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
When LCP is running on the peer and receives an ECHOREQ packet, it responds with an echo reply
(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.
Note Use the debug chdlc slarp packet command and other Cisco HDLC debug commands to display
information about the Serial Line Address Resolution Protocol (SLARP) packets that are sent to the peer
after the keepalive timer has been configured.
Frame Relay Encapsulation
When Frame Relay encapsulation is enabled on a serial interface, the interface configuration is
hierarchical and comprises the following elements:
1. 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.
2. 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.
3. 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.
Note The administrative state of a parent interface drives the state of the subinterface and its PVC. When the
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
command.
Frame Relay interfaces support two types of encapsulated frames:
• Cisco (default)
• IETF
Use the encap command in PVC configuration mode to configure Cisco or IETF encapsulation on a
PVC. If the encapsulation type is not configured explicitly for a PVC, then that PVC inherits the
encapsulation type from the main serial interface.
Note Cisco encapsulation is required on serial main interfaces that are configured for MPLS. IETF
encapsulation is not supported for MPLS.
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