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# Testing Interfaces

You can enable loopback tests at the SONET/SDH level. You can also test for connectivity between an interface and the SONET/SDH interface at the other end of the line.

## Loopback Testing

To configure loopback testing at the SONET/SDH level, use loopback.

loopback

- Use to configure the type of loopback at the SONET/SDH layer.
- Specify one of the following options:
  - local—Loops the data back toward the router
  - network—Loops the data toward the network before the data reaches the frame.
- Example
  `
  host1(config)#controller sonet 4/0
  host1(config-controller)#loopback network
  `
- Use the no version to disable loopback.
- See loopback.

## Testing Connectivity

Use the path overhead j1 command to check for connectivity between the router and a SONET/SDH device at the other end of the line. This command defines:

- A message that the router sends from the specified interface to the SONET/SDH device at the other end of the line.
- A message that the router expects to receive on the specified interface from the SONET/SDH device at the other end of the line.

When you define a message that the interface sends, you must monitor receipt of that message at the remote end.

When you define a message that the interface expects to receive, you should configure the remote device to transmit the same message to the interface. You can then use the show controllers sonet command to compare the expected and receive messages.

You must remove trace messages before you can change the port type from SONET to SDH or vice versa. Otherwise, you see the following error message:

    % Cannot set port mode (path trace message is set)

path overhead j1

- Use to define messages that the router sends to or expects to receive from a SONET/SDH device connected to one of its SONET interfaces.
- Do not specify a path identifier for unchannelized SONET interfaces.
- Specify the keyword msg for a message that the router transmits for this path.
- Specify the keyword exp-msg to define a message that the router expects to receive on this path.
- Define a message of up to 62 characters for SONET or up to 15 characters for SDH.
- Configure the remote device to send the same message that the router expects to receive on this path. You can then compare the expected and received messages in the display of the show controllers sonet command.
- Example for unchannelized SONET interfaces:
  `
  host1(config-controller)#path overhead j1 msg hello
  `
- Use the no version to restore the default situation, in which all the characters in the transmitted or expected message are zeros.
- See path overhead j1.

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