User guide

CHAPTER 6

Interface Diagnostics

•

Interface Diagnostics on page 85

•

Interface Diagnostics Operational Mode Commands on page 92

Interface Diagnostics

You can use two diagnostic tools to test the physical layer connections of interfaces:

loopback testing and bit error rate test (BERT) testing. Loopback testing enables you to

verify the connectivity of a circuit. BERT testing enables you to identify poor signal quality

on a circuit. This section contains the following topics:

•

Configuring Loopback Testing on page 85

•

Interface Diagnostics on page 87

Configuring Loopback Testing

Loopback testing allows you to verify the connectivity of a circuit. You can configure any

of the following interfaces to execute a loopback test: Aggregated Ethernet, Fast Ethernet,

Gigabit Ethernet, E1, E3, NxDS0, serial, SONET/SDH, T1, and T3.

The physical path of a network data circuit usually consists of segments interconnected

by devices that repeat and regenerate the transmission signal. The transmit path on one

device connects to the receive path on the next device. If a circuit fault occurs in the form

of a line break or a signal corruption, you can isolate the problem by using a loopback

test. Loopback tests allow you to isolate segments of the circuit and test them separately.

To do this, configure a line loopback on one of the routers. Instead of transmitting the

signal toward the far-end device, the line loopback sends the signal back to the originating

router. If the originating router receives back its own data link layer packets, you have

verified that the problem is beyond the originating router. Next, configure a line loopback

farther away from the local router. If this originating router does not receive its own data

link layer packets, you can assume the problem is on one of the segments between the

local router and the remote router’s interface card. In this case, the next troubleshooting

step is to configure a line loopback closer to the local router to find the source of the

problem.