Cisco Systems 3825 manual Usage Guidelines Keepalive Time Interval, Line Failure, Dropped Packets

Appendix A Cisco 3825 Mobile Wireless Edge Router RAN-O Command Reference

keepalive

Usage Guidelines	Keepalive Time Interval
	You can configure the keepalive time interval, which is the frequency at which the Cisco IOS software
	sends messages to itself (Ethernet and Token Ring) or to the other end (serial and tunnel), to ensure that
	a network interface is alive. The interval is adjustable in 1-second increments, down to a minimum of 1
	second. An interface is declared down after three update intervals have passed without receiving a
	keepalive packet unless the retry value is set higher.
	Setting the keepalive timer to a low value is very useful for rapidly detecting Ethernet interface failures
	(such as a transceiver cable disconnecting, or cable that is not terminated).
	Line Failure
	A typical serial line failure involves losing the Carrier Detect (CD) signal. Because this sort of failure is
	typically noticed within a few milliseconds, adjusting the keepalive timer for quicker routing recovery
	is generally not useful.
	Keepalive Packets with Tunnel Interfaces
	GRE keepalive packets may be sent either from both sides of a tunnel or from just one side. If they are
	sent from both sides, the period and retry parameters can be different at each side of the link. If you
	configure keepalives on only one side of the tunnel, the tunnel interface on the sending side might
	perceive the tunnel interface on the receiving side to be down because the sending interface is not
	receiving keepalives. From the receiving side of the tunnel, the link appears normal because no
	keepalives were enabled on the second side of the link.
	Dropped Packets
	Because keepalive packets are treated as ordinary packets, it is possible that they will be dropped. To
	reduce the possibility that dropped keepalive packets will cause the tunnel interface to be taken down,
	increase the number of retries.
Note	When adjusting the keepalive timer for a very-low-bandwidth serial interface, large datagrams can delay
	the smaller keepalive packets long enough to cause the line protocol to go down. You may need to
	experiment to determine the best values to use for the timeout and the number of retry attempts.
Examples	The following example shows how to set the keepalive interval to 3 seconds:
	Router(config)# interface ethernet 0
	Router(config-if)# keepalive 3
	The following example shows how to set the keepalive interval to 3 seconds and the retry value to 7:
	Router(config)# interface tunnel 1
	Router(config-if)# keepalive 3 7

	Cisco 3825 Mobile Wireless Edge Router Software Configuration Guide
OL-15667-03		A-57