Troubleshooting tools


Note: This information applies to the classic CLI.

Assessing problems in the distribution of IP multicast traffic can be difficult. The mtrace feature uses a tracing feature implemented in multicast routers that is accessed via an extension to the IGMP protocol. The mtrace feature is used to print the path from the source to a receiver; it does this by passing a trace query hop-by-hop along the reverse path from the receiver to the source. At each hop, information such as the hop address, routing error conditions and packet statistics should be gathered and returned to the requester.

Data added by each hop includes:

  • query arrival time

  • incoming interface

  • outgoing interface

  • previous hop router address

  • input packet count

  • output packet count

  • total packets for this source/group

  • routing protocol

  • TTL threshold

  • forwarding/error code

The information enables the network administrator to determine:

  • where multicast flows stop

  • the flow of the multicast stream

When the trace response packet reaches the first hop router (the router that is directly connected to the source’s net), that router sends the completed response to the response destination (receiver) address specified in the trace query.

If some multicast router along the path does not implement the multicast traceroute feature or if there is some outage, then no response is returned. To solve this problem, the trace query includes a maximum hop count field to limit the number of hops traced before the response is returned. This allows a partial path to be traced.

The reports inserted by each router contain not only the address of the hop, but also the TTL required to forward and some flags to indicate routing errors, plus counts of the total number of packets on the incoming and outgoing interfaces and those forwarded for the specified group. Taking differences in these counts for two traces separated in time and comparing the output packet counts from one hop with the input packet counts of the next hop allows the calculation of packet rate and packet loss statistics for each hop to isolate congestion problems.

Finding the last hop router

The trace query must be sent to the multicast router which is the last hop on the path from the source to the receiver. If the receiver is on the local subnet (as determined using the subnet mask), then the default method is to multicast the trace query to ( with a TTL of 1. Otherwise, the trace query is multicast to the group address because the last hop router is a member of that group if the receiver is. Therefore, it is necessary to specify a group that the intended receiver has joined. This multicast is sent with a default TTL of 64, which may not be sufficient for all cases.

When tracing from a multihomed host or router, the default receiver address may not be the wanted interface for the path from the source. In that case, the wanted interface should be specified explicitly as the receiver.

Directing the response

By default, mtrace first attempts to trace the full reverse path, unless the number of hops to trace is explicitly set with the hop option. If there is no response within a 3 second timeout interval, a "*" is printed and the probing switches to hop-by-hop mode. Trace queries are issued starting with a maximum hop count of one and increasing by one until the full path is traced or no response is received. At each hop, multiple probes are sent. The first attempt is made with the unicast address of the host running mtrace as the destination for the response. Because the unicast route may be blocked, the remainder of attempts request that the response be multicast to ( with the TTL set to 32 more than what’s needed to pass the thresholds seen so far along the path to the receiver. For the last attempts the TTL is increased by another 32.

Alternatively, the TTL may be set explicitly with the TTL option.

For each attempt, if no response is received within the timeout, a "*" is printed. After the specified number of attempts have failed, mtrace tries to query the next hop router with a DVMRP_ASK_NEIGHBORS2 request (as used by the mrinfo program) to determine the router type.

The output of mtrace is a short listing of the hops in the order they are queried, that is, in the reverse of the order from the source to the receiver. For each hop, a line is printed showing the hop number (counted negatively to indicate that this is the reverse path); the multicast routing protocol; the threshold required to forward data (to the previous hop in the listing as indicated by the up-arrow character); and the cumulative delay for the query to reach that hop (valid only if the clocks are synchronized). The response ends with a line showing the round-trip time which measures the interval from when the query is issued until the response is received, both derived from the local system clock.

Mtrace/mstat packets use special IGMP packets with IGMP type codes of 0x1E and 0x1F.


Note: This information applies to the classic CLI.

The mstat command adds the capability to show the multicast path in a limited graphic display and provide drops, duplicates, TTLs and delays at each node. This information is useful to the network operator because it identifies nodes with high drop and duplicate counts. Duplicate counts are shown as negative drops.

The output of mstat provides a limited pictorial view of the path in the forward direction with data flow indicated by arrows pointing downward and the query path indicated by arrows pointing upward. For each hop, both the entry and exit addresses of the router are shown if different, along with the initial ttl required on the packet to be forwarded at this hop and the propagation delay across the hop assuming that the routers at both ends have synchronized clocks. The output consists of two columns, one for the overall multicast packet rate that does not contain lost/sent packets and a column for the (S,G)-specific case. The S,G statistics do not contain lost/sent packets.


Note: This information applies to the classic CLI.

The simple mrinfo mechanism is based on the ask_neighbors igmp to display the configuration information from the target multicast router. The type of information displayed includes the Multicast of the router, code version, metrics, ttl-thresholds, protocols and status. This information, for instance, can be used by network operators to verify if bidirectional adjacencies exist. After the specified multicast router responds, the configuration is displayed.