Configuring Dynamic PMTU

Context

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 the S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-HI, S6730-H, S6730S-H, S6730-S, and S6730S-S support the IPv6 PMTU discovery function.

Procedure

Run system-view
The system view is displayed.
(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.
  
  Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support switching between Layer 2 and Layer 3 modes.
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 command to restart the interface and allow the changes to take effect.
5. Run quit
  
  Return to the system view.
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.
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.