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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 DHCP snooping.

Basic Principles

DHCP snooping provides the trusted interface and listening functions.

Trusted Interface

DHCP snooping supports the trusted interface and untrusted interfaces to ensure that DHCP clients obtain IP (Internet Protocol) addresses from an authorized DHCP server.

If a private DHCP server exists on a network, a DHCP client may obtain an incorrect IP address and network configuration parameters from it, leading to communication failure. The trusted interface controls the source of DHCP Reply messages to prevent bogus or unauthorized DHCP servers from assigning IP addresses and other configurations to other DHCP clients.

The trusted interface and untrusted interfaces process DHCP messages as follows:
  • The device forwards DHCP Reply messages on the trusted interface.
  • The device discards DHCP ACK messages, NAK messages, and Offer messages on untrusted interfaces.
NOTE:

The administrator configures the interface directly or indirectly connected to an authorized DHCP server as the trusted interface, and other interfaces as untrusted interfaces. This ensures that DHCP clients obtain IP addresses from authorized DHCP servers.

Listening

After DHCP snooping is enabled, the device generates a DHCP snooping binding table by listening to DHCP Request messages and Reply messages. A binding entry contains the MAC address, IP address, interface number, and Virtual Local Area Network (VLAN) ID of the DHCP client.

The DHCP snooping binding entries are aged out when the DHCP release expires, or the entries are deleted when users send DHCP Release packets to release IP addresses.

The administrator needs to record IP addresses of DHCP clients and identify the mappings between the IP addresses and MAC addresses of the DHCP clients. The DHCP snooping binding table helps the administrator conveniently record the mappings.

NOTE:

To ensure that the device obtains parameters such as MAC addresses for generating a DHCP snooping binding table, apply DHCP snooping to Layer 2 access devices or the first DHCP relay agent from the device to the DHCP server.

The DHCP snooping binding table records the mapping between IP addresses and MAC addresses of DHCP clients. The device can check DHCP messages against the DHCP snooping binding table to prevent bogus DHCP message attacks.

Option 82 Supported by DHCP Snooping

Overview

During the traditional dynamic IP address allocation, a DHCP server cannot detect the DHCP client location based on the received DHCP Request message. As a result, DHCP clients in the same VLAN have the same right to access network resources. The network administrator cannot control network access of clients in the same VLAN, which brings challenges to security control.

RFC 3046 defines DHCP Relay Agent Information Option, that is, the Option 82 field, which records the location of a DHCP client. A DHCP snooping-enabled device or a DHCP relay agent inserts the Option 82 field to a DHCP Request message to notify the DHCP server of the DHCP client location. The DHCP server can properly assign an IP address and other configurations to the DHCP client, ensuring DHCP client security.

The Option 82 field contains two commonly used suboptions: circuit ID and remote ID. The circuit ID distinguishes VLAN ID and interface number of a client, and the remote ID distinguishes the MAC address of the client.

NOTE:
  • As a DHCP relay agent, the device supports the Option 82 field no matter whether DHCP snooping is enabled on the device. However, as an access device on a Layer 2 network, the device supports the Option 82 field only after DHCP snooping is enabled.
  • The Option 82 field records the location of a DHCP client and is encapsulated in a DHCP Request message sent to the DHCP server. To deploy different IP addresses or security policies for different clients, the DHCP server must support the Option 82 field and be configured with IP address assignment or security policies.
  • The Option 82 field is different from parameters recorded in a DHCP snooping binding table. The device adds the Option 82 field to the DHCP Request message when the DHCP client requests an IP address. At this time, the client does not have an IP address. A DHCP snooping binding table is generated based on the DHCP ACK messages replied by the DHCP server. At this time, the client obtains an IP address.
Implementation
As a DHCP relay agent or an access device on the Layer 2 network, the device supports the Option 82 field after DHCP snooping is enabled. The device inserts the Option 82 field to a DHCP Request message in two modes:
  • Insert mode: Upon receiving a DHCP Request message without the Option 82 field, the device inserts the Option 82 field. If the DHCP Request message contains the Option 82 field, the device checks whether the Option 82 field contains the remote ID. If so, the device retains the Option 82 field; if not, the device inserts the remote ID.

  • Rebuild mode: Upon receiving a DHCP Request message without the Option 82 field, the device inserts the Option 82 field. If the DHCP Request message contains the Option 82 field, the device deletes the original Option 82 field and inserts the Option 82 field set by the administrator.

The device handles the reply packets from the DHCP server in the same way no matter whether the Insert or Rebuild method is used.

  • The DHCP reply packets contain Option 82 field:
    • If the DHCP request packets received by the device do not contain Option 82 field, the device deletes Option 82 field from the DHCP reply packets, and forwards the packets to the DHCP client.
    • If the DHCP request packets contain Option 82 field, the device changes the Option 82 format in the DHCP reply packets into the Option 82 format in the DHCP request packets, and then forwards the packets to the DHCP client.
  • If the DHCP reply packets do not contain Option 82 field, the device directly forwards the packets.
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Updated: 2019-08-09

Document ID: EDOC1000041694

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