Configuring Serial Interfaces on the Cisco ASR 9000 Series Router
Information About Configuring Serial Interfaces
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Cisco ASR 9000 Aggregation Services Router Interfaces and Hardware Component Configuration Guide
OL-26061-03
MLPPP provides the same features that are supported on PPP Serial interfaces with the exception of
QoS. It also provides the following additional features:
• Fragment sizes of 128, 256, and 512 bytes
• Long sequence numbers (24-bit)
• Lost fragment detection timeout period of 80 ms
• Minimum-active-links configuration option
• LCP echo request/reply support over multilink interface
• Full T1 and E1 framed and unframed links
For more information about configuring MLPPP on a serial interface, see the “Configuring PPP on the
Cisco ASR 9000 Series Router” module in this document.
Keepalive Timer
Cisco keepalives are useful for monitoring the link state. Periodic keepalives are sent to and received
from the peer at a frequency determined by the value of the keepalive timer. If an acceptable keepalive
response is not received from the peer, the link makes the transition to the down state. As soon as an
acceptable keepalive response is obtained from the peer or if keepalives are disabled, the link makes the
transition to the up state.
Note The keepalive command applies to serial interfaces using HDLC or PPP encapsulation. It does not apply
to serial interfaces using Frame Relay encapsulation.
For each encapsulation type, a certain number of keepalives ignored by a peer triggers the serial interface
to transition to the down state. For HDLC encapsulation, three ignored keepalives causes the interface
to be brought down. For PPP encapsulation, five ignored keepalives causes the interface to be brought
down. ECHOREQ packets are sent out only when LCP negotiation is complete (for example, when LCP
is open).
Use the keepalive command in interface configuration mode to set the frequency at which LCP sends
ECHOREQ packets to its peer. To restore the system to the default keepalive interval of 10 seconds, use
the keepalive command with no argument. To disable keepalives, use the keepalive disable command.
For both PPP and Cisco HDLC, a keepalive of 0 disables keepalives and is reported in the show
running-config command output as keepalive disable.
Note During Minimal Disruptive Restart (MDR) keepalives may fail. Therefore the keepalive timers, on both
ends, should be either disabled or set to an interval longer than the MDR time.
Note Before performing a Minimal Disruptive Restart (MDR) upgrade, we recommend disabling keepalives
on a Cisco XR 12000 Series Router.
Note Before performing a Minimal Disruptive Restart (MDR) upgrade, we recommend configuring a
keepalive interval of 10 seconds or more on a Cisco CRS-1 Router.
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