Cisco MWR 1941-DC - 1941 Mobile Wireless Router Software Configuration Manual page 85

Chapter 5 Cisco MWR 1941-DC Router IP-RAN Command Reference
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 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
(transceiver cable disconnecting, cable not terminated, and so on).
Line Failure
A typical serial line failure involves losing 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 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
Keepalive packets are treated as ordinary packets, so it is possible that they will be dropped. To reduce
the chance that dropped keepalive packets will cause the tunnel interface to be taken down, increase the
number of retries.
When adjusting the keepalive timer for a very low bandwidth serial interface, large datagrams can
Note
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
OL-11503-01
Cisco MWR 1941-DC Mobile Wireless Edge Router Software Configuration Guide
keepalive
5-5