9.1.4 Dynamic monitoring

The generation and deletion of static routes is governed according to the status of the interface directly connected to the gateway, or the presence or absence of a route to that gateway. Consequently, there is no guarantee that packets will reach the gateway even if a route has been created. The Switch provides functionality for dynamically monitoring packet delivery by polling gateways at regular intervals using ICMPv4 Echo Request and Echo Reply messages. By using this function, you can control the generation of static routes only if reachability to the corresponding gateway is ensured in addition to the route generation and deletion conditions described in 9.1.3 Specifying forwarding routes for static routes.

Even if a gateway that was unreachable becomes reachable, the route is not generated at that time. Rather, the route is generated after the reachability of the gateway is monitored for a set period and stability is confirmed.

<Structure of this section>

(1) Switching routes by dynamic monitoring of static routes

The following figure shows an example of monitoring static routes dynamically.

Figure 9-2: Example of dynamic monitoring of static routes

In this example, two static routes to network B have been set up in Switch A: a preferred route via Switch B and a next-best route via Switch C. Without dynamic monitoring, if an error occurs in the Switch B interface, the static route via Switch B would not be deleted because the interface on the Switch A side is still working normally. This stops communication between Switch A and Network B without switching to a static route through Switch C.

With dynamic monitoring, however, the monitoring functionality of Switch A detects that Switch B is unreachable although the interface on the Switch A side is normal. This enables switching to a static route via Switch C and ensures communication between Switch A and Network B.

(2) Timing for creating, deleting, and regenerating static routes through dynamic monitoring

The timing for generating, deleting, or restoring a static route by dynamic monitoring depends on the values set in the ip route static poll-interval and ip route static poll-multiplier configuration commands.

In the following description, the relevant settings are pollinterval in ip route static poll-interval, and invalidcount and restorecount in ip route static poll-multiplier.

(a) Route generation timing

The Switch polls a gateway when its interface comes online or some other triggering event occurs. If a response is received, a static route through that gateway is created when the next polling time arrives (pollinterval). The following figure shows an example of route generation by dynamically monitoring static routes.

Figure 9-3: Route generation by dynamic monitoring of static routes

(b) Route deletion timing

A static route through a gateway that is polled at regular intervals (pollinterval) will be deleted if no response is received after the number of consecutive poll attempts set in the invalidcount parameter. For example, if the invalidcount setting is 3, the route will be deleted if no response is received after three consecutive polls. If the interface goes down or some other trigger for route generation is lost, the static route will be deleted just as if the gateway were not being polled (poll parameter not specified). The following figure shows an example of route deletion by dynamically monitoring static routes.

Figure 9-4: Deleting static routes by dynamic-monitoring (invalidcount=3)

(c) Path Regeneration Timing

A static route deleted as a result of dynamic monitoring will be restored when the number of consecutive responses received from the polled gateway matches the number set in the restorecount parameter. For example, if the restorecount setting is 2, the route will be restored when responses are received for two consecutive polls. The following figure shows an example of restoring a static route as a result of dynamic monitoring.

Figure 9-5: Route regeneration by dynamically monitoring static routes (when restorecount =2)