Determining the effective MTU in a link

Kaspersky SD-WAN can determine the supported MTU (maximum transmission unit) size on links between two devices (a CPE device and an SD-WAN gateway or between two CPE devices).

Determining the maximum MTU size on links is necessary to ensure the passage of user traffic through the SD-WAN network when the MTU on the underlay network is too low, and fragmented packets are blocked on the subsequent hop (see the figure below).

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

Example of a link with a reduced MTU size and fragmented packet getting dropped

The supported MTU size is calculated by sending variable-payload LLDP packets through all links on the CPE device and the SD-WAN gateway. The minimum detectable MTU size is 1,280 bytes, and the maximum size is 1,500 bytes.

The supported MTU size is calculated:

When the CPE device is turned on.
With the frequency set in the topology.link.pmtud.scheduler.interval.sec property of the SD-WAN Controller. By default, the frequency is set to 86,400 seconds.
Manually when you request it.

You can calculate the supported MTU size on an individual link. All links built within the SD-WAN network are displayed in the overall table of links in the Tunnels section, as well as in the graphic topology in the Topology section. A table of links built using a particular CPE device is also displayed in the configuration of that CPE device, on the Tunnels tab.

Supported MTU sizes are displayed in the MTU column of the link table. If the value has not been calculated yet, the Unknown value is displayed.

To calculate the MTU on a link, use the following instructions:

Calculating the MTU on a link using the overall table of links.
To calculate the MTU on a link using the overall table of links:
1. In the menu, go to the Infrastructure section.
  The SD-WAN infrastructure management page is displayed. By default, the Network resources tab is selected, which displays the table of SD-WAN Controllers.
2. Click Management next to the SD-WAN Controller and in the drop-down list, select Configuration menu.
  This opens the SD-WAN Controller configuration menu. By default, you are taken to the Controller nodes section, which displays a table of Controller nodes.
3. Go to the Tunnels section.
  A table of links is displayed.
4. Click Management next to the link and in the drop-down list, select Check MTU.
The test result is displayed in the MTU column.
Calculating the MTU on a link in the configuration of a CPE device.
To calculate the MTU on a link in the configuration of a CPE device:
1. In the menu, go to the SD-WAN section.
  By default, the CPE subsection is displayed with a table of CPE devices.
2. Click the CPE device.
  The settings area is displayed in the lower part of the page. You can expand the settings area to fill the entire page by clicking the expand button .
3. Select the Tunnels tab.
  A table of links is displayed.
4. Click Management next to the link and in the drop-down list, select Check MTU.
The test result is displayed in the MTU column.

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