23.1.6 Linktrace

The linktrace functionality can be used at the Layer 2 level, and is equivalent to traceroute. The linktrace functionality collects information about switches that pass traffic between MEPs or between a MEP and a MIP of the same MA, and outputs routing information.

The linktrace functionality sends a linktrace message (a kind of CFM PDU) and collects the returned responses as routing information.

The following figure shows an example of sending a linktrace message to a destination.

Figure 23-23: Sending a linktrace message to a destination

A linktrace message is forwarded to the destination via MIPs. An MIP sends back information about the port of the local switch used to receive the MIP and the ports used to forward the MIP. The switch from which the message was sent (the source switch) keeps the information sent by the MIPs as routing information.

The following figure shows an example of forwarding a linktrace message to the destination.

Figure 23-24: Transferring a linktrace message to a destination

The MIP that sent back the information forwards the linktrace message to the destination. However, switch C in the above figure does not send back the information because MEPs or MIPs are not configured on switch C. At least one MIP must be configured on a switch in order to send back information.

When a linktrace message reaches the MEP or the MIP at the destination, a message containing information about the MEP or MIP at the destination to which the linktrace message was delivered and the port through which the message was received is delivered to the source switch.

The source switch outputs the information it has retained as route information that can be used to check the route to the destination.

The linktrace functionality provides information for each switch. For example, whether one or multiple MIPs are configured on a switch, the linktrace functionality provides information about the port used to receive the message and the port used to forward the message.

Because the linktrace functionality uses what the CC functionality learns, the CC functionality must be started beforehand. If you configure a MIP on the destination switch, you must note the MAC address of the port used as the MIP beforehand.

(a) Linktrace to isolate faults

You can use the execution results of the linktrace functionality to isolate the switch or port on which a failure has occurred.

- When a timeout is detected

The following figure shows an example of timeout detection by the linktrace functionality.

Figure 23-25: Linktrace detects a timeout.

In this example, when switch A detects a timeout by using the linktrace functionality, a receiving port on the network might not be able to communicate. A linktrace message is forwarded from switch B to switch C, but because switch C cannot communicate and cannot return a response, a timeout occurs.

- When a forwarding failure is detected

The following figure shows an example of a communication failure detected by the linktrace functionality.

Figure 23-26: Communication failure detected by Linktrace

If switch A detects a forwarding failure by using the linktrace functionality, a sending port on the network might not be able to communicate. The reason is that a linktrace message cannot be forwarded to switches C and D (destination), and therefore the linktrace functionality returns a message indicating that a sending port cannot communicate with switch A.

(b) About Linktrace Replies

Linktrace messages are multicast frames.

When forwarding linktrace messages between switches on which CFM is used, see the MIP CCM database and the MAC address table to determine the port used to forward linktrace messages.

Switches on which CFM is not used flood linktrace messages. As a result, if there is a switch on the network on which CFM is not used, responses are returned from switches that are not on the route to the destination.