Dell Configuration Guide for the S4048–ON System 9.14.2.5

Understanding Multicast Traceroute (mtrace)

Multicast Traceroute (mtrace) is a multicast diagnostic facility used for tracing multicast paths.

Mtrace enables you to trace the path that a multicast packet takes from its source to the destination. When you initiate mtrace from a source to a destination, an mtrace Query packet with IGMP type 0x1F is sent to the last-hop multicast router for the given destination. The mtrace query packet is forwarded hop-by-hop untill it reaches the last-hop router.

The last-hop router converts this query packet to a request packet by adding a response data block. This response data block contains the last-hop router’s interface address. The response data block inserted by the router also contains the following information:

• Incoming interface details
• Outgoing interface details
• Previous-hop router address
• Forwarding Code
• Query Arrival Time
• Routing Protocol details

The last-hop router calculates the path to reach the source in the reverse direction of the multicast data traffic. Based on this calculation, the last-hop router estimates the possible next-hop neighbor that is located in the direction towards the source and forwards the request packet to that neighbor.

Each router along the multicast path fills its response block in a similar manner. When the mtrace request reaches the first-hop router, it sends the response (with IGMP type 0x1E) to the response destination address specified in the mtrace query.

The response may be returned before reaching the first-hop router if a fatal error condition such as "no route" is encountered along the path.

If a multicast router along the path does not implement the mtrace feature or if there is an outage, no response is returned.

When the initiator does not get a response for a specified time interval, the system performs a hop-by-hop expanding-length search to pin point the location in the network where the problem has occurred.