Information About Configuring Serial Interfaces
The keepalive command applies to serial interfaces using HDLC or PPP encapsulation. It does not apply
Note
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 the no keyword. 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.
When LCP is running on the peer and receives an ECHOREQ packet, it responds with an ECHOREP
packet, regardless of whether keepalives are enabled on the peer.
Keepalives are independent between the two peers. One peer end can have keepalives enabled; the other
end can have them disabled. Even if keepalives are disabled locally, LCP still responds with ECHOREP
packets to the ECHOREQ packets it receives. Similarly, LCP also works if the period of keepalives at
each end is different.
Frame Relay Encapsulation
When Frame Relay encapsulation is enabled on a serial interface, the interface configuration is
hierarchical and comprises the following elements:
•
•
•
•
The administrative state of a parent interface drives the state of the subinterface and its PVC. When the
Note
administrative state of a parent interface or subinterface changes, so does the administrative state of any
child PVC configured under that parent interface or subinterface.
To configure Frame Relay encapsulation on serial interfaces, use the encapsulation (Frame Relay
VC-bundle) command.
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The serial main interface comprises the physical interface and port. If you are not using the serial
interface to support Cisco HDLC and PPP encapsulated connections, then you must configure
subinterfaces with permanent virtual circuits (PVCs) under the serial main interface. Frame Relay
connections are supported on PVCs only.
Serial subinterfaces are configured under the serial main interface. A serial subinterface does not
actively carry traffic until you configure a PVC under the serial subinterface. Layer 3 configuration
typically takes place on the subinterface.
When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type,
the configured serial subinterfaces on the main interface inherit the newly changed encapsulation
and they do not get deleted.
Point-to-point PVCs are configured under a serial subinterface. You cannot configure a PVC directly
under a main interface. A single point-to-point PVC is allowed per subinterface. PVCs use a
predefined circuit path and fail if the path is interrupted. PVCs remain active until the circuit is
removed from either configuration. Connections on the serial PVC support Frame Relay
encapsulation only.
Chapter 4
Configuring the Serial Interface
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