1. Presentation_ID
© 2003, Cisco Systems, Inc. All rights reserved. 1 1 1

2. Bidirectional PIM
Thom Bryant
June 2003

3. Bidirectional PIM
Bidirectional shared trees appeared in Core Based Trees (CBTs) proposal first
Added to “Simple” later
Idea: use the same tree for traffic from sources towards RP and from RP to receivers
Benefits:
Less state in routers (many sources for the same group produce one (*,G) only)
Traffic from sources / to receivers follows the same path if on the same branch of the RP

4. Bidirectional PIM–Sources
Traffic forwarded natively toward RP rather than registered
RP identified for bidir groups (static or auto-RP) RP
Source

5. Bidirectional PIM–Designated Forwarder
Designated Forwarder elected per subnet based on metric to RP RP 1 DF 2
Source

6. Bidirectional PIM–Receivers
Receivers join toward RP
Forwarding state created with RPF toward RP as the iif RP
Receiver DF
Source
Receiver
(*, G) Join
Forwarding state

7. Bidirectional PIM–Traffic Flow
Traffic flows natively toward RP and can be forwarded directly
on a branch toward interested receivers without first reaching RP.
Traffic forwarded by Designated Forwarder toward RP
Source RP DF
Receiver
Source
Receiver
Traffic for all sources in group G, forwarded based on same *,G entry

8. PIM Bidir Modifications
On each link, the router with the best path to the RP is elected to be the Designated Forwarder
The Designated Forwarder is responsible for forwarding upstream towards the RP
No special treatment is required for local sources
One election per bidir RP.
Election is performed at RP discovery time.
No constant control traffic.
Election is robust and enforces consistent view on all routers on link.

9. Questions?
Thank you

10. Presentation_ID
© 2003, Cisco Systems, Inc. All rights reserved. 10 10 10