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# Investigating Multicast Routes

You can use the mtrace command to trace the path that multicast packets take from a source to a destination through a multicast group address. This command is similar to the traceroute command for investigating unicast routes.

mtrace

- Use to trace the path that multicast packets take to a destination.
- Specify the unicast IP address of the source for the packets.
- To direct the packets to a particular destination, specify the unicast address for that destination. If you do not specify a destination, the router traces the route from the device on which you issue the command.
- To direct the packets through a particular multicast group address, specify that multicast group address. If you do not specify a multicast group address, the router traces the route through the MBone audio multicast group.
- To send the trace to a particular device, specify the IP address of that device. If you do not specify a response address, the router sends the trace to an IP address on the router.
- To investigate a problem at a particular point in the route, specify the maximum number of hops for the trace. The default number of hops is 64.
- The trace starts at the destination and works back to the source.
- Field descriptions
  - Tracing multicast route from a.a.a.a to b.b.b.b for group c.c.c.c using response address d.d.d.d—A description of the trace is as follows:
    - a.a.a.a—IP address of the source
    - b.b.b.b—IP address of the destination
    - c.c.c.c—IP address of the multicast group
    - d.d.d.d—IP address of the router to which the router sends the trace
  - Received mtrace response packet of length n—Length of the response packet, in bytes
  - Each line of the trace has the following format: hops. ip-address Protocol: protocol FwdingCode:forwarding code
    - hops—Number of hops from the destination to this intermediate router
    - ip-address—IP address of the intermediate router
    - protocol—Multicast protocol running on the intermediate router. A value of 12 indicates IGMP; other values comply with A “ traceroute” Facility for IP Multicast – draft-ietf-idmr-traceroute-ipm-07.txt.
    - FwdingCode—Forwarding information or error associated with this hop. For example, RPF iif indicates that the request arrived on the expected RPF interface for this source group. For more information about the forwarding information or error codes, see A “ traceroute” Facility for IP Multicast – draft-ietf-idmr-traceroute-ipm-07.txt.
- Example
  `
  host1#mtrace 100.4.4.4 40.1.1.1 232.1.1.1
  Tracing multicast route from 100.4.4.4 to 40.1.1.1 for group 232.1.1.1 using response address 10.6.129.56
  (Press ^c to stop.)
  Received mtrace response packet of length 88
  1. 40.1.1.1 Protocol: PIM(3) FwdingCode: RPF iif(9)
  2. 21.2.2.2 Protocol: PIM(3) FwdingCode: Reached RP(8)
  `
- There is no no version.
- See mtrace.

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