Brocade Multi-Service IronWare Multicast Configuration Guide (Supporting R05.6.00) - Manual (Page 21)

Multi-Service IronWare Multicast Configuration Guide 9
53-1003032-02
Mtrace overview
1
Multicast traceroute uses any information available to it in the router to determine a previous hop
to forward the trace towards the source. Multicast routing protocols vary in the type and amount of
state they keep; multicast traceroute endeavors to work with all of them by using whatever is
available. For example, if a PIM-SM router is on the (*,G) tree, it chooses the parent towards the RP
as the previous hop. In these cases, no source/group-specific state is available, but the path may
still be traced.
FIGURE 1 Network topology
Mtrace components
There are 3 main components in a multicast traceroute implementation. They are:
a. Mtrace Query
b. Mtrace Request
c. Mtrace Response
• Mtrace Query
The party requesting the traceroute sends a traceroute query packet to the last-hop multicast
router for the given destination. The query and request have the same opcode, the receiving router
can distinguish between a query and a request by checking the size of the packet. A query is a
request packet with none of the response fields filled up.
• Mtrace Request
The last-hop router turns the Query packet into a Request packet by adding a response data block
containing its interface addresses and packet statistics, and then forwards the Request packet via
unicast to the router that it believes is the proper previous hop for the given source and group.
Each hop adds its response data to the end of the Request packet, then unicast forwards it to the
previous hop.
• Mtrace Response
The first hop router (the router that believes that packets from the source originate on one of its
directly connected networks) changes the packet type to indicate a Response packet and sends
the completed response to the response destination address. 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.