What is the purpose of the mtrace command?

davidhuynh5 · ‎09-17-2010

Can someone please explain to me how the mtrace command works? Please provide an example. Thanks in advance.

mhorner · ‎09-17-2010

mtrace

To trace the path from a source to a destination branch for a multicast distribution tree, use the mtrace user EXEC command.

mtrace source [destination] [group]

Syntax Description


source	DNS name or the IP address of the multicast-capable source. This is a unicast address of the beginning of the path to be traced.
destination	(Optional) DNS name or address of the unicast destination. If omitted, the mtrace starts from the system at which the command is typed.
group	(Optional) DNS name or multicast address of the group to be traced. Default address is 224.2.0.1 (the group used for MBONE Audio). When address 0.0.0.0 is used, the software invokes a weak mtrace. A weak mtrace is one that follows the RPF path to the source, regardless of whether any router along the path has multicast routing table state.

Command Mode

EXEC

Usage Guidelines

This command first appeared in Cisco IOS Release 11.0.

The trace request generated by the mtrace command is multicast to the multicast group to find the last hop router to the specified destination. The trace then follows the multicast path from destination to source by passing the mtrace request packet via unicast to each hop. Responses are unicast to the querying router by the first hop router to the source. This command allows you to isolate multicast routing failures.

If no arguments are entered, the router will interactively prompt you for them.

This command is identical in function to the UNIX version of mtrace.

Sample Display

The following is sample output from the mtrace command:

Router# mtrace 171.69.215.41 171.69.215.67 239.254.254.254

Type escape sequence to abort.

Mtrace from 171.69.215.41 to 171.69.215.67 via group 239.254.254.254

From source (?) to destination (?)

Querying full reverse path...

 0  171.69.215.67

-1  171.69.215.67 PIM  thresh^ 0  0 ms

-2  171.69.215.74 PIM  thresh^ 0  2 ms

-3  171.69.215.57 PIM  thresh^ 0  894 ms

-4  171.69.215.41 PIM  thresh^ 0  893 ms

-5  171.69.215.12 PIM  thresh^ 0  894 ms

-6  171.69.215.98 PIM  thresh^ 0  893 ms

describes the fields shown in the display.

Table 71 Mtrace Field Descriptions
Field	Description
Mtrace from 171.69.215.41 to 171.69.215.67 via group 239.254.254.254	Name and address of source, destination, and group for which routes are being traced.
-3 171.69.215.57	Hops away from destination (-3) and address of intermediate router.
PIM thresh^ 0	Multicast protocol in use on this hop, and ttl threshold.
893 ms	Time taken for trace to be forwarded between hops.

davidhuynh5 · ‎09-17-2010

Thanks, but can you explain in simple english?

Mani Ganesan · ‎09-21-2010

Do you want to know the purpose of mtrace or how it works internally ?

When I dug up a littile bit on how it works , I got this :

Mtrace use special IGMP packet type (Queries and Response) IGMP 1F and 
1E. Requestor send packet to ther last-hop router. This router add some 
usefull parameters such as IF, OF, input packet count, out packet count, 
query time and many other. Then router send traceroute Request to the 
next-hop (out to Input Interface for SPT or shared tree) and each hop add 
same data to the packet. This packet extend as unicasts. At the end first-hop 
router send IGMP Response to the requestor.

HTH,
Mani