Use Loopback Testing for T1 Interfaces
This section includes the following information to assist you when troubleshooting T1 interfaces:
- Checklist for Using Loopback Testing for T1 Interfaces
- Diagnose a Suspected Hardware Problem with a T1 Interface
- Create a Loopback
- Set Clocking to Internal
- Verify That the T1 Interface Is Up
- Clear T1 Interface Statistics
- Force the Link Layer To Stay Up
- Verify the Status of the Logical Interface
- Ping the T1 Interface
- Check for T1 Interface Error Statistics
- Diagnose a Suspected Circuit Problem
Checklist for Using Loopback Testing for T1 Interfaces
Purpose
To use loopback testing for T1 interfaces.
Action
Table 1 provides commands for using loopback testing for T1 interfaces.
Table 1: Checklist for Using Loopback Testing for T1 Interfaces
Tasks | Command or Action |
|---|---|
| Diagnose a Suspected Hardware Problem with a T1 Interface | |
Connect a T1 loopback plug. | |
[edit interfaces interface-name t1-options] | |
[edit interfaces interface-name] | |
show interfaces t1-fpc/pic/port | |
clear interfaces statistics t1-fpc/pic/port | |
[edit interfaces interface-name] | |
[edit interfaces interface-name] | |
show interfaces t1-fpc/pic/port | |
ping interface t1-fpc/pic/port local-IP-address bypass-routing count 1000 rapid | |
show interfaces t1-fpc/pic/port extensive | |
| Diagnose a Suspected Circuit Problem | |
[edit interfaces interface-name t1-options] | |
Perform Steps 2 through 8 from Diagnose a Suspected Hardware Problem with a T1 Interface. | |
Diagnose a Suspected Hardware Problem with a T1 Interface
Problem
Take the following steps to verify if there is a hardware problem with a T1 interface.
Solution
To diagnose a suspected hardware problem with a T1 interface, follow these steps:
- Create a Loopback
- Set Clocking to Internal
- Verify That the T1 Interface Is Up
- Clear T1 Interface Statistics
- Force the Link Layer To Stay Up
- Verify the Status of the Logical Interface
- Ping the T1 Interface
- Check for T1 Interface Error Statistics
Create a Loopback
You can create a physical loopback or configure a local loopback to help diagnose a suspected hardware problem. Creating a physical loopback is recommended because it allows you to test and verify the T1 port. If a field engineer is not available to create the physical loopback, you can configure a local loopback for the interface. The local loopback creates a loopback internally in the Physical Interface Card (PIC).
Create a Physical Loopback
Action
To create a physical loopback at the T1 port, connect a T1 loopback plug to the T1 port. You can make a T1 loopback plug by connecting pin 1 to pin 4 and pin 2 to pin 5 on an RJ-48 plug.
Meaning
When you create and test a physical loopback, you are testing the T1 port. This action is recommended if a field engineer is available to create the physical loop as it provides a more complete test of the PIC.
Configure a Local Loopback
Action
To configure a local loopback without physically connecting the transmit port to the receive port, follow these steps:
- In configuration mode, go to the following hierarchy level:[edit]user@host# edit interfaces interface-name t1-options
- Configure the loopback:[edit interfaces interface-name t1-options]user@host# set loopback local
- Verify the configuration:user@host# showFor example:[edit interfaces t1-1/3/0 t1-options]user@host# show loopback local;
- Commit the change:user@host# commit
For example:
[edit interfaces t1-1/3/0 t1-options]user@host# commit commit complete
Meaning
When you create a local loopback, you create an internal loop on the interface being tested. A local loopback loops the traffic internally on that PIC. A local loopback tests the interconnection of the PIC but does not test the transmit and receive ports.
 | Note: Remember to delete the loopback statement after completing the test. |
Set Clocking to Internal
Purpose
You set clocking to internal because there is no external clock source in a loopback connection.
Action
To configure clocking to internal, follow these steps:
- In configuration mode, go to the following hierarchy level:[edit]user@host# edit interfaces interface-name
- Configure the clocking to internal:[edit interfaces interface-name]user@host# set clocking internal
- Verify the configuration:user@host# show
For example:
[edit interfaces t1-1/3/0]user@host# show clocking internal; - Commit the change:user@host# commit
For example:
[edit interfaces t1-1/3/0]user@host# commit commit complete
Meaning
This command saves the clocking change to the configuration database, activates the configuration on the router, and exits configuration mode.
Verify That the T1 Interface Is Up
Purpose
Display the status of the T1 interface to determine whether the physical link is up or down.
Action
To verify that the status of the T1 interface is up, use the following Junos OS command-line interface (CLI) operational mode command:
Sample Output
The following output is for a T1 interface with the physical link up:
user@host> show interfaces t1-1/1/0 Physical interface: t1-1/1/0, Enabled, Physical link is Up
Interface index: 24, SNMP ifIndex: 20
Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF
Device flags : Present Running Loop-Detected
Interface flags: Link-Layer-Down Point-To-Point SNMP-Traps
Link flags : Keepalives
Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
Keepalive: Input: 3 (00:00:06 ago), Output: 9 (00:00:06 ago)
Last flapped : 2002-01-06 00:59:00 UTC (00:00:40 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)
DS1 alarms : None
DS1 defects : None
Logical interface t1-1/1/0.0 (Index 9) (SNMP ifIndex 34)
Flags: Device-Down Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
Protocol inet, MTU: 1500, Flags: None
Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
Destination: 1.1.1.0/30, Local: 1.1.1.1
Meaning
The sample output shows that the physical link is up, the loop is detected, and there are no T1 alarms or defects.
Sample Output
If the physical link is down, there may be a problem with the port. The following output is an example of the show interfaces t1-fpc/pic/port command when the physical link is down:
user@host> show interfaces t1-1/1/0 Physical interface: t1-1/1/0, Enabled, Physical link is Down
Interface index: 24, SNMP ifIndex: 20
Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF
Device flags : Present Running Down
Interface flags: Hardware-Down Point-To-Point SNMP-Traps
Link flags : Keepalives
Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
Keepalive: Input: 32 (00:00:23 ago), Output: 35 (00:00:04 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)
DS1 alarms : LOF, LOS
DS1 defects : LOF, LOS
Logical interface t1-0/0/0.0 (Index 9) (SNMP ifIndex 34)
Flags: Device-Down Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
Protocol inet, MTU: 1500, Flags: None
Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
Destination: 1.1.1.0/30, Local: 1.1.1.1
Meaning
The sample output shows that the physical link is down, the device flags and interface flags are down, and that there are T1 alarms and defects. Verify that the fiber can successfully loop a known good port of the same type by checking for damage to the cable.
Clear T1 Interface Statistics
Purpose
You must reset T1 interface statistics before initiating the ping test. Resetting the statistics provides a clean start so that previous input/output errors and packet statistics do not interfere with the current diagnostics.
Action
To clear all statistics for the interface, use the following Junos OS CLI operational mode command:
Sample Output
user@host> clear interfaces statistics t1-1/1/0 user@host>
Meaning
This command clears the interface statistics counters for interface t1-1/1/0 only.
Force the Link Layer To Stay Up
To complete the loopback test, the link layer must remain up. However, Junos OS is designed to recognize that loop connections are not valid connections and to bring the link layer down. You need to force the link layer to stay up by making some configuration changes to the encapsulation and keepalives.
To force the link layer to stay up, follow these steps:
Configure Encapsulation to Cisco-HDLC
Action
To configure encapsulation on a T1 physical interface, follow these steps:
- In configuration mode, go to the following hierarchy level:[edit]user@host# edit interfaces interface-name
- Configure encapsulation to Cisco-HDLC:[edit interfaces interface-name]user@host# set encapsulation cisco-hdlc
- Verify the configuration:user@host# show
For example:
[edit interfaces t1-1/3/0]user@host# show encapsulation hdlc; - Commit the change:user@host# commit
For example:
[edit interfaces t1-1/3/0]user@host# commit commit complete
Meaning
This command sets the interface encapsulation to the Cisco High-level Data-Link Control (HDLC) transport protocol.
Configure No-Keepalives
Action
To disable the sending of link-layer keepalives on a T1 physical interface, follow these steps:
- In configuration mode, go to the following hierarchy level:[edit]user@host# edit interfaces interface-name
- Configure no-keepalives:[edit interfaces interface-name]user@host# set no-keepalives
For example:
[edit interfaces t1-1/3/0]user@host# set no-keepalives - Verify the configuration:user@host# show
For example:
[edit interfaces t1-1/3/0]user@host# show no-keepalives; - Commit the change:user@host# commit
For example:
[edit interfaces t1-1/3/0]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.
Verify the Status of the Logical Interface
Purpose
To verify the status of the logical interface, use the following two Junos OS CLI operational mode commands:
Action
Sample Output
The following output is for a logical interface that is up:
user@host> show interfaces t1-1/1/0 Physical interface: t1-1/1/0, Enabled,Physical link is Up
Interface index: 29, SNMP ifIndex: 20
Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF
Device flags : Present Running
Interface flags: Point-To-Point SNMP-Traps
Link flags : No-Keepalives
Last flapped : 2002-01-06 01:09:00 UTC (00:00:44 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)
DS1 alarms : None
DS1 defects : None
Logical interface t1-1/1/0.0 (Index 9) (SNMP ifIndex 34)
Flags: Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
Bandwidth: 0
Protocol inet, MTU: 1500, Flags: None
Addresses, Flags: Is-Preferred Is-Primary
Destination: 1.1.1.0/30, Local: 1.1.1.1
user@host> show interfaces terse t1-1/1/0
Interface Admin Link Proto Local Remote
t1-1/1/0 up up
t1-1/1/0.0 up up inet 1.1.1.1/30
Meaning
The sample output for the first command shows that the logical link is up because there are no flags indicating that the link layer is down. The output for the show interfaces terse command shows that logical interface t1-1/0/0 is up.
Sample Output
The following output is for a logical interface that is down:
user@host> show interfaces t1-1/1/0Physical interface: t1-1/1/0, Enabled, Physical link is Up
Interface index: 29, SNMP ifIndex: 20
Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF
Device flags : Present Running
Interface flags: Link-Layer-Down Point-To-Point SNMP-Traps
Link flags : Keepalives
Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
Keepalive: Input: 14 (00:01:01 ago), Output: 9 (00:00:05 ago)
Last flapped : 2002-01-06 01:09:00 UTC (00:03:39 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)
DS1 alarms : None
DS1 defects : None
Logical interface t1-1/1/0.0 (Index 9) (SNMP ifIndex 34)
Flags: Device-Down Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
Bandwidth: 0
Protocol inet, MTU: 1500, Flags: None
Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
Destination: 1.1.1.0/30, Local: 1.1.1.1
user@host> show interfaces terse t1-1/1/0
Interface Admin Link Proto Local Remote
t1-1/1/0 up down
t1-1/1/0.0 up down inet 1.1.1.1/30
Meaning
The sample output for both commands shows that the logical interface is down. The first command shows that the link layer, device, and destination route are all down. The second command shows that logical interface t1-1/1/0.0 is down.
Ping the T1 Interface
Purpose
Use the ping command to verify the loopback connection.
Action
To ping the local interface, use the following Junos OS CLI operational mode command:
Sample Output
user@host> ping interface t1-1/1/0 1.1.1.1 bypass-routing
count 1000 rapid PING 1.1.1.1 (1.1.1.1): 56 data bytes !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! --- 1.1.1.1 ping statistics --- 1000 packets transmitted, 1000 packets received, 0% packet loss round-trip min/avg/max/stddev = 2.036/2.120/9.809/0.681 ms
Meaning
This command sends 1000 ping packets out of the interface to the local IP address. The ping should complete successfully with no packet loss. If there is any persistent packet loss, open a case with the Juniper Networks Technical Assistance Center (JTAC) at support@juniper.net, or at 1-888-314-JTAC (within the United States) or 1-408-745-9500 (from outside the United States).
Check for T1 Interface Error Statistics
Purpose
Persistent interface error statistics indicate that you need to open a case with JTAC.
Action
To check the local interface for error statistics, use the following Junos OS CLI operational mode command:
Sample Output
user@host> show interfaces t1-1/1/0 extensive Physical interface: t1-1/1/0, Enabled, Physical link is Up
Interface index: 29, SNMP ifIndex: 20, Generation: 32
Link-level type: Cisco-HDLC, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF
Device flags : Present Running
Interface flags: Point-To-Point SNMP-Traps
Link flags : Keepalives
Hold-times : Up 0 ms, Down 0 ms
Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
Keepalive statistics:
Input : 28 (last seen 00:00:02 ago)
Output: 32 (last sent 00:00:06 ago)
Last flapped : 2002-01-06 01:09:00 UTC (00:07:19 ago)
Statistics last cleared: Never
Traffic statistics:
Input bytes : 84682 80 bps
Output bytes : 92685 0 bps
Input packets: 1031 0 pps
Output packets: 1077 0 pps
Input errors:
Errors: 0, Drops: 0, Framing errors: 0, Policed discards: 70, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0,
HS link CRC errors: 0, SRAM errors: 0
Output errors:
Carrier transitions: 1, Errors: 0, Drops: 0, Aged packets: 0
DS1 alarms : None
DS1 defects : None
T1 media: Seconds Count State
SEF 1 1 OK
BEE 0 0 OK
AIS 0 0 OK
LOF 1 1 OK
LOS 0 0 OK
YELLOW 1 2 OK
BPV 1 1
EXZ 1 1
LCV 1 2
PCV 1 6
CS 0 0
LES 1
ES 1
SES 1
SEFS 1
BES 1
UAS 0
HDLC configuration:
Policing bucket: Disabled
Shaping bucket : Disabled
Giant threshold: 1514, Runt threshold: 3
Timeslots : All active
Line encoding: B8ZS, Byte encoding: Nx64K, Data inversion: Disabled
Buildout : 0 to 132 feet
DS1 BERT configuration:
BERT time period: 10 seconds, Elapsed: 0 seconds
Induced Error rate: 10e-0, Algorithm: Unknown (0)
Packet Forwarding Engine configuration:
Destination slot: 1, PLP byte: 1 (0x00)
CoS transmit queue Bandwidth Buffer Priority Limit
% bps % bytes
0 best-effort 0 0 0 0 low none
1 expedited-forwarding 0 0 0 0 low none
2 assured-forwarding 0 0 0 0 low none
3 network-control 0 0 0 0 low none
Logical interface t1-1/1/0.0 (Index 9) (SNMP ifIndex 34) (Generation 14)
Flags: Point-To-Point SNMP-Traps Encapsulation: Cisco-HDLC
Bandwidth: 0
Protocol inet, MTU: 1500, Flags: None, Generation: 29 Route table: 0
Addresses, Flags: Is-Preferred Is-Primary
Destination: 1.1.1.0/30, Local: 1.1.1.1, Broadcast: Unspecified, Generation: 36
Meaning
Check for any error statistics that may appear in the output. There should not be any input or output errors. If there are any persistent input or output errors, open a case with JTAC at support@juniper.net, or at 1-888-314-JTAC (within the United States) or 1-408-745-9500 (from outside the United States).
Diagnose a Suspected Circuit Problem
When you suspect a circuit problem, it is important to work with the transport-layer engineer to resolve the problem. The transport-layer engineer may ask you to create a loop from the router to the network, or the engineer may create a loop to the router from various points in the network.
To diagnose a suspected circuit problem, follow these steps:
- Create a Loop from the Router to the Network
- Create a Loop to the Router from Various Points in the Network
Create a Loop from the Router to the Network
Purpose
Creating a loop from the router to the network allows the transport-layer engineer to test the router from various points in the network. This helps the engineer isolate where the problem is located.
Action
To create a loop from the router to the network, follow these steps:
- In configuration mode, go to the following hierarchy level:[edit]user@host# edit interfaces interface-name t1-options
- Configure remote loopback:[edit interfaces interface-name t1-options]user@host# set loopback remote
- Verify the configuration:user@host# show
For example:
[edit interfaces t1-1/3/0 t1-options]user@host# show loopback remote; - Commit the change:user@host# commit
For example:
[edit interfaces t1-1/3/0 t1-options]user@host# commit commit complete
Meaning
This command loops any traffic from the network back into the network.
Create a Loop to the Router from Various Points in the Network
Purpose
The transport-layer engineer creates a loop to the router from various points in the network. You can then perform tests to verify the connection from the router to that loopback in the network.
Action
After the transport-layer engineer has created the loop to the router from the network, you must verify the connection from the router to the loopback in the network. Follow Step 2 through Step 8 in Diagnose a Suspected Hardware Problem with a T1 Interface. Keep in mind that any problems encountered in the test indicate a problem with the connection from the router to the loopback in the network.
By performing tests to loopbacks at various points in the network, you can isolate the source of the problem.
