PIM Flooding Mechanism and Source Discovery Enhancements

3.2. PFM optimization Hello option

A PIM router indicates support for the forwarding optimization by including the PFM Optimization Hello option (OptionType TBD3) in PIM Hello messages.¶

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OptionType = TBD3       |           Length = 0          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

OptionType (16 bits):

TBD3¶

OptionLength (16 bits):

0¶

A router that supports this optimization MUST track, per interface, whether all neighbors support the option. This tracking is beneficial in heterogeneous networks where only certain routers support the optimization.¶

For each learned Router-ID, the router MUST maintain a set of interfaces, denoted as PFM_OPT_IF, that satisfy both of the following conditions:¶

• The neighbor with this Router-ID is the sole PIM neighbor on this interface.¶

• The neighbor is advertising the PFM optimization option TBD3 on this interface.¶

PFM message exchange MAY be optimized on interfaces in the PFM_OPT_IF set. This is discussed in Section 3.4.¶

3.3. Sample PFM Topology

                         Router C
                            |
                            | LAN 1
                            |
     Router A --------------------------------- Router B
        |                                          |
        |----------------- Link L1 ----------------|
        |                                          |
        |----------------- Link L2 ----------------|
        |                                          |
        |----------------- Link L3 ----------------|
        |__________________________________________|
                            |
                            | LAN 2
                            |
                         Router D

This figure is schematic and not intended to imply a specific physical layout. It depicts two routers (A and B) connected by three parallel point-to-point links (L1, L2, L3), with shared LAN segments (LAN 1 and LAN 2) attached to Routers C and D.¶

3.4. PFM message handling with Relaxed-RPF enabled

When two routers maintain multiple adjacencies and are the only neighbors on those links, PFM messages are typically transmitted on all links and filtered by RPF checks at the receiver. This results in redundant processing. If both routers advertise Router-IDs and support the optimization, each router forms a PFM_OPT_IF set containing eligible interfaces.¶

Implementations can experience transient states while link failures are being detected and propagated. During such intervals, a PFM message sent on the selected interface can be lost or delayed if that interface fails before the sender updates its PFM_OPT_IF state. This is not unique to the optimization in this document and can also occur in regular PFM operation when a message is sent on the selected RPF path and that path fails. Implementations SHOULD apply robust and timely link-failure detection and update forwarding/interface-selection state promptly. Even if one message is lost, subsequent periodic PFM messages refresh state.¶

When Relaxed-RPF is enabled:¶

• A sender MUST select a single interface from its PFM_OPT_IF set for PFM transmission to that neighbor. The selection method is implementation-specific.¶

• On shared LANs, the sender MUST send PFM messages as normal since optimization cannot be applied when there are more than two routers on the network segment.¶

• A receiver that supports Relaxed-RPF MUST:¶

  • Determine the expected RPF interface using standard procedures.¶

  • Accept a PFM message received on any interface in the PFM_OPT_IF set if both sender and receiver support the optimization.¶

  • Otherwise, perform standard RPF validation.¶

Referring to Figure 1, when Router A originates or forwards a PFM message, it transmits the message on one of links L1, L2, or L3. This behavior reduces processing overhead on point-to-point links. The selection of the interface from the PFM_OPT_IF set is implementation-specific. Router A also sends the message on both LAN 1 and LAN 2 so Routers C and D receive the message.¶

3.5. Maintenance of PFM_OPT_IF

Routers MUST update the PFM_OPT_IF set upon neighbor or capability changes:¶

• Neighbor Addition: If the new neighbor is the sole neighbor on the interface and advertises both a Router-ID and the optimization option, the interface MUST be added to the corresponding PFM_OPT_IF set. If no set exists, it MUST be created. If a second neighbor appears on the interface, the interface MUST be removed from the PFM_OPT_IF set.¶

• Neighbor Removal: After a neighbor removal event, if exactly one neighbor remains and it advertises both a Router-ID and the optimization option, the interface MUST be added to the PFM_OPT_IF set for that Router-ID. If no set exists, it MUST be created.¶

• Router-ID Changes: If a neighbor starts advertising a Router-ID and satisfies all conditions, the interface MUST be added to the PFM_OPT_IF set. If a neighbor stops advertising a Router-ID, the interface MUST be removed from the PFM_OPT_IF set for that Router-ID. If the set becomes empty, it MUST be deleted.¶

• Optimization Capability Changes: If a neighbor starts advertising the optimization option and satisfies all conditions, the interface MUST be added to the PFM_OPT_IF set. If a neighbor stops advertising the optimization option, the interface MUST be removed from the PFM_OPT_IF set for that Router-ID. If the set becomes empty, it MUST be deleted.¶

These procedures apply during topology changes, configuration updates, and software upgrades or downgrades. Routers MUST maintain accurate PFM_OPT_IF state for each Router-ID.¶

3.6. PFM message forwarding to sender

When the TBD3 optimization is enabled on a PIM router, the router MUST NOT forward a PFM message on a link if both of the following conditions are true: (1) the link has only one neighbor, and (2) that neighbor's Router-ID matches the Router-ID of the router that originated the PFM message. It is sufficient for the neighbor to advertise only the Router-ID, without any additional optimization options, since this information alone ensures the message is not sent back to its original sender, thereby reducing unnecessary PFM message forwarding.¶