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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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Specifying an IP Address for the DHCP Server on a DHCP Relay Agent

Specifying an IP Address for the DHCP Server on a DHCP Relay Agent

Context

A device functioning as a DHCP relay agent transparently transmits DHCP Discovery messages to the destination DHCP server. The DHCP server can then allocate network parameters including IP addresses to clients. After an IP address is specified for the DHCP server on the DHCP relay agent, clients can send DHCP Discovery messages to the DHCP server to apply for IP addresses.

The device supports two configuration methods:
  • In the interface view: Specify an IP address for the DHCP server in the interface view. This method is recommended for configuring DHCP relay on an interface or multiple interfaces on which DHCP servers have different IP addresses.

  • In the DHCP server group view: Create a DHCP server group in the system view, add members (IP addresses of specified DHCP servers) to the group, and apply the DHCP server group to an interface. This method is recommended for configuring DHCP relay on multiple interfaces that map the same DHCP server.

A maximum of 16 DHCP relay agents are allowed between a DHCP server and a DHCP client. If there are more than 16 DHCP relay agents, DHCP messages are discarded.

You can configure multiple DHCP server addresses on an interface of a device functioning as a DHCP relay agent. The device functioning as a DHCP relay agent processes a received broadcast DHCP Discover message as follows:
  • The device forwards the message to all DHCP servers by default, and does not check whether the DHCP server status is Down. The device changes the forwarding order each time it forwards a DHCP Discover message.
  • If the ip relay address cycle command is configured, the device forwards a DHCP Discover message to one DHCP server at a time and does not check whether the DHCP server status is Down. Each time the device receives a DHCP Discover message, it forwards the message to a different DHCP server to implement load balancing among DHCP servers.

Procedure

  • Specify an IP address for the DHCP server in the interface view.
    1. Run system-view

      The system view is displayed.

    2. (Optional) Run ip relay address cycle

      The DHCP server polling function is configured on the DHCP relay agent.

      By default, DHCP server polling is disabled on a DHCP relay agent.

    3. (Optional) Run dhcp set ttl { unvaried | ttl-value }

      The TTL value is set for DHCP Discovery messages after they are forwarded by the DHCP relay agent at Layer 3.

      By default, the TTL value of DHCP Discovery messages decreases by 1 after they are forwarded by the DHCP relay agent at Layer 3.

      NOTE:
      If the DHCP relay agent connects to a special client whose TTL value of DHCP Discovery messages is 1, and if there are routing devices between the DHCP relay agent and DHCP server, run the dhcp set ttl ttl-value command to specify a fixed TTL value (16 is recommended) for DHCP Discovery messages after they are forwarded by the DHCP relay agent at Layer 3.

    4. Run interface interface-type interface-number[.subinterface-number ]

      The interface view or sub-interface view is displayed.

      NOTE:

      Only the S5720EI, S5720HI, S5730HI, S6720EI, S6720HI, and S6720S-EI support sub-interfaces.

    5. (Optional) On an Ethernet interface, run undo portswitch

      The interface is switched to Layer 3 mode.

      By default, an Ethernet interface works in Layer 2 mode.
      NOTE:

      Only the S5720EI, S5720HI, S5730HI, S6720EI, S6720HI, and S6720S-EI support switching between Layer 2 and Layer 3 modes.

    6. Run dhcp relay server-ip ip-address

      An IP address is specified for a DHCP server.

      By default, no IP addresses are specified.

      Each interface supports a maximum of 20 IP addresses of DHCP servers.

    7. (Optional) Run dhcp relay gateway-switch enable

      DHCP relay gateway switching is enabled.

      By default, DHCP relay gateway switching is disabled.

      After primary and secondary IP addresses are configured on an interface, the primary IP address functions as the gateway address in most cases. If clients cannot use the primary IP address to apply for IP addresses, configure DHCP relay gateway switching to allow the clients to use secondary IP addresses to apply for IP addresses. After DHCP relay gateway switching is enabled, configure address pools on the same network segment as the secondary IP addresses on the connected DHCP server.

      • The gateway address switches from the primary IP address to a secondary IP address only when a user fails at least three times to obtain an IP address using the primary IP address and the interval between the last failure and first failure exceeds 24 seconds.
      • Each interface supports one primary IP address and 31 secondary IP addresses. The system tries the secondary IP addresses one by one based on the IP address configuration sequence until users successfully obtain IP addresses.

  • Specify an IP address for the DHCP server in the DHCP server group view.
    1. Run system-view

      The system view is displayed.

    2. (Optional) Run ip relay address cycle

      The DHCP server polling function is configured on the DHCP relay agent.

      By default, DHCP server polling is disabled on a DHCP relay agent.

    3. (Optional) Run dhcp set ttl { unvaried | ttl-value }

      The TTL value is set for DHCP Discovery messages after they are forwarded by the DHCP relay agent at Layer 3.

      By default, the TTL value of DHCP Discovery messages decreases by 1 after they are forwarded by the DHCP relay agent at Layer 3.

      NOTE:
      If the DHCP relay agent connects to a special client whose TTL value of DHCP Discovery messages is 1, and if there are routing devices between the DHCP relay agent and DHCP server, run the dhcp set ttl ttl-value command to specify a fixed TTL value (16 is recommended) for DHCP Discovery messages after they are forwarded by the DHCP relay agent at Layer 3.

    4. Run dhcp server group group-name

      A DHCP server group is created, and its view is displayed.

      By default, no DHCP server group is configured.

      A maximum of 32 DHCP server groups can be configured on a device.

    5. Run dhcp-server ip-address [ ip-address-index ]

      DHCP server members are configured in the DHCP server group.

      By default, no DHCP server member is configured in a DHCP server group.

      A maximum of 20 DHCP servers can be added to a DHCP server group.

    6. (Optional) Run gateway ip-address

      A gateway address is specified for clients.

      If the IP address of the interface that connects the DHCP relay agent to clients functions as the gateway address, skip this step.

      The gateway address specified in this step must be the same as the egress gateway address of clients specified on the DHCP server. If the device functions as the DHCP server, refer to Configuring a Gateway Address for Clients for details about how to specify the egress gateway address for clients.

    7. (Optional) Run vpn-instance vpn-instance-name

      The DHCP server group is bound to a VPN instance.

      By default, the DHCP server group is not bound to a VPN instance.

      Only the S1720GF, S1720GW, S1720GW-E, S1720GWR, S1720GWR-E, S1720GFR-P, S1720X, S1720X-E, S2720EI, S5720EI, S5720HI, S5720I-SI, S5720LI, S5720S-LI, S5720S-SI, S5720SI, S5730HI, S5730S-EI, S5730SI, S6720EI, S6720HI, S6720LI, S6720S-EI, S6720S-LI, S6720S-SI, and S6720SI support this step.

      If the DHCP relay agent is deployed on a VPN network, bind the DHCP server group to the VPN instance that is also bound to the address pool of the DHCP server. Otherwise, clients cannot obtain IP addresses.

    8. Run quit

      Return to the system view.

    9. Run interface interface-type interface-number[.subinterface-number ]

      The interface view or sub-interface view is displayed.

      NOTE:

      Only the S5720EI, S5720HI, S5730HI, S6720EI, S6720HI, and S6720S-EI support sub-interfaces.

    10. (Optional) On an Ethernet interface, run undo portswitch

      The interface is switched to Layer 3 mode.

      By default, an Ethernet interface works in Layer 2 mode.
      NOTE:

      Only the S5720EI, S5720HI, S5730HI, S6720EI, S6720HI, and S6720S-EI support switching between Layer 2 and Layer 3 modes.

    11. Run dhcp relay server-select group-name

      A DHCP server group is configured on the interface.

    12. (Optional) Run dhcp relay gateway-switch enable

      DHCP relay gateway switching is enabled.

      By default, DHCP relay gateway switching is disabled.

      After primary and secondary IP addresses are configured on an interface, the primary IP address functions as the gateway address in most cases. If clients cannot use the primary IP address to apply for IP addresses, configure DHCP relay gateway switching to allow the clients to use secondary IP addresses to apply for IP addresses. After DHCP relay gateway switching is enabled, configure address pools on the same network segment as the secondary IP addresses on the connected DHCP server.

      • The gateway address switches from the primary IP address to a secondary IP address only when a user fails at least three times to obtain an IP address using the primary IP address and the interval between the last failure and first failure exceeds 24 seconds.
      • Each interface supports one primary IP address and 31 secondary IP addresses. The system tries the secondary IP addresses one by one based on the IP address configuration sequence until users successfully obtain IP addresses.

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Updated: 2018-12-24

Document ID: EDOC1100038342

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