PIM SSM and Bi-Dir PIM?

Unanswered Question
Feb 4th, 2009

I have two quick questions ...

What is the best use and reason for implementing PIM SSM?

What is the best use and reason for implementing Bi-Dir PIM?

Thank you!

I have this problem too.
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Mohamed Sobair Wed, 02/04/2009 - 09:04


Bellow are the reasons:

Pim SSM: Cisco highly recommend the use of Data-mdt (SSM) to optimize multicast traffic and to support high multicast bandwidth applications. In Source Specific multicast , Hosts sends IGMP Source specific along with its group. The IGMP Query is send to the address (

Pim Bi-Dir: Pim Bi-directional is implemented in a Network which has sources that are also recievers for different Groups (In between). In Pim Bi-dir the source of the tree is rooted at the shared tree. Pim Designated Forwarder is elected based on the highest ip address on a segment, In Pim Bi-dir the Pim Register and Register-message is the removed to prevent added overhead.



Harold Ritter Wed, 02/04/2009 - 09:54


Just one clarification on the Designated Forwarder election (DF). The DF is elected based on

(1) the preference of its route to the RP.

(2) the metric of its route to the RP.

(3) Highest IP address (tie breaker)


Giuseppe Larosa Wed, 02/04/2009 - 09:07

Hello Rashida,

Bi-Dir PIM is a good solution when there are multiple sources for the multicast groups in use the capability to build bidirectional trees provide more forwarding efficiency.

PIM SSM is source aware so it allows for security (do not propagate traffic from arbitrary ip addresses) and for reuse of multicast ip addresses: a different ip source is seen as a different session users with IGMP v3 request a group and can specify a source.

Hope to help


Mohamed Sobair Wed, 02/04/2009 - 13:09

Yes Harold Correct, I missed that ..

The prefrence "first" and the least metric toward the RP "secondly" and then the highest IP address.

Actually, the Designated Router in Pim Sparse mode is selected based on the highest IP address on a segment.



Harold Ritter Wed, 02/04/2009 - 18:02

Hi Mohamed,

You are correct about the DR election. By the way, The DF election was based on the PIM assert mechanism.



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