Quick on PIM Multicast State Flags

 Today I am going to talk about the Multicast State Flags used in the PIM protocol. These flags can be seen when you are trying to check the show mroute command. Many of you already knew about the flags used in the PIM Multicast state. As some of you asked this question I am explaining all the states one by one.

Fig 1.1- Basic Topology using Multicast Data Packets

Above is just an example of the topology showing the Multicast traffic passing but here we are just going to discuss about the flags used in the multicast environment. Lets talk about the flags here.

So we are using 13 different flags in the PIM Multicast and let me explain all these states one by one.

Dense mode flag 

This indicates that the multicast group is a dense mode group. Appears on (*,G) entries only.

Sparse mode flag

This indicates that the multicast group is a sparse mode group. Appears on (*,G) entries only.

Connected

The C flag appears on both (S,G) and (*,G) entries and indicates that there is a directly connected member for this multicast group.

Local flag

The L flag appears on both (S,G) and (*,G) entries and indicates that the router itself is a member of this group. Therefore, the router will process all multicast traffic sent to this group at process level.A good example of a group that the router itself would join is the PIM RP-Discovery (224.0.1.40) group that distributes Auto-RP information.

Pruned flag

The P flag indicates that the outgoing interface is Null. This condition causes a Prune to be sent to the upstream (RPF) neighbor for this (*,G) or (S,G) group.

SPT flag

The T flag (or SPF flag) appears only on (S,G) entries and indicates that this traffic is being forwarded via the (S,G) entry (that is, the SPT). This flag is set whenever the first (S,G) packet is received corresponding to this mroute table entry. In Dense mode the T flag is always set on (S,G) entries.

Proxy-Join timer flag

The X flag appears only on (S,G) entries and indicates that the Proxy-Join timer is running.When this timer is running, the router will continue to send (S,G) Joins in the direction of the source even if the OIL is NULL.This is used to handle the special turn-around router situation which occurs when the SPT to the RP and the Shared Tree merge.

MSDP entry flag 

This flag only appears on (S,G) entries and only on the router that is the active RP for group G.The flag indicates that the RP has learned of this particular source via an MSDP "Source Active" message.

MSDP Advertisement flag

This flag only appears on (S,G) entries and only on the router that is the active RP for group G.The A flag indicates that this source is in the local PIM-SM domain and that it is a candidate for being announced to the RP's in other networks via MSDP SA messages.

The source is considered to be in the local domain if an (S,G) Register message was received for this source or the source is directly connected to the RP or the (S,G) traffic was received on a Dense mode interface that has been designated as a dense mode boundary interface.

Join SPT flag

For Sparse mode (*,G) entries, the J flag is set whenever the rate of traffic flowing down the shared tree is exceed the SPT-threshold. (This calculation is done once a second). When the J flag has been set to the (*,G) entry, the next (S,G) packet received down the shared tree will cause source S to be cut over the SPT. (This switchover occurs only if the C flag is also set.)

For Dense mode groups, the J flag appears only on the (*,G) entry and is simply used internally to tell PIM that an (S,G) state should be automatically created for any arriving (S,G) traffic. (This flag can be ignored for dense mode groups.)

Join SPT flag 

The J flag is set on sparse mode (S,G) entries to indicate that this source was previously cut over to the SPT. This flag tells PIM to check the traffic rate flowing down the SPT against SPT-Threshold to see whether this source should be switched back to the shared tree. (This calculation is done once per minute to prevent flip-flop condition.)

Register flag

The F flag is used on (S,G) entries to indicate that we must send Register message for this traffic. This indicates that this router is "first-hop" router and triggers it to send Register messages to the RP to inform the RP of this active source.

This flag can also be set for arriving (S,G) entries created at a border router such as a router that borders on a DVMRP or other dense mode cloud. This causes the router to perform a proxy-register operation and send (S,G) Register messages to the RP on behalf of the downstream DVMRP routers. This proxy-register operation follows the same rules as for directly connected sources. 

If the F flag persists, then your router is most likely not receiving the PIM Register-Stop messages back from the RP, and thus there are sources that has not switched to the SPT tree.

RP-bit flag

The R flag is the RP-bit flag and indicates that the information in the (S,G) entry is applicable to the shared tree. (The shared tree is sometimes called the RP Tree, hence the name, "RP-bit".)RP-bit entries are used to prune redundant (S,G) traffic from the shared tree after a downstream router has joined SPT for source S.Note: 

When the RP-bit is set, the (S,G) incoming interface is switched to the point toward the RP instead of the source, S. This is done so that the RPF check will succeed for (S,G) traffic flowing down the shared tree.