Glossary

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.
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Multicast Features