Cisco IOS XE, Configuration Manual, Page: 280 / 358

Configuring ISG Policies for Session Maintenance
Information About Configuring Policies for Session Maintenance
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The scope of the session timers and connection timers is determined by the type of service within which
the timer is specified. If specified in a service profile for which no traffic class is defined, the timer action
will be to terminate the session or connection. If a traffic class specifier resides in the service profile, the
timer action will be to deactivate the service.
Benefits of Session Maintenance Timers
The PPP idle timeout functionality has been replaced by the ISG idle timeout feature. The idle timer is
a generic feature that can be set to detect idle traffic in both PPP and IP sessions.
You set the idle timer in a service profile that is installed on a session to control how long that service
stays installed before it is removed from the session because no traffic is flowing through that service.
If the service has traffic class parameters associated with it, that traffic class is terminated when this
timer expires, or when the session itself is terminated.
The same is true for the session timer, except that this timer determines how long the session or service
stays up, regardless of traffic flowing through it.
Monitoring Sessions
The IP subscriber session’s data traffic in the upstream direction can be monitored for idleness using a
keepalive feature configured for the subscriber. If a session is idle for a configured period of time,
keepalive requests are sent to the subscriber. This action verifies that the connection is still active. The
protocol to use for the keepalive request and response can be configured based on the IP subscriber
session type. If it is a directly connected host (Layer #2 connection), ARP is used. For routed host (Layer
3 connected) subscribers, ICMP is used. If the access interface does not support ARP, the keepalive
protocol defaults to ICMP.
Using ARP for Keepalive Messages
When a session is established and the keepalive feature is configured to use ARP, the keepalive feature
saves the ARP entry as a valid original entry for verifying future ARP responses.
Note In cases where the access interface does not support ARP, the protocol for keepalives defaults to ICMP.
When ARP is configured, the ARP unicast request is sent to the subscriber. After a configured interval
of time, the ARP response (if received) is verified. If the response is correct and matches the original
entry that was saved when the subscriber was initially established, the keepalive feature continues
monitoring the data plane for the configured interval of time. If the response is not correct, the keepalive
feature resends the ARP request until a correct response is received or the configured maximum number
of attempts is exceeded.
Using ICMP for Keepalive Messages
If ICMP is configured, the ICMP “hello” request is sent to the subscriber and checked for a response,
until the configured maximum number of attempts is exceeded.