15.1.6 Tracking function

The Switch supports fault monitoring interfaces and VRRP polling as a feature that monitors network failures and dynamically manipulates virtual router priorities (tracking function).

If a failure occurs on an interface on which a virtual router is configured, a backup router takes over as the master router. However, if a failure occurs on an interface on which no virtual router is configured, such as an IP interface, a port channel interface, or an Ethernet interface that is the destination of packet routing, no backup router takes over as the master even if communication is disabled.

As a unique additional functionality, the Switch provides functionality for monitoring the VLAN interfaces, port channel interfaces, and Ethernet interfaces on it and for lowering the priorities of virtual routers if the interfaces go down. This tracking functionality is called failure monitoring interfaces. Note that an IP address must be assigned to a VLAN interface if you want to monitor it for failures.

The failure monitoring interfaces cannot detect failures that occur beyond the routers because they can only monitor the failures that are manifested as interface-down failures. The Switch has another special functionality that can be used as tracking functionality. VRRP polling monitors the specified VLAN interfaces, checks the reachability of the specified destinations by pinging them, and lowers the priorities of virtual routers if no reply is returned. This tracking functionality is called VRRP polling.

You can use the failure monitoring interfaces to monitor the failures that occur between the Switch and neighboring devices. You can use VRRP polling to monitor the failures that occur between the Switch and devices located beyond the routers.

Two methods are provided for changing the priorities of virtual routers.

One method is priority switching. Priority switching allows you to change the priority of a virtual router to the value specified in the vrrp track priority configuration command when the tracking functionality detects a failure on it.

The other is priority decrement. Priority decrement subtracts the value specified in the vrrp track decrement configuration command for the failure monitoring interfaces from the priority value of a virtual router when the tracking functionality detects a failure.

For priority switching, you can specify one failure monitoring interface or one instance of VRRP polling. For priority decrement, you can specify multiple failure monitoring interfaces and multiple instances of VRRP polling.

When the priority of a virtual router becomes 0 as a result of executing tracking functionality, the IP interface on which the virtual router is configured goes down.

Table 15-2: Combination of priority operation method and monitoring method
Method for changing priority	Failure monitoring interfaces	VRRP polling
Priority switching	Only one instance of polling can be specified.	Only one instance of polling can be specified.
Priority decrement	Multiple instances of polling can be specified.	Multiple instances of polling can be specified.

<Structure of this section>

(1) Failure monitoring interface

The following figure shows failure monitoring interfaces for a virtual router.

Figure 15-6: Fault monitoring interface

In this example, VLAN interfaces are specified as failure monitoring interfaces. VLAN interface Ia and VLAN interface Ib are assigned to Switch A. The virtual router is configured on VLAN interface Ia. In normal VRRP operation, if VLAN interface Ib goes down due to a VLAN failure, the operation of the virtual router is not affected. However, on the Switch, you can change the operating status of a virtual router by specifying failure monitoring interfaces and a priority switching value or priority decrement value to be applied if a failure monitoring interface goes down.

Specify VLAN interface Ib as the failure monitoring interface for the virtual router on Switch A. Specify 0 as the priority to be applied if the failure monitoring interface goes down.If VLAN interface Ib goes down, Switch B automatically takes over for Switch A and becomes the master.

Similarly, you can change the operating status of a virtual router by assigning a port channel interface or Ethernet interface as a failure monitoring interface.

(2) VRRP polling

The following figure shows the difference between when VRRP polling is configured and when VRRP polling is not configured.

Figure 15-7: Comparing VRRP polling with no polling configured

If a failure occurs on the device that is the destination of VRRP polling or if no reply is returned due to a network failure, VRRP polling lowers the priority based on the predefined switching priority or priority decrement.

The following table describes the VRRP status and the corresponding priority and intervals of polling attempts.

Table 15-3 Combining State, Priority, and Polling Attempt Interval for VRRP Polling
Status	Priority	Polling attempt interval
Normal	Priority set by the vrrp priority configuration command	track check-status-interval
Failure detection inspection	Priority set by the vrrp priority configuration command	track failure-detection-interval
Problem	Based on the switching priority set by the vrrp track priority configuration command or the priority decrement set by the vrrp track decrement configuration command, lowers the priority	track check-status-interval
Failure recovery inspection		track recovery-detection-interval

The following figure shows the status transitions of VRRP polling and transition conditions.

Figure 15-8: State transitions during VRRP polling

No reply was made, and a timeout occurred.
A response that satisfies the polling success count#2 is received for the polling attempt count#1.
When it is found that the response that satisfies the polling success count#2 cannot be received for the polling attempt count#1
Received a response
When it is found that the response that satisfies the polling success count#3 cannot be received for the polling attempt count#1
A response that satisfies the polling success count#3 is received for the polling attempt count#1.

#1: Set by using the track check-trial-times configuration command.

#2: Set by using the track failure-detection-times configuration command.

#3: Set by using the track recovery-detection-interval configuration command.

<>Failure Verification Operation

The following figure shows the failure detection inspection sequence.

Figure 15-9: Error verification sequence

In failure detection inspection, polling is performed at the special intervals. When the Switch determines that it is not possible to satisfy the polling success condition within the number of polling retries (in this figure when the nth response timed out), the Switch determines a failure has occurred and lowers the priority.

In the factory default configuration, the number of polling retries is set to 4. The Switch determines that polling will not succeed within the number of polling retries when two responses time out (four seconds after the failure detection operation started) and lowers the priority.

<>Failure recovery verification operation

The following figure shows the failure recovery inspection sequence.

Figure 15-10: Error recovery verification sequence

Failure recovery verification performs polling at special intervals. When the Switch satisfies the polling success condition within the number of polling retries (in this figure when the nth response is received), the Switch determines that it has recovered from a failure and returns the priority of the Switch to normal.

In the factory default configuration, the number of polling retries is set to 4. The Switch determines that polling is successful when the Switch receives three responses (six seconds after the failure recovery inspection started) and returns its priority to normal.

If an interface goes down, VRRP polling assumes that a failure has occurred and waits until the interface is enabled. When the interface is enabled, VRRP polling restarts the polling and performs failure recovery verification.When VRRP polling determines that operation is normal, switch-back is performed.

When the IP address of the VRRP polling destination is on the network beyond the routers, the routing tables of the routers are used to determine the IP address. For this reason, the interface receiving VRRP poll response might not match the interface sending VRRP poll, as shown in Figure 15-11: Incoming and outgoing interfaces do not match. In this case, specify the receiving interface check (track check-reply-interface configuration command) to check the sending interface and receiving interface. Packets are dropped when the sending interface and receiving interface do not match. If there is a mismatch in an interface on a network that is not under the control of the local device, such as "Figure 15-12: Interface mismatch on a network that is not under the control of the local device", no guarantees are made.

Figure 15-11: Transmit and receive interfaces do not match

Figure 15-12: Interface mismatch on a network that is not under the control of the local device