TechLibrary

Navigation Back up to About Overview

Use Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Purpose

To use loopback testing to isolate Fast Ethernet and Gigabit Ethernet interface problems.

Action

Table 1 provides links and commands for using loopback testing for Fast Ethernet and Gigabit Ethernet interfaces.

Table 1: Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Tasks

Command or Action

Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface
  1. Create a Loopback
 

Connect the transmit port to the receive port.

Cross pin 1 (TX+) and pin 3 (RX+) together, and pin 2 (TX-) and pin 6 (RX-) together.

[edit interfaces interface-name (fastether-options | gigether-options)]
set loopback
show
commit

  1. Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up

show interfaces (fe-fpc/pic/port | ge-fpc/pic/port)

  1. Configure a Static Address Resolution Protocol Table Entry

show interfaces ge-fpc/pic/port
[edit interfaces interface-name unit logical-unit-number family inet address address]
set arp ip-address mac mac-address
show
commit
run show arp no-resolve

  1. Clear Fast Ethernet or Gigabit Ethernet Interface Statistics

clear interfaces statistics fe-fpc/pic/port | ge-fpc/pic/port

  1. Ping the Fast Ethernet or Gigabit Ethernet Interface

ping remote-IP-address bypass-routing interface (fe-fpc/pic/port | ge-fpc/pic/port count 100 rapid

  1. Check for Fast Ethernet or Gigabit Ethernet Interface Error Statistics

show interfaces (fe-fpc/pic/port | ge-fpc/pic/port ) extensive

Diagnose a Suspected Circuit Problem

Perform Steps 2 through 8 from Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface.

Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface

Problem

When you suspect a hardware problem, take the following steps to help verify if there is a problem.

Solution

To diagnose a suspected hardware problem with the Ethernet interface, follow these steps:

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 transmit and receive ports. 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).

  1. Create a Physical Loopback for a Fiber-Optic Interface
  2. Create a Loopback Plug for an RJ-45 Ethernet Interface
  3. Configure a Local Loopback

Create a Physical Loopback for a Fiber-Optic Interface

Action

To create a physical loopback at the port, connect the transmit port to the receive port using a known good fiber cable.

Note: Make sure you use single-mode fiber for a single-mode port and multimode fiber for a multimode port.

Meaning

When you create and then test a physical loopback, you are testing the transmit and receive ports of the PIC. 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.


Create a Loopback Plug for an RJ-45 Ethernet Interface

Action

To create a loopback plug, cross pin 1 (TX+) and pin 3 (RX+) together, and cross pin 2 (TX-) and pin 6 (RX-) together. You need the following equipment to create the loopback:

  • A 6-inch long CAT5 cable
  • An RJ-45 connector
  • A crimping tool

Figure 1 illustrates how to create a loopback plug for an RJ-45 Ethernet interface.

Figure 1: RJ-45 Ethernet Loopback Plug

RJ-45 Ethernet Loopback Plug

Meaning

When you create and then test a physical loopback, you are testing the RJ-45 interface of the PIC. 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:

  1. In configuration mode, go to the following hierarchy level:
    [edit]user@host# edit interfaces interface-name (fastether-options | gigether-options)
  2. Configure the local loopback:
    [edit interfaces interface-name (fastether-options | gigether-options)] user@host# set loopback
  3. Verify the configuration:
    user@host# show

    For example:

    [edit interfaces fe-1/0/0 fastether-options]user@host# show loopback;
  4. Commit the change:
    user@host# commit

    For example:

    [edit interfaces fe-1/0/0 fastether-options]user@host# commit commit complete

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. On an Ethernet interface, you cannot create a remote loopback, therefore there is no option to use a local or remote statement. Simply including the loopback statement at the [edit interfaces interface-name (fastether-options | gigether-options] hierarchy level, places the interface into local loopback mode.

Note: Remember to delete the loopback statement after completing the test.

Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up

Purpose

Display the status of the Fast Ethernet or Gigabit Ethernet interface to provide the information you need to determine whether the physical link is up or down.

Action

To verify that the status of the Fast Ethernet or Gigabit Ethernet interface is up, use the following Junos OS command-line interface (CLI) operational mode command:

user@host> show interfaces (fe-fpc/port | ge-fpc/pic/port)

Sample Output

user@host# show interfaces fe-1/3/0
Physical interface: fe-1/3/0, Enabled, Physical link is Up
  Interface index: 44, SNMP ifIndex: 35
  Link-level type: Ethernet, MTU: 1514, Source filtering: Disabled
  Speed: 100mbps,  Loopback: Disabled , Flow control: Enabled
  Device flags   : Present Running
  Interface flags: SNMP-Traps
  Link flags     : None
  Current address: 00:90:69:8d:2c:db, Hardware address: 00:90:69:8d:2c:db
  Input rate     : 0 bps (0 pps), Output rate: 0 bps (0 pps)
  Active alarms  : None
  Active defects : None
  MAC statistics:
    Input octets: 0, Input packets: 0, Output octets: 0, Output packets: 0
  Filter statistics:
    Filtered packets: 0, Padded packets: 0, Output packet errors: 0
  Autonegotiation information:
    Negotiation status: Incomplete, Link partner status: OK
    Link partner: Full-duplex, Flow control: None

Meaning

The sample output shows that the link is up and there are no alarms in this loopback configuration. When an internal loopback is configured, the physical loopback should come up without an alarm.

Sample Output

When you see that the physical link is down, there may be a problem with the port. The following output is an example of the show interfaces fe-fpc/pic/port command when the physical link is down:

user@router> show interfaces fe-1/3/0
Physical interface: fe-1/3/0, Enabled, Physical link is Down
  Interface index: 44, SNMP ifIndex: 35
  Link-level type: Ethernet, MTU: 1514, Source filtering: Disabled
  Speed: 100mbps, Loopback: Disabled, Flow control: Enabled
  Device flags   : Present Running Down
  Interface flags: Hardware-Down SNMP-Traps
  Link flags     : None
  Current address: 00:90:69:8d:2c:db, Hardware address: 00:90:69:8d:2c:db
  Input rate     : 0 bps (0 pps), Output rate: 0 bps (0 pps)
  Active alarms  : LINK
  Active defects : LINK
  MAC statistics:
    Input octets: 0, Input packets: 0, Output octets: 0, Output packets: 0
  Filter statistics:
    Filtered packets: 0, Padded packets: 0, Output packet errors: 0
  Autonegotiation information:
    Negotiation status: Incomplete, Link partner status: Down
    Reason: Link partner autonegotiation failure
    Link partner: Half-duplex, Flow control: None

Meaning

The sample output shows that the physical link is down and there are active alarms and defects.

Table 2 presents problem situations and actions for a physical link that is down.

Table 2: Problems and Solutions for a Physical Link That Is Down

Problem

Action

Cable mismatch

Verify that the fiber connection is correct.

Damaged and/or dirty cable

Verify that the fiber can successfully loop a known good port of the same type.

Too much or too little optical attenuation

Verify that the attenuation is correct per the PIC optical specifications.

The transmit port is not transmitting within the dBm optical range per the specifications

Verify that the Tx power of the optics is within range of the PIC optical specification.

Mismatch between the cable type and the port

Verify that a single-mode fiber cable is connected to a single-mode interface and that a multimode fiber cable is connected to a multimode interface. (This problem does not always cause the physical link to go down; errors and dropped packets are sometimes the result.)

Configure a Static Address Resolution Protocol Table Entry

Purpose

Configure a static Address Resolution Protocol (ARP) entry to allow a packet to be sent out of a looped Ethernet interface.

Note: Remove the static ARP entry at the end of the loop test after you have completed the ping test, checked interface statistics, and monitored interface traffic.

Action

To configure a static ARP table entry for a Gigabit Ethernet interface, follow these steps. You can follow the same procedure to configure a static ARP entry for a Fast Ethernet interface.

  1. Find the Media Access Control (MAC) address for the Gigabit Ethernet interface:

    user@host> show interfaces ge-fpc/pic/port

  2. In configuration mode, go to the following hierarchy level:
    [edit]user@host# edit interfaces interface-name unit logical-unit-number family inet address address
  3. Configure the static ARP entry:
    user@host# set arp ip-address mac mac-address

    Note: The MAC address used should be the same as the physical address of the port being tested because this allows the port to receive the frames when you run the ping test.

  4. Verify the configuration:
    user@host# show
  5. Commit the configuration:
    user@host# commit
  6. Verify that the static ARP entry is installed:
    user@host# run show arp no-resolve

Sample Output

user@host> show interfaces ge-7/2/1
Physical interface: ge-7/2/1, Enabled, Physical link is Down
  Interface index: 44, SNMP ifIndex: 35
  Link-level type: Ethernet, MTU: 1514, Source filtering: Disabled
  Speed: 100mbps, Loopback: Disabled, Flow control: Enabled
  Device flags   : Present Running Down
  Interface flags: Hardware-Down SNMP-Traps
  Link flags     : None
  Current address: 00:90:69:8d:2c:db, Hardware address: 00:90:69:8d:2c:db
  Input rate     : 0 bps (0 pps), Output rate: 0 bps (0 pps)
[edit interfaces ge-7/2/1 unit 0 family inet address 10.108.120.1/30]

user@host#  set arp 10.108.120.2 mac 00:90:69:8d:2c:db
[edit interfaces ge-7/2/1 unit 0 family inet address 10.108.120.1/30]

user@host# show  
arp 10.108.120.2 mac 00:90:69:8d:2c:db;
[edit interfaces ge-7/2/1 unit 0 family inet address 10.108.120.1/30]

user@host# commit  
commit complete
[edit interfaces ge-7/2/1 unit 0 family inet address 10.108.120.1/30]

user@host# run show arp no-resolve  
MAC Address       Address         Interface     Flags
00:90:69:8d:2c:db 10.108.120.2    ge-7/2/1.0    permanent
00:e0:34:bb:8c:40 209.211.135.1   fxp0.0        none
00:a0:a5:28:0c:70 209.211.135.8   fxp0.0        none
00:a0:a5:12:12:c7 209.211.135.10  fxp0.0        none
00:90:ab:3c:68:a0 209.211.135.31  fxp0.0        none
08:00:20:a1:53:15 209.211.135.65  fxp0.0        none
00:a0:cc:66:3e:85 209.211.135.98  fxp0.0        none
Total entries: 7

Meaning

The sample output is for Step 1 through Step 6 and shows that a static ARP entry was configured on Gigabit Ethernet interface ge-7/2/1. The MAC address used is the same as the physical address of the port being tested because this allows the port to receive the frames when you run the ping test. The port is working as expected if you see that the time to live (TTL) expired; if you do not receive a response to your ping test, it indicates a hardware problem.

Clear Fast Ethernet or Gigabit Ethernet Interface Statistics

Purpose

You must reset the Fast Ethernet and Gigabit Ethernet 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:

user@host> clear interfaces statistics (fe-fpc/pic/port | ge-fpc/pic/port)

Sample Output

user@host> clear interfaces statistics ge-7/2/0
user@host>

Meaning

This command clears the interface statistics counters for the Gigabit Ethernet interface only.

Ping the Fast Ethernet or Gigabit Ethernet Interface

Purpose

Use the ping command to verify the loopback connection.

Action

To send ping packets from the Ethernet interface, use the following Junos OS CLI operational mode command:

user@host> ping remote-IP-address bypass-routing interface (fe-fpc/pic/port | ge-fpc/pic/port) count 100 rapid

Sample Output

user@router> ping 10.108.120.2 bypass-routing interface ge-7/2/1 count 100 rapid
PING 10.108.120.2 (10.108.120.2): 56 data bytes
36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e871   0 0000  01  01 cc5c 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e874   0 0000  01  01 cc59 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e878   0 0000  01  01 cc55 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e87c   0 0000  01  01 cc51 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e880   0 0000  01  01 cc4d 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 e884   0 0000  01  01 cc49 10.108.120.1  10.108.120.2 
.36 bytes from 10.108.120.1: Time to live exceeded

Meaning

The sample output shows that the time to live (TTL) expired, indicating that the link is receiving the frames from the ping test. The MAC address used is the same as the physical address of the port being tested because this allows the port to accept the frames from the ping test. As the packet is looped over the link, you expect to receive a TLL exceeded message for each ping sent. These messages are generated because the ping packets are repeatedly looped between the router and the physical loopback. When the packet is sent to the other end of the link, which does not exist, the loopback returns the packet back to the same interface, where it is again subjected to the Packet Forwarding Engine fabric for routing. After the route lookup, the TTL is decremented, and the packet is again sent out of the looped interface. This process repeats until the packed is either lost, or the TLL expires with subsequent TTL expired message displayed. Should any errors occur, the packet is discarded and a time-out error is displayed, rather than the expected TTL expired message. Note that the default TTL for ICMP echo packets in Junos OS is 64. This means a given test packet must be successfully sent and received 63 times before a TTL expired message can be generated. You can alter the TTL value to adjust the tolerance for loss, for example, a value of 255 is the most demanding test because now the packet must be sent and received error free 254 times.

Check for Fast Ethernet or Gigabit Ethernet Interface Error Statistics

Purpose

Persistent interface error statistics indicate that you need to open a case with the Juniper Networks Technical Assistance Center (JTAC).

Action

To check the local interface for error statistics, use the following Junos OS CLI operational mode command:

user@host> show interfaces (fe-fpc/pic/port | ge-fpc/pic/port) extensive

Sample Output

user@router> show interfaces ge-7/2/1 extensive
Physical interface: ge-7/2/1, Enabled, Physical link is Up
  Interface index: 25, SNMP ifIndex: 32, Generation: 41
  Description: Test
  Link-level type: Ethernet, MTU: 4470, Speed: 1000mbps, Loopback: Disabled,
  Source filtering: Disabled, Flow control: Disabled
  Device flags   : Present Running
  Interface flags: SNMP-Traps
  Link flags     : None
  Hold-times     : Up 0 ms, Down 0 ms
  Current address: 00:90:69:4c:17:b1, Hardware address: 00:90:69:4c:17:b1
  Statistics last cleared: 2002-01-07 17:53:19 UTC (2w2d 03:20 ago)
  Traffic statistics:
   Input  bytes  :        3799515503823                    0 bps
   Output bytes  :           7325566425                    0 bps
   Input  packets:           4628009535                    0 pps
   Output packets:             30678225                    0 pps
  Input errors:
    Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 0, L3 incompletes: 0,
    L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0
  Output errors:
    Carrier transitions: 14, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0,
    FIFO errors: 0, HS link CRC errors: 0
  Active alarms  : None
  Active defects : None
  MAC statistics:                      Receive         Transmit
    Total octets                 3883579444813       7880356346
    Total packets                   4628009534         30678237
    Unicast packets                 4627879788         29893563
    Broadcast packets                       30              464
    Multicast packets                   129716           784210
    CRC/Align errors                         0                0
    FIFO errors                              0                0
    MAC control frames                       0                0
    MAC pause frames                         0                0
    Oversized frames                         0
    Jabber frames                            0
    Fragment frames                          0
    VLAN tagged frames                       0
    Code violations                          0
  Filter statistics:
    Input packet count              4628009244
    Input packet rejects                     0
    Input DA rejects                         0
    Input SA rejects                         0
    Output packet count                                30678237
    Output packet pad count                              856248
    Output packet error count                                 0
    CAM destination filters: 9, CAM source filters: 0
  Autonegotiation information:
    Negotiation status: Complete, Link partner status: Ok, Link partner: Full-duplex,
    Flow control: None
  PFE configuration:
    Destination slot: 7
    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 ge-7/2/1.0 (Index 23) (SNMP ifIndex 48) (Generation 38)
    Description: To Cosine Left 23/1
    Flags: SNMP-Traps Encapsulation: ENET2
    Protocol inet, MTU: 4456, Flags: None, Generation: 85 Route table: 0
      Addresses, Flags: Is-Preferred Is-Primary
        Destination: 10.108.120.0/30, Local: 10.108.120.1, Broadcast: 10.108.120.3,
        Generation: 81
    Protocol iso, MTU: 4453, Flags: None, Generation: 86 Route table: 0

Meaning

Check for any error statistics. There should not be any input or output errors. If there are any persistent input or output errors, 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).

Diagnose a Suspected Circuit Problem

Purpose

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 create 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 Fast Ethernet or Gigabit Ethernet 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.

Published: 2011-11-03