Passive multicast route insertion

To prevent unwanted multicast traffic from being sent to the CPU, PIM routing and passive multicast route insertion (PMRI) can be used together to ensure that multicast streams are forwarded out only ports with interested receivers and unwanted traffic is dropped in hardware on Layer 3 switches.

PMRI enables a Layer 3 switch running PIM Sparse to create an entry for a multicast route (for example, (S,G)), with no directly attached clients or when connected to another PIM device (transit network).

When a multicast stream has no output interfaces (OIF), the Layer 3 switch can drop packets in hardware if the multicast traffic meets either of the following conditions:

In PIM-SM:

  • The route has no OIF and
  • If directly connected source passed source reverse-path forwarding (RPF) check and completed data registration with reverse path (RP) or
  • If non directly connected source passed source RPF check.

In PIM-DM:

  • The route has no OIF and
  • passed source RPF check and
  • Device has no downstream PIM neighbor.

If the OIF is inserted after the hardware-drop entries are installed, the hardware entries will be updated to include the OIFs.

Note: Disabling hardware-drop does not immediately take away existing hardware-drop entries, they will go through the normal route aging processing when the traffic stops.