You can enable polling with the device-tracking tracking command in global configuration mode. After you do, you still have the flexibility to turn polling on or off for a particular interface or VLAN. For this, configure the tracking enable or tracking disable keywords in the policy (the device-tracking configuration mode). When polling is enabled, the switch polls the host at the specified interval, thus reconfirming its reachability for the duration of its reachable lifetime. When polling is enabled, the switch sends up to three polling requests, after the reachable lifetime expires, at system-determined intervals.
You can also configure this interval with the device-tracking tracking retry-interval seconds command in global configuration mode. The figure below depicts the lifecycle of an entry where the host is polled. Default reachable and stale lifetimes, and retry intervals are used in figure: An event (E) is detected and a REACHABLE entry is created. If an event is detected during the reachable lifetime, the reachable lifetime timer is reset. The switch sends a polling request after the reachable lifetime expires. The switch polls the host up to three times at fixed, system-determined intervals.
The polling request may be in the form of a unicast Address Resolution Protocol (ARP) probe or a Neighbor Solicitation message. During this time the state of the entry changes to VERIFY. If a polling response is received (thus confirming reachability of the host), the state of the entry changes back to REACHABLE. If the switch does not receive a polling response after three attempts, the entry changes to the STALE state. It remains in this state for 24 hours. If an event is detected during the stale lifetime, the state of the entry is changed back to REACHABLE. At expiry of the stale lifetime, the device sends one final polling to ascertain reachability.
If this final polling attempt receives a reply, the state of the entry is changed back to REACHABLE. If the final polling attempt does not receive a response, the entry is deleted. Figure 5: Lifecycle of an Entry Where the Host is Polled (Click on the image to see the details more clearly.) The DHCP lease time determines how long a DHCP-originated binding remains valid in the table. When the DHCP lease time expires, the entry is removed from the table without additional reachability probes. Binding Table Sources The following are the sources of information and events that cause the creation and update of a binding table entry: • Learning events that dynamically populate the binding table:
• Dynamic Host Configuration Protocol (DHCP) negotiation (DHCP REQUEST, and DHCP REPLY). This includes DHCPv4 and DHCPv6. Configuring Switch Integrated Security Features Binding Table Sources Configuring Switch Integrated Security Features 5