Connectivity Fault Management (CFM)

The Connectivity Fault Management (hereinafter also CFM) functionality allows detecting unavailable links between CPE devices. When CFM is enabled, the CPE device sends Continuity Check Message (CCM) control packets over its links at the specified intervals, and listens for response control packets on opposite-direction links. If response control packets do not arrive, the CPE device considers the link unavailable and starts transmitting traffic over a randomly selected available link.

The response timeout is equal to the control packet sending interval times 3.5. The time it takes to switch a CPE device over to a new connection when no response packets are received is equal to the control packet sending interval multiplied by a number in the range from 3.5 to 7. For example, if you specified 300 milliseconds as the time interval for sending control packets, the response timeout is 1.05 seconds, and the time it takes to switch the CPE device to a new link is 1.05 to 2.1 seconds.

You can enable CFM on a CPE device or link:

• A CPE device with CFM enabled sends control packets over all of its links, including new links that will be established in the future. If you specify different time intervals for sending control packages on CPE devices, the largest of the specified time intervals is used. For example, if you specified 300 milliseconds as the control packet interval on CPE 1, and 1 second on CPE 2, the control packet interval is 1 second.

  When CFM is enabled on one CPE device and disabled on the other CPE device, the CPE device with CFM enabled takes precedence. In this case, CPE devices send control packets to each other over all of their links.

• When CFM is enabled for a link, the CPE device sends control packets over that link only. When you enable CFM for a link, you must also enable CFM on the opposite-direction link. The time interval for sending control packets set for the link overrides the time intervals for sending control packets set on CPE devices. For example, if you specified 300 milliseconds as the control packet interval on CPE 1, 1 second on CPE 2, and 10 seconds on the link between CPE 1 and CPE 2, the resulting time interval for sending control packets is 10 seconds.

For example, you can enable CFM on a CPE device and disable CFM on one of the links of that CPE device. In this case, the CPE device sends control packets over all its links, except for the link on which CFM is disabled. By default, CFM is disabled on CPE devices and links.

Enabling CFM on a CPE device

A CPE device with CFM enabled sends control packets over all of its links, including new links that will be established in the future. You can enable or disable CFM in a CPE template or on a CPE device. CFM settings specified in the CPE template are automatically propagated to all CPE devices that use this CPE template.

When CFM is enabled on one CPE device and disabled on the other CPE device, the CPE device with CFM enabled takes precedence. In this case, CPE devices send control packets to each other over all of their links.

To enable CFM on a CPE device:

1. Enable CFM on the CPE device in one of the following ways:
   • If you want to enable CFM in a CPE template, go to the SD-WAN → CPE templates menu section, click the CPE template, and select the CFM tab.
   • If you want to enable CFM on a CPE device, go to the SD-WAN → CPE menu section, click the CPE device, select the CFM tab, and select the Override check box.
2. In the CFM drop-down list, select Enabled. The default value is Disabled.
3. In the Interval drop-down list, select the time interval for sending control packets:
   • 300 ms.
   • 1 s. Default value.
   • 10 s.
   • 1 min.

   The response timeout is equal to the control packet sending interval times 3.5. The time it takes to switch a CPE device over to a new connection when no response packets are received is equal to the control packet sending interval multiplied by a number in the range from 3.5 to 7. For example, if in the Interval drop-down list, you selected 300 ms, the response timeout is 1.05 seconds, and the time it takes to switch the CPE device to a new link is 1.05 to 2.1 seconds.

   If you specified different control packet intervals on CPE devices, the largest of the specified intervals is used. For example, if in the Interval drop-down list for CPE 1, you selected 300 ms, and for CPE 2 you selected 1 s, the time interval for sending control packets is 1 second.

4. In the upper part of the settings area, click Save to save the settings of the CPE template or CPE device.

Enabling CFM on a link

When CFM is enabled for a link, the CPE device sends control packets over that link only. When you enable CFM for a link, you must also enable CFM on the opposite-direction link.

To enable CFM on a link:

1. Enable CFM on the link in one of the following ways:
   • If you want to enable CFM for a link that was established from a CPE device, go to the SD-WAN → CPE section, click the CPE device, select the Links tab, and click Management → Set CFM next to the link.
   • If you want to enable CFM 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 Links section, and click Management → Set CFM next to the link.
2. If you want to enable CFM for one of the links in the graphic topology with all links, go to the Infrastructure section, click Management → Configuration menu next to the controller, go to the Topology section, click the link, and click Set CFM.
3. This opens a window, in that window, select the Override check box. This check box is cleared by default.
4. Select Save for both links to enable CFM on the opposite-direction link. This check box is cleared by default.
5. In the CFM drop-down list, select Enabled. The default value is Disabled.
6. In the Interval drop-down list, select the time interval for sending control packets:
   • 300 ms.
   • 1 s. Default value.
   • 10 s.
   • 1 min.

   The response timeout is equal to the control packet sending interval times 3.5. The time it takes to switch a CPE device over to a new connection when no response packets are received is equal to the control packet sending interval multiplied by a number in the range from 3.5 to 7. For example, if in the Interval drop-down list, you selected 300 ms, the response timeout is 1.05 seconds, and the time it takes to switch the CPE device to a new link is 1.05 to 2.1 seconds.

   The time interval for sending control packets set for the link overrides the time intervals for sending control packets that you specified when enabling CFM on CPE devices. For example, if in the Interval drop-down list for CPE 1, you selected 300 ms, for CPE 2 you selected 1 s, and for the link between CPE 1 and CPE 2 you selected 10 s, the resulting time interval for sending control packets is 10 seconds.

7. Click Save.

   CFM is enabled for the link.

8. If you enabled CFM for a link established from the CPE device, click Save in the upper part of the settings area to save the CPE device settings.