ZXDSL 9210 (V3.1) Broadband Universal Access Equipment Technical Manual
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hierarchical DVMRP is proposed, which divides multicast network into areas;
the multicast in each area can be executed according to any protocol; trans-area
multicast can be executed by a border router via the DVMRP. Routing overhead
can be greatly reduced.
The PIM is designed by the Inter-Domain Multicast Route (IDMR) workgroup.
As defined by its name, the PIM is independent of any specific unicast routing
protocol; it uses the unicast routing table established by any unicast routing
protocol to check the RPF instead of maintaining an independent multicast
routing table for multicast forwarding. As the PIM does not need to receive or
send any multicast route updating information, it requires much less overhead in
comparison with other multicast protocols. The design idea of the PIM is to
support both the SPT and the shared tree within Internet and to enable flexible
conversion between them. This integrates the advantages of both and improves
multicast efficiency. PIM defines 2 modes: Dense-Mode and Sparse-Mode.
2) Regarding the sparse protocol mode, the core-based trees multicast routing
protocol is adopted as an example.
The Core-Based Trees (CBT) multicast routing protocol is to reduce the router
multicast status in the network so as to provide multicast scalability. To this end,
the CBT is designed as the sparse mode (similar to PIM-SM). The CBT adopts a
bi-directional shared tree, which is rooted on a core router and permits
bi-directional flows of multicast information. This is different from the PIM-SM
(The shared tree in the PIM-SM is uni-directional; it adopts the SPT between the
RP and the multicast source to forward multicast data to the RP). Hence, the
CBT cannot adopt the RPF in checking but uses the destination group address of
the IP packet header as the check and forwarding buffer. This requires that the
CBT shared tree must be carefully maintained to prevent any multicast route
cycling.
3) Regarding the link status protocol, the multicast open shortest path first protocol
is adopted as an example.
Multicast Open Shortest Path First (MOSPF) is a routing protocol based on link
status and is the extended unicast OSPF protocol. Similar to the OSPF, the
MOSPF defines three levels of routes:
