Determining the MTU value

You can determine the MTU value of a link to find out why fragmented packets are being blocked on the link (see the figure below).

Diagram of IP packets passing through devices on the network, where fragmented packets are dropped

Links with a reduced MTU size and fragmented packet getting dropped

The MTU value is determined by sending LLDP packets with a variable payload size through the link. The minimum detectable MTU size is 1280 bytes, and the maximum is 1500 bytes. The MTU value is determined automatically when CPE devices are enabled and at an interval specified in the topology.link.pmtud.scheduler.interval.sec controller property.

You can determine the MTU value manually.

To manually determine the MTU value,

Determine the MTU value in one of the following ways:

If you want to manually determine the MTU value for a link that was established from a CPE device, go to the SD-WAN → CPE section, click the CPE device, select the Tunnels tab, and click Management → Check MTU next to the link.
If you want to manually determine the MTU value for one of the links in the table of all links, go to the Infrastructure section, click Management → Configuration menu next to the controller, go to the Tunnels section, and click Management → Check MTU next to the link.

The MTU value is displayed in the MTU column.

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