PIM Source Fanout Trace for multicast flow
Cisco Systems
821 Alder Drive
Milpitas, CA 95035
United States of America
mankamis@cisco.com
Routing
PIM Working Group
Mtrace version 2 traces an IP multicast path by walking from a
last-hop router or rendezvous point toward the source. That model is
efficient for a single receiver path, but it does not directly answer
the operational question of where a multicast source fans out
downstream without issuing separate traces from many receiver-side
locations.
defines Mtrace version 2 (Mtrace2), which
allows a multicast path to be traced by starting at a Last-Hop Router
(LHR) or Rendezvous Point (RP) and walking upstream toward the source.
This receiver-to-source model is useful for diagnosing a single
receiver path, but it does not directly provide source-rooted
visibility into the downstream multicast fanout tree.
Operationally, there are cases where an operator needs to start from
the source side and discover the downstream fanout for an exact
multicast flow. Examples include finding all receiver branches for a
high-rate IPTV flow, identifying the branch where forwarding stops, or
validating whether a source-rooted tree reaches all expected receiver
edges. With only the receiver-to-source model, the operator must know
candidate receivers or last-hop routers in advance and issue many
separate traces. This is inefficient and can miss branches that are
unknown to the operator.
This limitation also affects automated tree health monitoring. If an
operator wants to continuously probe the health of a multicast tree, an
Mtrace2-only approach requires originating one or more receiver-side
probes for each candidate branch during every polling interval. As the
number of sources, groups, receivers, VRFs, and polling frequency grow,
this can result in N probes every T seconds, with each probe requiring
control-plane processing on routers along the traced path. Repeated
per-branch probing can therefore become CPU intensive and does not
scale as a general mechanism for monitoring whole-tree health.
A source-to-receiver fanout tracing mechanism is therefore needed to
complement Mtrace2 by allowing an operator to discover downstream
forwarding branches, termination points, and failure points for an
exact multicast flow from the source-side root. The desired approach
is one where a single source-rooted probe can provide enough
information about the downstream tree to identify reachable branches and
failure points, while reducing the amount of repeated control-plane
work required from the network.