Stateful autoconfiguration,that is DHCPv6,DCHPv6 allocation has the following two methods:
    • DHCPv6 stateful autoconfiguration: DHCPv6 servers automatically provide IPv6 addresses, PD prefixes, and other network configuration parameters, such as addresses of the DNS, NIS, and SNTP servers.
    • DHCPv6 stateless autoconfiguration: IPv6 addresses are generated based on RA packets. A DHCPv6 server does not provide IPv6 addresses but provides other configuration parameters about the DNS, NIS, and SNTP servers.
DHCPv6 Architecture

Figure 6-59 shows the DHCPv6 architecture.

Figure 6-59 DHCPv6 architecture

DHCPv6 involves the following roles:

  • DHCPv6 client

    A DHCPv6 client applies to a DHCPv6 server for IPv6 addresses, prefixes, and network configuration parameters to complete its address configuration.

  • DHCPv6 relay

    A DHCPv6 relay agent relays DHCPv6 packets between a DHCPv6 client and a DHCPv6 server to help the DHCPv6 client complete its address configuration. Generally, a DHCPv6 client communicates with a DHCPv6 server through the link-local multicast address to obtain IPv6 addresses, prefixes, and other network configuration parameters. If a DHCPv6 server and a DHCPv6 client are on different links, a DHCPv6 relay agent is required to forward DHCPv6 packets. In this case, you do not need to deploy a DHCPv6 server on each link, which saves costs and facilitates centralized management.

    A DHCPv6 relay agent is optional. If a DHCPv6 client and a DHCPv6 server are on the same link or a DHCPv6 client communicates with a DHCPv6 server in unicast mode to complete address allocation or information configuration, you do not need to deploy a DHCPv6 relay agent. A DHCPv6 relay agent is required only when a DHCPv6 client and a DHCPv6 server are located on different links or a DHCPv6 client cannot communicate with a DHCPv6 server in unicast mode.

  • DHCPv6 server

    A DHCPv6 server processes requests of address allocation, address lease extension, and address release from a DHCPv6 client or a DHCPv6 relay agent, and assigns IPv6 addresses and other network configuration parameters to the DHCPv6 client.

Basic DHCPv6 Concepts
  1. Multicast address
    • In DHCPv6, a DHCPv6 client does not need to be configured with the IPv6 address of a DHCPv6 server. Instead, the DHCPv6 client locates DHCPv6 servers by sending Solicit packets with multicast addresses as destination addresses.
    • In DHCPv4, a DHCP client locates DHCP servers by broadcasting DHCP packets. To prevent broadcast storms, IPv6 does not use broadcast packets. Instead, IPv6 uses multicast packets. DHCPv6 uses the following two multicast addresses:
      • FF02::1:2 (All DHCP Relay Agents and Servers): indicates the multicast address of all the DHCPv6 servers and DHCPv6 relay agents. The address is a link-local multicast address and is used for communication between a DHCPv6 client and its neighboring servers or between a DHCPv6 client and DHCPv6 relay agents. All DHCPv6 servers and relay agents are members of this multicast group.
      • FF05::1:3 (All DHCP Servers): indicates the multicast address of all the DHCPv6 servers. The address is a site-local address and is used for communication between DHCPv6 relay agents and DHCPv6 servers within a site. All DHCPv6 servers within a site are members of this multicast group.
  2. UDP port number
    • DHCPv6 packets are transmitted through UDPv6.
    • DHCPv6 clients only process DHCPv6 packets with UDP port number 546.
    • DHCPv6 servers and relay agents only process DHCPv6 packets with UDP port number 547.
  3. DHCPv6 Unique Identifier (DUID)
    • A DUID identifies a DHCPv6 device. Each DHCPv6 server or client has a unique DUID. DHCPv6 servers use DUIDs to identify DHCPv6 clients and DHCPv6 clients use DUIDs to identify DHCPv6 servers.
    • The DUIDs of a DHCPv6 client and a DHCPv6 server are carried in the Client Identifier option and the Server Identifier option respectively. The Client Identifier option and the Server Identifier option have the same format and are distinguished by the option-code field value.
  4. Identity association (IA)
    • An IA enables a DHCPv6 server and a DHCPv6 client to identify, group, and manage IPv6 addresses. Each IA consists of an identity association identifier (IAID) and associated configuration information.
    • A DHCPv6 client must associate at least one IA with each of its network interfaces for which the DHCPv6 client requests IPv6 addresses from a DHCP server. The DHCPv6 client uses IAs associated with network interfaces to obtain configuration information from a DHCPv6 server. Each IA must be associated with at least one interface.
    • The IAID identifies an IA, and IAIDs on the same DHCPv6 client must be unique. The IAID is not lost or changed because of factors such as DHCPv6 client reboot.
    • The configuration information in an IA consists of one or more IPv6 addresses along with the lifetimes T1 and T2. Each address in an IA has a preferred lifetime and a valid lifetime.
    • An interface must be associated with at least one IA; an IA can contain information about one or more addresses.

DHCPv6 Packets

DHCPv6 Packet Format

Figure 6-60 shows the DHCPv6 packet format.

Figure 6-60 DHCPv6 packet format

Table 6-18 Description of each field in a DHCPv6 packet

Field

Length

Description

msg-type

1 byte

Indicates the packet type. The value ranges from 1 to 13. For details, see the DHCPv6 Packet Type.

transaction-ID

3 bytes

Identifies packet transaction between DHCPv6 clients and servers. For example, a DHCPv6 client initiates a Solicit/Advertise transaction or a Request/Reply transaction. Their transaction IDs are different. Transaction IDs have the following characteristics:
  • The transaction ID is randomly generated by a DHCPv6 client.
  • Transaction IDs of request and reply packets must be the same.
  • The transaction ID of a packet initiated by a DHCPv6 server is 0.

Options

Variable

Indicates the option field in a DHCPv6 packet. The option field contains configurations that the DHCPv6 server assigns to IPv6 hosts, such as the IPv6 address of the DNS server.

DHCPv6 Packet Type

DHCPv6 defines 13 types of packets. A DHCPv6 server and a DHCPv6 client communicate by exchanging these types of packets. Table 6-19 lists DHCPv6 packets and their corresponding DHCPv4 packets and describes the DHCPv6 packets.

Table 6-19 Comparisons between DHCPv6 packets and DHCPv4 packets
DHCP Packet Type DHCPv6 Packet DHCPv4 Packet Description
1 SOLICIT DHCP DISCOVER A DHCPv6 client sends a Solicit packet to locate DHCPv6 servers.
2 ADVERTISE DHCP OFFER A DHCPv6 server sends an Advertise packet in response to a Solicit packet to declare that it can provide DHCPv6 services.
3 REQUEST DHCP REQUEST A DHCPv6 client sends a Request packet to request IPv6 addresses and other configuration parameters from a DHCPv6 server.
4 CONFIRM - A DHCPv6 client sends a Confirm packet to any available DHCPv6 server to check whether the obtained IPv6 address applies to the link that the DHCPv6 client is connected to.
5 RENEW DHCP REQUEST A DHCPv6 client sends a Renew packet to the DHCPv6 server that provides the IPv6 addresses and other configuration parameters to extend the lifetime of the addresses and to update configuration parameters.
6 REBIND DHCP REQUEST A DHCPv6 client sends a Rebind packet to any available DHCPv6 server to extend the lifetime of the assigned IPv6 address and to update configuration parameters when the client does not receive a response to its Renew packet.
7 REPLY DHCP ACK/NAK

A DHCPv6 server sends a Reply packet in the following situations:

  1. A DHCPv6 server sends a Reply packet containing IPv6 addresses and configuration parameters in response to a Solicit, Request, Renew or Rebind packet received from a DHCPv6 client.
  2. A DHCPv6 server sends a Reply packet containing configuration parameters in response to an Information-Request packet.
  3. A DHCPv6 server sends a Reply packet in response to a Confirm, Release, or Decline packet received from a DHCPv6 client.
8 RELEASE DHCP RELEASE A DHCPv6 client sends a Release packet to the DHCPv6 server that assigns IPv6 addresses to the DHCPv6 client, indicating that the DHCPv6 client will no longer use the obtained addresses.
9 DECLINE DHCP DECLINE A DHCPv6 client sends a Decline packet to a DHCPv6 server, indicating that the IPv6 addresses assigned by the DHCPv6 server are already in use on the link to which the DHCPv6 client is connected.
10 RECONFIGURE - A DHCPv6 server sends a Reconfigure packet to a DHCPv6 client, informing the DHCPv6 client that the DHCPv6 server has new addresses or updated configuration parameters.
11 INFORMATION-REQUEST DHCP INFORM A DHCPv6 client sends an Information-Request packet to a DHCPv6 server to request configuration parameters except for IPv6 addresses.
12 RELAY-FORW - A DHCPv6 relay agent sends a Relay-Forward packet to relay Request packets to DHCPv6 servers.
13 RELAY-REPL - A DHCPv6 server sends a Relay-Reply packet to a DHCPv6 relay agent. The Relay-Reply packet carries a packet that the DHCPv6 relay agent needs to deliver to a DHCPv6 client.

Working Principle of the DHCPv6 Relay Agent

Figure 6-61 shows the working process of a DHCPv6 relay agent. A DHCPv6 client sends packets to a DHCPv6 server through a DHCPv6 relay agent to obtain IPv6 addresses, prefixes, and other network configuration parameters, such as IPv6 addresses of DNS servers.

Figure 6-61 Working principle of a DHCPv6 relay agent

The working process of a DHCPv6 relay agent is as follows:

  1. A DHCPv6 client sends DHCPv6 Request packets with the destination multicast address FF02::1:2 to all DHCPv6 servers and DHCPv6 relay agents.
  2. A DHCPv6 relay agent processes packets in the following two ways:
    • If a DHCPv6 relay agent and a DHCPv6 client are located on the same link, that is, the DHCPv6 relay agent is the first-hop relay agent of the DHCPv6 client, the DHCPv6 relay agent is the IPv6 gateway of the DHCPv6 client. After receiving a packet from the DHCPv6 client, the DHCPv6 relay agent encapsulates the packet in the Relay Message option of a Relay-Forward packet. Then the DHCPv6 relay agent sends the Relay-forward packet to a DHCPv6 server or the next hop relay agent.

    • If the DHCPv6 relay agent and DHCPv6 client are on different links, the DHCPv6 relay agent receives Relay-Forward packets sent from other relay agents. The DHCPv6 relay agent constructs a new Relay-Forward packet and sends the packet to the DHCPv6 server or the next hop relay agent.
  3. The DHCPv6 server parses the request of the DHCPv6 client in the Relay-Forward packet and selects IPv6 addresses and other network configuration parameters to construct a reply packet. Then the DHCPv6 server encapsulates the reply packet in the Relay Message option in a Relay-Reply packet and sends the Relay-reply packet to the DHCPv6 relay agent.
  4. The DHCPv6 relay agent parses the reply packet of the DHCPv6 server in the Relay-Reply packet and forwards the reply packet to the DHCPv6 client. If the DHCPv6 client receives reply packets from multiple DHCPv6 servers, the DHCPv6 client selects the DHCPv6 server with the highest priority, and obtains the IPv6 address and other network configuration parameters from the DHCPv6 server.
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Updated: 2019-08-09

Document ID: EDOC1000041694

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