Qtech QSW-3900-24-Т-DC [198/313] Brief introduction of pim

Qtech QSW-3900-24-Т-AC [198/313] Brief introduction of pim
QTECH Software Configuration Manual
12-197
Delete SSM-Mapping static group address mapping rule
no ssm-mapping static { access-control-list source-address | all }
By default, there is no SSM-Mapping static group address mapping rule.
Caution: SSM multicast group address range is specified by configuring ssm multicast
group range in PIM mode.
12.9 Brief introduction of PIM
PIM-DM (Protocol Independent Multicast-Dense Mode) is intensive multicast route protocol. PIM-DM suits small
scaled network and multicast members are intensive.
12.9.1 Working theory of PIM-DM
The working process of PIM-DM are: Neighbor Discovering, DM forwarding to pruning and grafting.
12.9.1.1 Neighbor Discovering
When enabling PIM-DM router, it needs Hello packet to be neighbor discovering. Each network node
running PIM-DM uses Hello packet to keep connection. PIM-DM sends Hello packet periodically.
12.9.1.2 Forwarding &Pruning
PIM-DM supposes all hosts in network are ready to receive multicast data. When some multicast source S
sends data to multicast group G, router will be RPF examining after receiving multicast packet. If the examination is
passed, router will create a (S,G) item and forward data to all downlink PIM-DM nodes. If the examination fails, that
is, multicast packet inputted from error interface, the packet is dropped. After this process, a (S,G) item will be
created in PIM-DM multicast area.
If there is no multicast member in downlink node, Prune packet will be sent to uplink node which informs
the uplink not to transmit data to downlink node. After receiving Prune packet, uplink node will delete corresponded
interface from the outputting interface list of its multicast transmission item (S,G) to set up a SPT (Short Path Tree)
with the source S being the root. Prune is originated by leave router.
This process is called Forwarding and Pruning. Each pruned node provides overtime mechanism. When
pruning is overtime, each router restarts Forwarding and Pruning. The process of PIM-DM Forwarding and Pruning
is periodically running.
In this process, PIM-DM adopts RPF examination to establish a multicast transmission tree originated with
data source by using current unicast route table. When a multicast packet arrives, router will judge the correctness of
arrival path. If arrival interface is the one to multicast source demonstrated by unicast route, the multicast packet is
from the correct path; or, this multicast packet will be dropped as redundant packet without transmission. The unicast
route information as path judging can be from any unicast route protocol, such as RIP or route information found by
OSPF, but not dependent on specified unicast route protocol.
12.9.1.3 Assert mechanism
As following picture, if there are two multicast routers A and B in a LAN network interface and they have
their own receiving paths to multicast source S, they will transmit this multicast packet to LAN after receiving
multicast packet from multicast source S. multicast router C in downlink node will receive two same multicast
packet.
Multicast packet router from uplink node detect this situation, it needs Assertmechanism to select a unique
transferrer. By sending Assert packet, select a best path. If the priority and metric of the two or more paths are the
same, the one with larger IP address will be the uplink neighbor of (S, G) item, which is responsible for the
transmission of (S, G) multicast packet.

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