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Checking the SONET Frame Checksum

 

This section includes the following information to assist you when troubleshooting SONET interfaces:

Checklist for Checking the SONET Frame Checksum

Purpose

To check and configure SONET frame checksum.

Action

Table 1 provides the links and commands for SONET frame checksum.

Table 1: Checklist for Checking the SONET Frame Checksum

Tasks

Command or Action

Understand the SONET Frame Checksum
Checking the SONET Frame Checksum
  1. Examining Output for Framing Errors

show interfaces interface-name extensive

  1. Checking the FCS Configuration

show configuration interfaces | interface-name

show interfaces interface-name

Configuring a SONET Frame Checksum
  1. Returning to the Default 16-Bit Checksum

[edit]

edit interfaces so-fpc/pic/port
sonet-options

delete fcs 32

show

commit

  1. Configuring a 16-Bit Checksum

[edit]

edit interfaces so-fpc/pic/port
sonet-options

set fcs 16

show

commit

  1. Configuring a 32-Bit Checksum

[edit]

edit interfaces so-fpc/pic/port
sonet-options

set (fcs 32 | rfc-2615)

show

commit

Understand the SONET Frame Checksum

Problem

Description: The SONET frame checksum is a calculation that is added to a frame for error control purposes. SONET frame checksum is used in High-Level Data Link Control (HDLC), Frame Relay, and other data-link layer protocols. For example, Router A calculates the frame check sequence (FCS) and adds it to the outgoing message. Router B, on receiving the message recalculates the FCS and compares it to the FCS from Router A. If there is a difference, both sides of the connection might not match in relation to the FCS configuration.

Solution

If you are having problems with a connection, check whether the FCS matches on both sides of the connection. To check the SONET frame checksum, see Checking the SONET Frame Checksum.

After you have checked the FCS and determined that a problem exists, you must configure a SONET frame checksum. For more information, see Configuring a SONET Frame Checksum.

Checking the SONET Frame Checksum

If you are having problems with a connection, check that the FCS matches on both sides of the connection.

To check the SONET frame checksum, follow these steps:

  1. Examining Output for Framing Errors

  2. Checking the FCS Configuration





Examining Output for Framing Errors

Purpose

By examining the output for an interface, you can determine if framing errors are incrementing in the absence of any SONET alarms or defects.

Action

From the Junos OS command-line interface (CLI) operational mode, use the following command to check for framing errors:

Sample Output

user@router1> show interfaces so-1/0/0 extensive

Meaning

The sample output shows that Router 1 is configured for FCS 16, that framing errors have incremented to 27, and that there are no SONET alarms or defects. Incrementing framing errors, in the absence of any SONET alarms or defects, are a symptom of SONET frame checksum errors.



Checking the FCS Configuration

Purpose

If you are having problems with a connection, check your router’s FCS configuration and, if possible, the FCS configuration on the router on the other side of the connection.

Action

From the Junos OS CLI operational mode, use one of the following two commands to check the SONET frame checksum:

or

Note

The option to display a specific configuration with the show configuration command hierarchy was introduced in Junos OS Release 5.3.

Sample Output 1

user@host> show configuration interfaces so-0/0/0

Sample Output 2

user@host> show interfaces so-0/0/1

Meaning

Sample output 1 shows that FCS 32 is configured. If you use the show configuration or show configuration interfaces command, you must scroll to the particular interface for the FCS configuration status.

Meaning

Sample output 2 shows that FCS 32 is configured. To change the FCS configuration, see Returning to the Default 16-Bit Checksum, Configuring a 16-Bit Checksum, or Configuring a 32-Bit Checksum.

Configuring a SONET Frame Checksum

After you have checked the FCS and determined that a problem exists, you might need to do one of the following, depending on the situation:

Note

By default, SONET interfaces use a 16-bit frame checksum. You can configure a 32-bit checksum, which provides more reliable packet verification. However, some older equipment may not support 32-bit checksums.



Returning to the Default 16-Bit Checksum

Action

To return to the default 16-bit frame checksum, follow these steps:

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

  2. Delete the fcs 32 statement from the configuration.

  3. Verify the configuration.

  4. Commit the configuration.



Configuring a 16-Bit Checksum

Action

To explicitly configure the 16-bit checksum, follow these steps:

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

  2. Configure the 16-bit checksum.

  3. Verify the configuration.

  4. Commit the configuration.



Configuring a 32-Bit Checksum

Action

To explicitly configure the 32-bit checksum, follow these steps:

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

  2. Configure the 32-bit checksum.

    Note

    The rfc-2615 statement automatically configures the interface to use FCS 32 and changes the C2 byte to 0x16, as per the RFC.

  3. Verify the configuration.

    or

  4. Commit the configuration.

    Note

    On a Channelized OC12 interface, the sonet-options fcs statement is not supported. To configure FCS on each DS3 channel, you must include the t3-options fcs statement in the configuration for each channel.