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Configuration Guide - IP Service

S7700 and S9700 V200R013C00

This document describes the configurations of IP service, including IP address, ARP, DHCP, 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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Configuring Dynamic PMTU

Configuring Dynamic PMTU


When the switch functions as an intermediate node, IPv6 PMTU discovery can be enabled. After this function is enabled, if the length of an IPv6 packet is larger than the IPv6 MTU of the outbound interface, the switch discards the packet, and uses the IPv6 PMTU discovery protocol to dynamically negotiate the PMTU with the source node for packet fragmentation.

After the PMTU ages out, the dynamic PMTU is deleted. The PMTU needs to be renegotiated.


When both static and dynamic PMTUs are configured, only static PMTU takes effect. Static PMTU entries never age.

Only X series cards support the IPv6 PMTU discovery function.


  1. Run system-view

    The system view is displayed.

  2. (Optional) Configure the IPv6 MTU for an interface.

    1. Run interface interface-type interface-number

      The specified interface view is displayed.

    2. (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.

    3. Run ipv6 enable

      The IPv6 function is enabled on the interface.

      By default, the IPv6 function is disabled on an interface.

    4. Run ipv6 mtu mtu

      The MTU of IPv6 packets on an interface is set.

      By default, the MTU of IPv6 packets on an interface is 1500 bytes.


      The switch supports MTU setting, so packets sent by the protocol stack are fragmented based on the configured MTU. However, packets forwarded by the hardware chip are not fragmented based on the configured MTU. The maximum length of packets that can be forwarded by the hardware chip is 12K bytes.

      After the MTU value is changed, run the shutdown and undo shutdown commands or the restart (interface view) command to restart the interface and allow the changes to take effect.

    5. Run quit

      Return to the system view.

  3. Run ipv6 packet-too-big drop

    The switch is configured to discard packets with the length larger than the IPv6 MTU of the interface, and IPv6 PMTU discovery is enabled.

    By default, the switch properly forwards packets with the length larger than the IPv6 MTU of the outbound interface and IPv6 PMTU discovery is disabled.


    On interfaces of X series cards except X1E, X5E, X5H, X5L, and X5S cards, the MTU value for sending IPv6 packets is set to 1500 bytes by default, and the MTU value modified through the ipv6 mtu command does not take effect. Therefore, for interfaces on these cards, the switch uses MTU values modified through the ipv6 mtu command to negotiate the PMTU with source nodes, but the effective PMTU is 1500 bytes.

  4. Run ipv6 pathmtu age age-time

    The aging time is set for dynamic PMTU entries.

    By default, the aging time of dynamic PMTU entries is 10 minutes.

Updated: 2019-04-20

Document ID: EDOC1100065743

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