Configuring Bidirectional PIM
Bidir-PIM Overview
IPC-472
Cisco IOS IP Configuration Guide
Membership to a bidirectional group is signalled via explicit join messages. Traffic from sources is
unconditionally sent up the shared tree toward the RP and passed down the tree toward the receivers on
each branch of the tree.
Bidir-PIM is designed to be used for many-to-many applications within individual PIM domains.
Multicast groups in bidirectional mode can scale to an arbitrary number of sources without incurring
overhead due to the number of sources.
Bidir-PIM is derived from the mechanisms of PIM-SM and shares many shortest-path tree (SPT)
operations. Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the
shared tree, but no registering process for sources as in PIM-SM. These modifications are necessary and
sufficient to allow forwarding of traffic in all routers solely based on the (*, G) multicast routing entries.
This feature eliminates any source-specific state and allows scaling capability to an arbitrary number of
sources. Figure 76 and Figure 77 show the difference in state created per router for a unidirectional
shared tree and source tree versus a bidirectional shared tree.
Figure76 Unidirectional Shared Tree and Source Tree
Receiver Source
RP
(*, G) (*, G)
(*, G)
(S, G)
(*, G)
Receiver
PIM source
register message
Multicast
data flow
Register
Receiver Source
Source treeShared tree
from RP
(A) (B)
RP
(*, G)
(*, G)
(S, G)
(*, G)
(S, G)
(*, G)
Receiver
33355
(*, G) (*, G)
(S, G)