Black Box LR1102A-T1/E1 - Manual (Page 115)

21
MTRACE
C
ONFIGURATION
21.1 Multicast Traceroute Facility
With multicast distribution trees, tracing from a source to a multicast destination is difficult, since the branch of the
multicast tree on which the destination lies is unknown. The technique used by the traceroute tool to trace unicast
network paths will not work for IP multicast because traceroute (ICMP) responses are specifically forbidden for
multicast traffic. Thus, you have to flood the whole tree to find the path from one source to one destination. However,
walking up the tree from destination to source is easy, as most existing multicast routing protocols know the previous
hop for each source. Tracing from destination to source involves only routers on the direct path.
To request a traceroute (which does not have to be the source or the destination), send a traceroute query packet to the
last-hop multicast router for the given destination. The last-hop router turns the query into a request packet by adding a
response data block containing its interface addresses and packet statistics, and then forwards the request packet using
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. 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.
Multicast traceroute uses any information available to it in the router to try to determine a previous hop to forward the
trace towards. Multicast routing protocols vary in the type and amount of state they keep; multicast traceroute tries to
work with all of them by using whatever is available. For example, if a DVMRP router has no active state for a
particular source but does have a DVMRP route, it chooses the parent of the DVMRP route as the previous hop. 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.
Black Box supports the following PIM related feature—a “traceroute” facility for IP multicast, as defined in
draft-ietf-idmr-traceroute-ipm-05.
The mtrace command for multicast traffic is similar to the traceroute command used for unicast traffic. Unlike
traceroute, however, mtrace traces traffic backwards, from the receiver to the source. mtrace uses other unicast
routing tables for RPF. For these, mtrace relies on Black Box Networks’ implementation of the mtrace protocol is
manageable through the CLI and can be executed from any command sub-tree of the Black Box CLI.
21.1.1 mtrace Command
mtrace
21.1.2 Restrictions
In this release, configuring Maximum Hops & TTL is not permitted.