3Bidirectional PIMBidirectional shared trees appeared in Core Based Trees (CBTs) proposal firstAdded to “Simple” laterIdea: use the same tree for traffic from sources towards RP and from RP to receiversBenefits: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
4Bidirectional PIM–Sources Traffic forwarded natively toward RP rather than registeredRP identified for bidir groups (static or auto-RP)RPSource
5Bidirectional PIM–Designated Forwarder Designated Forwarder elected per subnet based on metric to RPRP1DF2Source
6Bidirectional PIM–Receivers Receivers join toward RPForwarding state created with RPF toward RP as the iifRPReceiverDFSourceReceiver(*, G) JoinForwarding state
7Bidirectional PIM–Traffic Flow Traffic flows natively toward RP and can be forwarded directlyon a branch toward interested receivers without first reaching RP.Traffic forwarded by Designated Forwarder toward RPSourceRPDFReceiverSourceReceiverTraffic for all sources in group G, forwarded based on same *,G entry
8PIM Bidir Modifications On each link, the router with the best path to the RP is elected to be the Designated ForwarderThe Designated Forwarder is responsible for forwarding upstream towards the RPNo special treatment is required for local sourcesOne 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.