If you use IP multicast in mission-critical networks, you should avoid the use of PIM-DM (dense mode).
Dense mode fallback describes the event of the PIM mode changing (falling back) from sparse mode (which requires an RP) to dense mode (which does not use an RP). Dense mode fallback occurs when RP information is lost.
If all interfaces are configured with the ip pim sparse-mode command, there is no dense mode fallback because dense mode groups cannot be created over interfaces configured for sparse mode.
Use the no ip pim dm-fallback command to disable PIM-DM flooding on sparse-dense interfaces.
Cause and Effect of Dense Mode Fallback
PIM determines whether a multicast group operates in PIM-DM or PIM-SM mode based solely on the existence of RP information in the group-to-RP mapping cache. If Auto-RP is configured or a bootstrap router (BSR) is used to distribute RP information, there is a risk that RP information can be lost if all RPs, Auto-RP, or the BSR for a group fails due to network congestion. This failure can lead to the network either partially or fully falling back into PIM-DM.
If a network falls back into PIM-DM and AutoRP or BSR is being used, dense mode flooding will occur. Switches that lose RP information will fallback into dense mode and any new states that must be created for the failed group will be created in dense mode.
Effects of Preventing Dense Mode Fallback
Prior to the introduction of PIM-DM fallback prevention, all multicast groups without a group-to-RP mapping would be treated as dense mode.
With the introduction of PIM-DM fallback prevention, the PIM-DM fallback behavior has been changed to prevent dense mode flooding. By default, if all of the interfaces are configured to operate in PIM sparse mode (using the ip pim sparse-mode command), there is no need to configure the no ip pim dm-fallback command (that is, the PIM-DM fallback behavior is enabled by default). If any interfaces are not configured using the ip pim sparse-mode command (for example, using the ip pim sparse-dense-mode command), then the PIM-DM fallback behavior can be explicitly disabled using the no ip pim dm-fallback command.
When the no ip pim dm-fallback command is configured or when ip pim sparse-mode is configured on all interfaces, any existing groups running in sparse mode will continue to operate in sparse mode but will use an RP address set to 0.0.0.0. Multicast entries with an RP address set to 0.0.0.0 will exhibit the following behavior:
- Existing (S, G) states will be maintained.
- No PIM Join or Prune messages for (*, G) or (S, G, RPbit) are sent.
- Received (*, G) or (S, G, RPbit) Joins or Prune messages are ignored.
- Received registers are answered with register stop.
- Asserts are unchanged.
- The (*, G) outgoing interface list (olist) is maintained only for the Internet Group Management Protocol (IGMP) state.
- Multicast Source Discovery Protocol (MSDP) source active (SA) messages for RP 0.0.0.0 groups are still accepted and forwarded.