Ssm Operations - Cisco Catalyst 3850 series Configuration Manual

SSM Operations

response to the issues associated with ISM and is intended to coexist in the network with the protocols
developed for ISM. In general, SSM provides IP multicast service for applications that utilize SSM.
ISM service is described in RFC 1112. This service consists of the delivery of IP datagrams from any source
to a group of receivers called the multicast host group. The datagram traffic for the multicast host group
consists of datagrams with an arbitrary IP unicast source address S and the multicast group address G as the
IP destination address. Systems will receive this traffic by becoming members of the host group. Membership
in a host group simply requires signaling the host group through IGMP Version 1, 2, or 3.
In SSM, delivery of datagrams is based on (S, G) channels. Traffic for one (S, G) channel consists of datagrams
with an IP unicast source address S and the multicast group address G as the IP destination address. Systems
will receive this traffic by becoming members of the (S, G) channel. In both SSM and ISM, no signaling is
required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels
to receive or not receive traffic from specific sources. In other words, receivers can receive traffic only from
(S, G) channels to which they are subscribed, whereas in ISM, receivers need not know the IP addresses of
sources from which they receive their traffic. The proposed standard approach for channel subscription
signaling utilizes IGMP INCLUDE mode membership reports, which are supported only in IGMP Version
3.
SSM can coexist with the ISM service by applying the SSM delivery model to a configured subset of the IP
multicast group address range. The Internet Assigned Numbers Authority (IANA) has reserved the address
range from 232.0.0.0 through 232.255.255.255 for SSM applications and protocols. The software allows SSM
configuration for an arbitrary subset of the IP multicast address range from 224.0.0.0 through 239.255.255.255.
When an SSM range is defined, an existing IP multicast receiver application will not receive any traffic when
it tries to use addresses in the SSM range unless the application is modified to use explicit (S, G) channel
subscription or is SSM-enabled through a URL Rendezvous Directory (URD).
SSM Operations
An established network in which IP multicast service is based on PIM-SM can support SSM services. SSM
can also be deployed alone in a network without the full range of protocols that are required for interdomain
PIM-SM. That is, SSM does not require an RP, so there is no need for an RP mechanism such as Auto-RP,
MSDP, or bootstrap router (BSR).
If SSM is deployed in a network that is already configured for PIM-SM, then only the last-hop routers must
be upgraded to a software image that supports SSM. Routers that are not directly connected to receivers do
not have to upgrade to a software image that supports SSM. In general, these non-last-hop routers must only
run PIM-SM in the SSM range. They may need additional access control configuration to suppress MSDP
signaling, registering, or PIM-SM shared-tree operations from occurring within the SSM range.
The SSM mode of operation is enabled by configuring the SSM range using the ip pim ssm global configuration
command. This configuration has the following effects:
• For groups within the SSM range, (S, G) channel subscriptions are accepted through IGMPv3 INCLUDE
• PIM operations within the SSM range of addresses change to PIM-SSM, a mode derived from PIM-SM.
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mode membership reports.
In this mode, only PIM (S, G) Join and Prune messages are generated by the router. Incoming messages
related to rendezvous point tree (RPT) operations are ignored or rejected, and incoming PIM register
messages are immediately answered with Register-Stop messages. PIM-SSM is backward-compatible
with PIM-SM unless a router is a last-hop router. Therefore, routers that are not last-hop routers can run
PIM-SM for SSM groups (for example, if they do not yet support SSM).
Configuring SSM
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