Switching from RPT to SPT
Once receiver-side DR receives the multicast data from RP to multicast group G, the switching
process from RPT to SPT will be triggered:
(1)
The receiver-side DR sends (S, G) join message to the multicast source S hop-by-hop, and
the join message finally arrives at the source-side DR. All routers the join message passes
will generate the (S, G) entry in their forwarding tables, thus building up a branch of SPT;
(2)
The receiver-side DR sends prune message toward the RP hop-by-hop. The RP will
forward the received prune message toward the multicast source. The switching process
from RPT to SPT is then accomplished.
After the switching from RPT to SPT, the multicast data will be sent from multicast source to
the receivers directly. Through this switching process from RPT to SPT, PIM SM constructs the
SPT in a more economical way than PIM DM does.
Asserting
The assert mechanism of PIM SM and PIM DM is the same. For more details, refer to
BSR Administrative Domain
BSR is the administrative core in the PIM SM domain. The BSR is exclusive in one PIM SM
domain and it advertises the RP-Set information in the whole PIM SM domain. All the multicast
group information is forwarded inside the BSR’s administrative network scope. When the PIM
SM domain is relatively large, you can consider dividing the PIM SM domain into multiple BSR
administrative domains, thus sharing the administrative pressure of single BSR and providing
specialized services for specific multicast groups.
In geographical space, the BSR administrative domains are separated with each other and one
router cannot belong to more than one BSR domain. In other words, the routers contained by
the BSR domains are different from each other.
In multicast address, each BSR administrative domain provides services for specific multicast
groups. These multicast group addresses usually have no intersection with each other, but
they may also have crossings and overlaps, as shown in Figure 11-19.
Figure 11-17 BSR Domain Divided by Multicast Address
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