Qtech QSW-3900-48-Т-DC Руководство пользователя онлайн [173/313] 481615

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QTECH Software Configuration Manual
12-172
multicast routing algorithms and creates multicast routing state entries on
multicast routers, which forward multicast traffic as per the routing state entries.
Based on the inter-domain multicast routing policy configured in the network,
an inter-domain multicast routing protocol propagates multicast source
information and exchanges multicast routing information among autonomous
systems (ASs), thus ensuing multicast forwarding among different domains.
12.2.5.1 Intra-Domain Multicast Routing Protocols
Among a variety of intra-domain multicast routing protocols, Protocol Independent Multicast (PIM) is a popular one.
Based on the forwarding mechanism, PIM falls into two modes dense mode (referred to as PIM-DM) and sparse
mode (referred to as PIM-SM).
(1) PIM-DM
In a PIM-DM domain, routers use periodical PIM Hello messages for PIM neighbor discovery, determination
of leaf routers and leaf networks, and designated router (DR) election on a multi-access network. Although PIM-DM
does not require a DR, one must be elected among multiple routers on a multi-access network running IGMPv1 in a
PIM-DM domain to act as the IGMPv1 querier on that multi-access network.
As a dense mode multicast routing protocol, PIM-DM uses the “push” mode for multicast forwarding, and is
suitable for small-sized networks with densely distributed multicast members. PIM-DM works as follows:
PIM-DM assumes that at least one multicast group member exists on each
subnet of a network, and therefore multicast data is flooded to all nodes on the
network. Then, branches without receivers downstream are pruned from the
forwarding tree, leaving only those branches with receivers. This “flood and
prune” process takes place periodically, that is, pruned branches resume
multicast forwarding when the pruned state times out and then data is
re-flooded down these branches, and then are pruned again.
When a host attached to a pruned node joins the multicast group, the node sends
a graft message toward the upstream node. Then the node resumes multicast
traffic forwarding.
(2) PIM-SM
In a PIM-SM domain, routers periodically sends PIM Hello messages for PIM neighbor discovery and DR
election on a multi-access network, where the DR sends join/prune messages toward the root of the multicast
forwarding tree for the receiver host attached to it, or forwards multicast traffic from the directly connected multicast
source onto the multicast distribution tree.
As a sparse mode multicast routing protocol, PIM-SM uses the pull” mode for multicast forwarding, and is
suitable for large- and medium-sized networks with sparsely and widely distributed multicast members. The basic
implementation of PIM-SM is as follows:
PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM
mode, and delivers multicast data only to those hosts that have explicitly
requested for the data. The core task for PIM-SM in multicast forwarding is to
build and maintain rendezvous point trees (RPTs). An RPT is rooted at a router
in the PIM domain as the common node, referred to as the rendezvous point
(RP), through which the multicast data travels down the RPT to the receivers.
When a receiver is interested in the multicast data addressed to a specific
multicast group, the router connected to this receiver sends a join message to
the RP for that multicast group. The path along which the message goes hop by
hop to the RP forms a branch of the RPT.

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