Administrator Guide

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.
NOTE: If the system initiating the mtrace is the last-hop router, then the Query message will not be initiated. Instead,
the router sends the request message to it previous 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.
NOTE: You cannot configure the wait time. It is fixed to 3 seconds.
Important Points to Remember
• Destination address of the mtrace query message can be either a unicast or a multicast address.
NOTE:
When you use mtrace to trace a specific multicast group, the query is sent with the group's address as the
destination. Retries of the query use the unicast address of the receiver.
• When you issue an mtrace without specifying a group address (weak mtrace), the destination address is considered as the unicast
address of the receiver.
• If the CLI session is terminated after the mtrace command is issued, then the response is ignored.
• System ignores any stray mtrace responses that it receives.
• Duplicate query messages as identified by the IP source, and Query ID (tuple) are ignored. However, duplicate request messages are
not ignored in a similar manner.
• The system supports up to a maximum of eleven mtrace clients at a time.
NOTE: The maximum number of clients are subject to performance restrictions in the new platform.
• Mtrace supports only IPv4 address family.
Printing Multicast Traceroute (mtrace) Paths
Dell EMC Networking OS supports Multicast traceroute.
MTRACE is an IGMP-based tool that prints the network path that a multicast packet takes from a source to a destination, for a particular
group. Dell EMC Networking OS has mtrace client and mtrace transit functionality.
• MTRACE Client — an mtrace client transmits mtrace queries and print the details from received responses.
• MTRACE Transit — when a Dell EMC Networking system is an intermediate router between the source and destination in an
MTRACE query, Dell EMC Networking OS computes the RPF neighbor for the source, fills in the request, and forwards the request to
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Multicast Features