4.1.2 Policy-based routing group

You can group multiple routes in a policy-based routing group. Priority is assigned to each route according to the specified application order. This allows a route with a higher priority to be dynamically selected from multiple routes according to the status of the destination interface and forwarding destination route. Information that groups multiple routes is called policy-based routing list information.

Specifying multiple routes in policy-based routing list information provides redundant routes. If the highest priority route can no longer be used for forwarding due to a failure, operation continues by switching to the next highest priority route in the same policy-based routing list information.

The following figure shows an example of a policy-based routing group configuration.

Figure 4-2: Example of a policy-based routing group configuration

The policy-based routing group can also be linked with the polling monitoring tracking function, to monitor routes to a location subject to polling monitoring. Polling monitoring of the tracking function monitors whether communication with devices on the network is possible. Monitoring results are used to determine the route to be selected based on the policy-based routing group. This allows route switching in response to failures that occur between the Switch and neighboring devices and in other routes.

The following figure shows a configuration example when a policy-based routing group is linked with the tracking functionality.

Figure 4-3: Example of how a policy-based routing group and tracking functions work together

<Structure of this section>

(1) Route selection for a policy-based routing group

The policy-based routing group selects a route from the routes registered in the policy-based routing list information based on the following information:

Monitoring results to determine priority and whether forwarding is possible
Default operation specification
Path switch-back operation specification

(a) Monitoring results and priority of forwarding availability

The results of the following monitored items are used to determine whether the routes registered in the policy-based routing list information can be used for forwarding:

Monitoring of the destination interface VLAN status
Polling monitoring based on tracking functionality

The route with the highest priority is selected from the routes that can be used for forwarding.

<>Monitoring VLAN status of the destination interfaces

If the forwarding destination route is specified by using the following configuration command, the possibility of forwarding is determined based on the status of the destination interface VLAN:

policy-interface command

VLAN ID (vlan parameter) and next-hop address (next-hop parameter) of the destination interface

Forwarding is possible only when the destination interface VLAN status is Up.

<>Polling monitoring using the tracking function

If the forwarding destination route is specified by using the following configuration command, the possibility of forwarding is determined based on the results of polling monitoring by the tracking functionality in addition to the status of the destination interface VLAN:

policy-interface command

VLAN ID (vlan parameter), next-hop address (next-hop parameter), and track ID (track-object parameter) of the destination interface

Forwarding is determined to be possible only when both the destination interface VLAN status and the result of polling monitoring are Up.

For details about the tracking functionality, see 4.1.5 Tracking functionality for policy-based routing.

<>Determination by priority

Route selection is based on the results of monitoring the destination interface VLAN status or polling monitoring by the tracking functionality. The route with the highest priority is selected based on the application order specified in the configuration, from the routes that can be used for forwarding in the policy-based routing list information.

(b) Default action specification

If all routes registered in the policy-based routing list information cannot be forwarded, or if no routes are registered, the default operation is called the default operation. The default operation can be specified in the default configuration command. The following table describes the possible default operation specifications.

Table 4-1: Default action specifications
Specification in the configuration command	Default operation	Description
permit	Forward normally	The target packet is forwarded over Layer 3 according to the routing protocol.
deny	Discard	The target packet is discarded.
Not specified	Discard	The target packet is discarded.

Packets that are forwarded over Layer 3 according to the routing protocol or discarded by the default operation are counted in statistics for the access list of the specified target policy-based routing list information.

The following packets cannot be discarded by the default operation:

An IPv4 packet exceeding the MTU
A packet whose TTL is set to 1
An IPv4 packet with an unknown destination

(c) Path switchback operation specification

If a route with a higher priority becomes available in the policy-based routing list information and is forwarded on a route with a lower priority, and the route with a higher priority becomes available for forwarding, the operation is called path switchback operation. The path switch-back operation can be specified in the recover configuration command. The following table describes the possible path switch-back operation specifications.

Table 4-2: Route switchback operation specifications
Specification in the configuration command	Path switch-back operation	Description
on	Switch back	If the route with a higher priority becomes available for forwarding, the route is switched back.
off	Do not switch back	Even if the route with a higher priority becomes available for forwarding, the route is not switched back.
Not specified	Switch back	If the route with a higher priority becomes available for forwarding, the route is switched back.

If Switch back is specified for the path switch-back operation, the route with the highest priority is always selected from the forwarding-enabled routes in the policy-based routing list information.

If Do not switch back is specified for the path switch-back operation, the route is not switched back even if a route with a higher priority than the currently selected route becomes available for forwarding. If the selected route can no longer be used for forwarding, the route is always switched to a route with a lower priority. If none of the routes registered in the policy-based routing list information can be used for forwarding, the default operation is performed instead of switching back the route. However, the route with the highest priority among the forwarding-enabled routes in the policy-based routing list information is selected again in the following cases:

The reset policy-list operation command is executed.
The recover configuration command is used to change the path switch-back operation to Switch back.
The policy-based program is restarted.

(2) Policy-based routing group at startup

When the Switch starts or restarts, monitoring of forwarding availability and route switching are stopped for a certain period of time after the policy-based program starts. This is because the switch status is checked after startup, and the results of the forwarding availability monitoring are not stable for the following reasons.

The VLAN interface is not in the Up state.
The result of polling monitoring by the tracking functionality is not Up.

The state of a route after the policy-based program starts until the policy-based routing group starts monitoring the forwarding availability is called the starting state. In the starting state, any packets subject to policy-based routing are discarded.

If you use any of the following configuration commands to change the policy-based routing list information while in the starting state, the change is applied when monitoring of the forwarding availability starts:

default (policy-list)
policy-interface (policy-list)
policy-list
recover (policy-list)

You can use the policy-list default-init-interval configuration command to change the time that can elapse from when the policy-based program starts, until the start of forwarding availability monitoring (no-forwarding availability monitoring period). Specify the time required to obtain stable results of forwarding availability monitoring after the program starts. The following table describes the transitions of the starting state and describes the transition conditions.

Table 4-3: Status transitions and transition conditions during startup
State transition	Transition conditions
Starting state exited	The starting state exits when the no-forwarding availability monitoring period expires. This state also exits when an operation to interrupt the startup is performed.
Starting state interrupted	The starting state is interrupted when any of the following is performed: The reset policy-list operation command is executed. The no-forwarding availability monitoring period is changed to be shorter than the currently elapsed time in the configuration. The policy-based program is restarted. Switching the master switch
Starting state extended	The starting state is extended when the no-forwarding availability monitoring period is changed to a longer time in the configuration. In this case, forwarding availability monitoring starts when the time obtained by subtracting the currently elapsed time from the new time period has elapsed.

If the starting state exits or is interrupted, the route with the highest priority is selected from the routes that can be used for forwarding in the policy-based routing list information.

(3) Policy-based routing group when switching the master switch

When a stack is configured with the Switch, monitoring of relay availability and switching of routes are stopped for a certain period of time after switching the master switch. This is because the status of the switch after the master switch has been switched is checked, and the monitoring result of forwarding availability is not stable during that time.

The state of a route from when the master switch is switched until the policy-based routing group starts monitoring forwarding availability is called switching. During switching, the route selected before the master switch is switched is inherited without changing the route.

If the policy-based routing list information is changed by the following configuration command during switching, the change is applied when monitoring of forwarding availability starts.

default (policy-list)
policy-interface (policy-list)
policy-list
recover (policy-list)

You can use the configuration command policy-list default-aging-interval to change the time between switching the master switch and the start of forwarding availability monitoring (the time for stopping forwarding availability monitoring). Specify the time required for the monitoring result of forwarding availability to stabilize after the master switch is switched. The following table describes the state transitions and transition conditions during switching.

Table 4-4: State transitions and transition conditions during switching
State transition	Transition conditions
End during switching	When the time for stopping the forwarding availability monitoring has elapsed, switching is terminated. Also, when switching is interrupted, switching is terminated in the same way.
Interrupt switching	Interrupts switching in the following cases: The reset policy-list operation command is executed. The no-forwarding availability monitoring period is changed to be shorter than the currently elapsed time in the configuration. The policy-based program is restarted.
Extension during switching	If the time for stopping the forwarding availability monitoring is changed longer in the configuration, the switching status is extended. In this case, forwarding availability monitoring starts when the time obtained by subtracting the currently elapsed time from the new time period has elapsed.

If switching is terminated or aborted, the route with the highest priority among the routes that can be forwarded in the policy-based routing list information is selected.