Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring Bidirectional PIM
Bidir-PIM Overview
IPC-473
Cisco IOS IP Configuration Guide
Figure 77 Bidirectional Shared Tree
When packets are forwarded downstream from the RP toward receivers, there are no fundamental
differences between bidir-PIM and PIM-SM. Bidir-PIM deviates substantially from PIM-SM when
passing traffic from sources upstream toward the RP.
PIM-SM cannot forward traffic in the upstream direction of a tree, because it only accepts traffic from
one Reverse Path Forwarding (RPF) interface. This interface (for the shared tree) points toward the RP,
therefore allowing only downstream traffic flow. In this case, upstream traffic is first encapsulated into
unicast register messages, which are passed from the designated router (DR) of the source toward the
RP. In a second step, the RP joins an SPT that is rooted at the source. Therefore, in PIM-SM, traffic from
sources traveling toward the RP does not flow upstream in the shared tree, but downstream along the SPT
of the source until it reaches the RP. From the RP, traffic flows along the shared tree toward all receivers.
In bidir-PIM, the packet forwarding rules have been improved over PIM-SM, allowing traffic to be
passed up the shared tree toward the RP. To avoid multicast packet looping, bidir-PIM introduces a new
mechanism called designated forwarder (DF) election, which establishes a loop-free SPT rooted at the
RP.
DF Election
On every network segment and point-to-point link, all PIM routers participate in a procedure called DF
election. The procedure selects one router as the DF for every RP of bidirectional groups. This router is
responsible for forwarding multicast packets received on that network upstream to the RP.
The DF election is based on unicast routing metrics and uses the same tie-break rules employed by PIM
assert processes. The router with the most preferred unicast routing metric to the RP becomes the DF.
Use of this method ensures that only one copy of every packet will be sent to the RP, even if there are
parallel equal cost paths to the RP.
A DF is selected for every RP of bidirectional groups. As a result, multiple routers may be elected as DF
on any network segment, one for each RP. In addition, any particular router may be elected as DF on
more than one interface.
Receiver
Source
RP
(*, G)
(*, G)
(*, G)
(*, G)
Receiver
33354
(*, G)