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PIM Sparse Mode Packet Deep Dive

PIM Sparse Mode Packet Deep Dive

Posted on Jan 24, 2020 (0)

PIM Sparse Mode Packet Deep Dive

Configuring PIM-SM: Configuring PIM-SM can be done by following commands in global mode and interface mode.

Global Mode:

Router-1)#ip multicast-routing

Enable PIM-SM on every interface in every router in the network, using the following interface command:

Router-1)#int fa0/0
Router-1(config-int)#ip pim sparse-mode

However, PIM-SM mode works when the Presence of RP is defined on routers participating in multicast traffic. So we can define RP via static, Auto-RP and another method. We will discuss these method in details:

But for here, we can define you as static way:

Router-1(config)# ip pim rp-address

Here, the recommended method for enabling multicast is to use pim sparse-dense mode. Router can work well in sparse mode but depending on existence of RP it can switch to dense mode to ensure that multicast packet should be delivered to destination.

PIM-SM States Rule:

PIM-SM stares rules can be broadly defined as per following criteria:

Apart from all general rule we discussed in PIM-DM, fooling are the PIM_SM rules which is following while preparing the mroute table.

PIM-SM (*, G) state Rule:  

PIM-SM (*, G) is created only when directly connected is joining the group by sending the IGMP membership report or when router receives the (*, G) join receipt. So we can say that (*, G) entry is created only on demand.

PIM-SM Rule 1: PIM-SM (*, G) is created when Explicit Join operation is done.

PIM-SM Rule 2: The Incoming interface of (*, G) will always points up the shared tree towards RP.

In PIM-SM mode, the IP address of RP is used to find the RPF interface.

PIM-SM(S, G) State Rule:

PIM-SM Rule 3:

There are some following condition which causes (S, G) entry to be created on router:

  • (S, G) entry is created when router receives the (S, G) join/Prune message
  • (S, G) entry is created on last-hop router when it switches to SPT
  • (S, G) entry is created when there is unexpected arrival of (S, G) traffic and when there is no (*,G)
  • (S, G) entry is created at RP when Register Message from Source is received.

PIM-SM Outgoing interface rules:

Addition of interface in OIL of (*, G) or (S, G) can be achieved by PIM-SM Rule 4 in either of following condition:

  • When (*, G) or (S, G) join is received via this interface
  • When there is a directly connected member of the group exists on the interface.

Removal of interface in OIL of (*, G) or (S, G) can be achieved by PIM-SM Rule 5 in either of following condition:

  • When (*, G) or (S, G) prune is received via this interface or no directly connected member of this group on this interface
  • Interface expiration timer count goes to zero

PIM-SM state Maintenance Rule:

Maintenance rule basically relates to expiration timer of interface which is reset to 3 min as result of following conditions:

PIM-SM Rule 6

  • When (*, G) or (S, G) join is received on interface
  • An IGMP membership report is received from directly connected member on interface.


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