# Configuration Tasks

To configure a Cisco HDLC interface:

1. Configure the physical interface on which you want to configure Cisco HDLC.
   host1(config)#interface serial 3/1:2/1
2. Select Cisco HDLC as the encapsulation method for the interface.
   host1(config-if)#encapsulation hdlc
3. Assign a local IP address to the interface.
   host1(config-subif)#ip address 192.32.10.2 255.255.255.0
4. (Optional) Use the appropriate show hdlc interface to verify that the configuration changes are correct.

encapsulation hdlc

- Use to specify Cisco HDLC as the encapsulation method for the interface.
- Example
  host1(config-if)#encapsulation hdlc
- Use the no version to disable Cisco HDLC on the interface.
- See encapsulation hdlc.

interface serial

- Use to configure a serial interface in the appropriate format by selecting a previously configured physical interface on which you want to configure Cisco HDLC. For example, to specify a channelized T3 interface, use the format slot/port:channel/subchannel.subinterface.
  - slot—Router chassis slot
  - port—Port on CT3, T3, or E3 I/O module
  - channel—T1 (DS1) channel
  - subchannel —Set of DS0 subchannels. For information about T1 subchannels, see Fractional T1 in JunosE Physical Layer Configuration Guide.
  - subinterface—User-assigned number that identifies a subinterface
- Example
  host1(config)#interface serial 3/1:2/1
- Use the no version to remove the interface or subinterface.
- See interface serial.

ip address

- Use to assign an IP address and subnet mask to the interface.
- Example
  host1(config-subif)#ip address 192.32.10.2 255.255.255.0
- Use the no version to remove the IP address of the interface.
- See ip address.

## Optional Tasks

The following tasks are optional.

1. Configure the SLARP keepalive interval.
   host1(config-if)#hdlc keepalive 10
2. Enable loopback detection on an interface.
   host1(config-if)#hdlc down-when-looped
3. Disable an interface.
   host1(config-if)#hdlc shutdown

hdlc down-when-looped

- Use to enable loopback detection on a Cisco HDLC interface.
- By default, loopback detection is disabled.
- Example
  host1(config-if)#hdlc down-when-looped
- Use the no version to disable loopback detection on a Cisco HDLC interface.
- See hdlc down-when-looped.

hdlc keepalive

- Use to specify the keepalive timeout value.
- When the keepalive timer expires, the interface increments its own counter; then it compares the value of this counter with the last value received from a peer. If the difference between the values of the two counters is greater than three, the Cisco HDLC interface is declared down. After that, the interface sends a keepalive message containing the value of its counter and the last received value of the peer’s counter.
- The router stores the values received in keepalive messages from a peer interface. If the interface is down, the router compares the received value of its own counter with the value from the peer. If the difference between the values of the two counters is less than four, the router declares the interface to be up. Both sides have to configure the same value for the keepalive interval.
- If the keepalive interval is 10 seconds, then a failed link is detected between 30 and 40 seconds after failure.
- The range is 0–6553 seconds. A value of 0 turns keepalive off.
- The default is 10 seconds.
- Example
  host1(config-if)#hdlc keepalive 10
- Use the no version to turn off the keepalive feature.
- See hdlc keepalive.

hdlc shutdown

- Use to terminate a Cisco HDLC session.
- This command administratively disables the interface.
- Example
  host1(config-if)#hdlc shutdown
- Use the no version to restart a disabled session. The default for each hdlc shutdown command is the no version.
- See hdlc shutdown.

## Configuration Example

This example shows how to configure Cisco HDLC over an unchannelized DS3 interface on a cOCx/STMx line module. The example shows the complete configuration procedure, from configuring the SONET interface to assigning an IP address to the Cisco HDLC interface.

1. Create or select a virtual router, vr1.
   host1(config)#virtual-router vr1
2. Configure SONET controller, slot 13, port 0.
   host1:vr1(config)#controller sonet 13/0
3. Set the SONET clock source to internal.
   host1:vr1(config-controll)#clock source internal module
4. Create an OC1 path.
   host1:vr1(config-controll)#path 1 oc1 1
5. Create an unchannelized DS3 interface.
   host1:vr1(config-controll)#path 1 ds3 1 unchannelized
6. Set the DS3 interface clock source to internal.
   host1:vr1(config-controll)#path 1 ds3 1 clock source internal module
7. Exit Controller Configuration mode.
   host1:vr1(config-controll)#exit
8. Create or select a serial interface over the DS3 interface.
   host1:vr1(config)#interface serial 13/0:1/1
9. Set the encapsulation to Cisco HDLC.
   host1:vr1(config-if)#encapsulation hdlc
10. Enable loopback detection on the interface.
    host1:vr1(config-if)#hdlc down-when-looped
11. Assign an IP address to the interface.
    host1:vr1(config-if)#ip address 160.1.0.1 255.255.255.0