Force the Link Layer to Stay Up

Purpose

To complete the loopback test, the link layer must remain up. However, JUNOS software is designed to recognize that loop connections are not valid connections and to bring the link layer down. If you have the Point-to-Point protocol (PPP) configured, you need to change the encapsulation to Cisco High-Level Data Link Control (HDLC) and reconfigure the keepalives in order to force the link layer to stay up.

Force the link layer to stay up, follow these steps:

1. Configure Encapsulation to Cisco-HDLC
2. Configure No-Keepalives

Configure Encapsulation to Cisco-HDLC

Action

To set the encapsulation on a T1 physical interface, follow these steps:

1. In configuration mode, go to the following hierarchy level:

   [edit]

   user@host# edit interfaces interface-name

2. Configure Cisco-HDLC:

   [edit interfaces interface-name]

   user@host# set encapsulation cisco-hdlc

3. Verify the configuration:

   user@host# show

   For example:

   [edit interfaces t1-0/1/1:8]

   user@host# show

   encapsulation hdlc;

4. Commit the change:

   user@host# commit

   For example:

   [edit interfaces t1-0/1/1:8]

   user@host# commit

   commit complete

5. Check the interface configuration
   
   

   user@host# run show interfaces t1-2/1/0:20

   Physical interface: t1-2/1/0:20, Enabled, Physical link is Up
   Interface index: 210, SNMP ifIndex: 173
   Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1,   Loopback: Local, FCS: 16,
   Mode: C/Bit parity, Framing: ESF
   Device flags   : Present Running
   Interface flags: Point-To-Point SNMP-Traps
   Link flags     : Keepalives
   Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
   Keepalive: Input: 39 (00:00:06 ago), Output: 0 (never)
   CoS queues     : 4 supported
   Last flapped   : 2004-05-20 21:46:27 UTC (00:14:28 ago)
   Input rate     : 16 bps (0 pps)
   Output rate    : 160 bps (0 pps)
   DS1   alarms   : None
   DS3   alarms   : None
   DS1   defects  : None
   DS3   defects  : None
   Logical interface t1-2/1/0:20.0 (Index 74) (SNMP ifIndex 213)
   Flags: Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
   Protocol inet, MTU: 1500
   Flags: None
   Addresses, Flags: Is-Preferred Is-Primary
   Destination: 10.10.1.1, Local: 10.10.1.2

Meaning

This command sets the interface encapsulation to the Cisco HDLC transport protocol. You must configure the interface with Cisco HDLC to ensure that the logical interface remains up in preparation for the ping test.

Configure No-Keepalives

Action

To disable the sending of link-layer keepalives on a channelized DS3 interface, follow these steps:

1. In configuration mode, go to the following hierarchy level:

   [edit]

   user@host# edit interfaces interface-name

2. Configure no-keepalives:

   [edit interfaces interface-name]

   user@host# set no-keepalives

3. Verify the configuration:

   user@host# show

   For example:

   [edit interfaces t1-2/1/0:20]

   user@host# show

   no-keepalives;

4. Commit the change:

   user@host# commit

   For example:

   [edit interfaces t1-2/1/0:20]

   user@host# commit

   commit complete

Meaning

By setting no-keepalives, the link layer is forced to stay up. If the setting remains at keepalive, the router will recognize that the same link-layer keepalives are being looped back and will bring the link layer down.