Allied Telesis Rapier 16Fi-FX Instalační příručka stáhnout pdf (Strana 103)

IP Multicasting 9

PIM Sparse Mode

PIM Sparse Mode (PIM-SM) provides efficient communication between

members of sparsely distributed groups - the type of groups that are most

common in wide-area internetworks. It is designed on the principle that

several hosts wishing to participate in a multicast conference does not justify

flooding the entire internetwork with periodic multicast traffic. PIM-SM is

designed to limit multicast traffic so that only those routers interested in

receiving traffic for a particular group receive the traffic.

The switch supports PIM Sparse Mode as specified in Internet Draft Protocol

Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised),

1 March 2002 (draft-ietf-pim-sm-v2-new-05).Routers with directly attached or

downstream members are required to join a Sparse Mode distribution tree by

transmitting explicit join messages. If a router does not become part of the

predefined distribution tree, it does not receive multicast traffic addressed to

the group. In contrast, dense mode multicast routing protocols assume

downstream group membership and continue to forward multicast traffic on

downstream links until explicit prune messages are received. The default

forwarding action of a sparse mode multicast routing protocol is to block traffic

unless it is explicitly requested, while the default action of the dense mode

multicast routing protocols is to forward traffic.

PIM-SM employs the concept of a rendezvous point (RP) where receivers

“meet” sources. The initiator of each multicast group selects a primary RP and

a small ordered set of alternative RPs, known as the RP-list. For each multicast

group, there is only a single active RP. Each receiver wishing to join a multicast

group contacts its directly attached router, which in turn joins the multicast

distribution tree by sending an explicit join message to the group’s primary RP.

A source uses the RP to announce its presence and to find a path to members

that have joined the group. This model requires Sparse Mode routers to

maintain some state information (the RP-list) prior to the arrival of data

packets. In contrast, Dense Mode multicast routing protocols are data driven,

since they do not define any state for a multicast group until the first data

packet arrives.

Roles in PIM Sparse Mode

A multicast sender does not need to know the addresses of the members of the

group in order to send to them, and the members of the group need not know

the address of the sender. Group membership can change at any time. When

PIM is enabled on the switch, and before the switch can route multicast traffic,

it must establish which of the PIM routers in the network are performing some

key roles: designated router (DR), rendezvous point (RP), and bootstrap router

(BSR).

Designated router There must be one PIM designated router (DR) in the subnetwork to which the

IP hosts are connected. Any PIM-SM interfaces on the subnetwork elect the

designated router with the highest DR priority. If there is more than one router

with the same priority, or no priority, they choose the interface with the highest

IP address number. The DR performs all the PIM functionality for the

subnetwork. If the current DR becomes unavailable, the remaining routers elect

a new DR on the interface by DR priority or IP address.

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