Edge-Core AS5700-54X, Справочное руководство CLI, Страница: 1041 / 1116

Chapter 30
| Multicast Routing Commands
PIM Multicast Routing
– 1041 –
Command Mode
Global Configuration
Command Usage
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For multicast group addresses that fall within fall within the default SSM range
of 232/8 or within a range set by this command, source-specific multicast
service mode is used. For all other multicast addresses, any-source multicast
service mode is used.
◆
SSM requires the client to specify the multicast source address in registration
messages. Only IGMPv3 currently supports the ability to designate a specific
source in join messages sent to the last hop router in the multicast delivery
tree. Therefore, IP multicast receiver applications will not receive any traffic if
they try to use addresses in the SSM range, unless the host operating system
and multicast application both support IGMPv3 explicit (S,G) channel
subscription.
◆
Both Any-source Multicast (ASM) and SSM can be used on the same network
interface because the multicast services address range used for SSM is
restricted by default to 232/8 (that is 232.0.0.0 to 232.255.255.255). Therefore,
any service requests from IGMPv1 or IGMPv2 clients within this address range
will be denied. To use SSM in a network, the edge routers should therefore be
made SSM-capable by enabling IGMPv3. The only requirement for core routers
is that they are capable of forwarding IGMPv3 messages. However, when PIM-
SM is used by either edge or core routers, the Rendezvous Point (RP) must not
be configured to accept any registration messages for addresses within the
configured SSM address range.
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SSM provides the following advantages over ASM:
■
SSM is suitable to dissemination-style applications with well-known
senders. It defines service delivery channels on a per-source basis (for
example, (S1,G)), and thereby avoids the problem of having to globally
allocate multicast service addresses across the Internet.
■
Because SSM designates a specific source/group channel for each service
request, it eliminates the problem of simultaneous delivery from multiple
sources which can easily occur in the ASM delivery model. This makes it
much more difficult to spam an SSM channel.
■
In contrast to PIM-SM, PIM-SSM uses only source-based forwarding. This
thereby eliminates the need to collect and distribute information about
local multicast devices using a bootstrap router, select a rendezvous point
for coordinating service requests, and then build a shared tree for
distributing multicast traffic to local clients.
◆
SSM has the following limitation — Because no mechanism in PIM-SSM notifies
a receiver when a source is active, the network must maintain the (S,G) state as
long as receivers are requesting receipt of that channel. Hence, as long as
receivers send (S,G) subscriptions, the shortest path tree (SPT) state from the