Protocol Atlas

Comparison

PIM-SM versus Bidir-PIM

Both use an RP and a shared tree. Only SM registers sources and builds (S,G) state. Bidir elects a Designated Forwarder and keeps many-to-many groups on (*,G).

PIM-SMBidir-PIM

Side-by-side walkthrough

One shared step index advances both scenarios. Share the URL — ?step= is part of the link.

Step 1 / 6

PIM-SM

Meeting at a point neither end chose

Source · 198.51.100.7Source’s DRRP · 10.255.0.1Last-hop routerReceiverWants: 239.1.1.1
  • Link
  • Blocking
  • Packet in flight
  • Discarded
  • Emphasis
Select a device to read its state. Arrow keys walk the topology.
Text equivalent of this diagram
Devices and links at this step
ElementKindState
Source · 198.51.100.7host—
Source’s DRrouter—
RP · 10.255.0.1router—
Last-hop routerrouter—
ReceiverhostWants: 239.1.1.1
Source · 198.51.100.7 — Source’s DRlinkup
Source’s DR — RP · 10.255.0.1linkup
RP · 10.255.0.1 — Last-hop routerlinkup
Last-hop router — Receiverlinkup
1 / 6

The receiver reports membership of 239.1.1.1 by IGMP. It names a group and no source, because in this model it has no way to know one.

What changed

  • Receiver: Wants → 239.1.1.1
  • Emphasis on Last-hop router ↔ Receiver

Bidir-PIM

Shared tree both ways, no Register

SourceR1 · DF uplinkDF elected per linkDF on src LAN: yesToward RPAR2 · last hopDF toward RP: yesReceiver
  • Link
  • Blocking
  • Packet in flight
  • Discarded
  • Emphasis
Select a device to read its state. Arrow keys walk the topology.
Text equivalent of this diagram
Devices and links at this step
ElementKindState
Sourcehost—
R1 · DF uplinkrouterDF on src LAN: yes
Toward RPArouter—
R2 · last hoprouterDF toward RP: yes
Receiverhost—
Source — R1 · DF uplinklinkup
R1 · DF uplink — Toward RPAlinkup
Toward RPA — R2 · last hoplinkup
R2 · last hop — Receiverlinkup
1 / 3

On every link, routers elect a Designated Forwarder for the Rendezvous Point Address. The DF is the only router that forwards bidir traffic onto that link — toward the RP and away from it.

What changed

  • R1 · DF uplink: DF on src LAN → yes
  • R2 · last hop: DF toward RP → yes
  • DF elected per link

Side by side

PIM-SM versus Bidir-PIM
MechanismPIM-SMBidir-PIM
Source traffic onto the treeThe source DR encapsulates the first packets in Register toward the RP until native (S,G) state exists, then the RP sends Register-Stop. Sending Register Messages from the DRRFC 7761 · INTERNET STANDARD · March 2016No Register. The Designated Forwarder on the source LAN forwards native multicast toward the RP. IntroductionRFC 5015 · PROPOSED STANDARD · October 2007
State(*,G) for the shared tree, then (S,G) after SPT switchover. RPF is toward the source for (S,G) and toward the RP for (*,G). Data Packet Forwarding RulesRFC 7761 · INTERNET STANDARD · March 2016(*,G) only. There is no per-source state and no SPT switchover. IntroductionRFC 5015 · PROPOSED STANDARD · October 2007
RPF / forwarding on a LANA datagram arriving off the reverse-path interface is dropped. That is the usual operational break. Data Packet Forwarding RulesRFC 7761 · INTERNET STANDARD · March 2016The DF is the only forwarder on the link, in both directions, toward and away from the RP. Asserts still pick the DF. Protocol SpecificationRFC 5015 · PROPOSED STANDARD · October 2007
When to use whichFew sources, many receivers, or when an SPT is wanted. SSM is a further restriction: no RP, (S,G) only. Phase Three: Shortest-Path TreeRFC 7761 · INTERNET STANDARD · March 2016Many-to-many groups where (S,G) state would not fit. There is no source registration to look for in a capture. IntroductionRFC 5015 · PROPOSED STANDARD · October 2007

What does not carry over

If the group is bidir, “I don’t see Register packets” is not a failure. If the group is SM, “I only have (*,G)” after the SPT threshold is a failure.

RFC 7761 §4.2 is the SM forwarding rule, including RPF. RFC 5015 is a different protocol, not an SM mode bit.

An RPF failure in SM is diagnosed from the unicast table toward S or the RP. In bidir, look at who won DF on the incoming interface instead.