Overview

The Path MTU (PMTU) discovery protocol dynamically discovers the MTU value of each hop along a given transmission path, which saves IPv6 device processing resources and helps IPv6 networks run more efficiently.

The PMTU protocol is accomplished through the Packet Too Big message of ICMPv6. The basic idea is that a source node initially assumes that the PMTU of a path is the MTU of its outgoing interface. If any of the IPv6 packets sent on that path are too large to be forwarded by some node along the path, that node will discard them and return ICMPv6 Packet Too Big messages to the source node with its own MTU value.

Upon receipt of such a message, the source node reduces its assumed PMTU for the path based on the MTU of the constricting hop as reported in the Packet Too Big message. The decreased PMTU causes the source to send smaller packets. This process is repeated until the message reaches the destination, then the source node knows the minimum MTU in the path of two communicating hosts which is called the Path MTU value.

The PMTU of a path may change over time, due to changes in the routing topology. Reductions of the PMTU are detected by Packet Too Big messages. To detect increases in a path's PMTU, a node periodically restores its assumed PMTU to the originally configured MTU value of the system (configured by using command set l3-interface vlan-interface <interface-name> mtu <mtu-value> or set l3-interface routed-interface <interface-name> mtu <mtu-value>).

Enabling Path MTU Discovery

The following commands can be used to enable or disable PMTU function on a VLAN interface or a routed interface.

set l3-interface vlan-interface <vlan-interface> pmtu-discovery disable <true | false>

set l3-interface routed-interface <routed-interface> pmtu-discovery disable <true | false>

Enable PMTU function on all the nodes in the path where you want to apply the PMTU function.

• The following example enables PMTU function on VLAN interface vlan100.

admin@PICOS# set l3-interface vlan-interface vlan100 pmtu-discovery disable falseadmin@PICOS# commit

• The following example enables PMTU function on routed interface rif-te19.

admin@PICOS# set l3-interface routed-interface rif-te19 pmtu-discovery disable false
admin@PICOS# commit