Which three statements are true about Source Specific Multicast? (Choose three.)
A.
Is best suited for applications that are in the one-to-many category.
B.
SSM uses shortest path trees only.
C.
The use of SSM is recommended when there are many sources and it is desirable to keep the amount of mroute state in the routers in the network to a minimum
D.
There are no RPs to worry about
Explanation:
The Source Specific Multicast feature is an extension of IP multicast where datagram traffic is forwarded to receivers from only those multicast sources to which the receivers have explicitly joined. For multicast groups configured for SSM, only source-specific multicast distribution trees (no shared trees) are created.The current IP multicast infrastructure in the Internet and many enterprise intranets is based on the PIM-SM protocol and Multicast Source Discovery Protocol (MSDP). These protocols have proven to be reliable, extensive, and efficient. However, they are bound to the complexity and functionality limitations of the Internet Standard Multicast (ISM) service model. For example, with ISM, the network must maintain knowledge about which hosts in the network are actively sending multicast traffic. With SSM, this information is provided by receivers through the source address(es) relayed to the last hop routers by IGMP v3lite or URD. SSM is an incremental 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 a more advantageous 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 to a host group simply requires signalling 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 signalling 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 that they are subscribed to, 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 signalling utilizes IGMP INCLUDE mode membership reports, which are only supported in Version 3 of IGMP.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 232.0.0.0 through 232.255.255.255 for SSM applications and protocols. Cisco IOS software allows SSM configuration for an arbitrary subset of the IP multicast address range 224.0.0.0 through 239.255.255.255. When an SSM range is defined, existing IP multicast receiver applications will not receive any traffic when they try to use addresses in the SSM range (unless the application is modified to use explicit (S, G) channel subscription or is SSM enabled through URD).
Surely this isn’t correct as SSM is suited to the many sources model not the one source model. In a single source model only a single mroute state exists in all the hops while in a many source model only maintaining mroute states for specific sources IS one of the reasons to solve scalability (and security and reliability).
I advise you to not answer as above but B, C and D.
Thanks.
In this question the simplest solution is to find the wrong answer (the rest three is correct).
Answer C is surely wrong, because it applies to Bidir-PIM which is the opposite of SSM.
So – ABD.
I’m not so sure that A,B & D aren’t the correct choices. This question initially stumped
me too. But RFC 4607 says;
“SSM is particularly well-suited to dissemination-style applications
with one or more senders whose identities are known before the
application begins. For instance, a data dissemination application
that desires to provide a secondary data source in case the primary
source fails over might implement this by using one channel for each
source and advertising both of them to receivers.”
And RFC 3569;
“Source-Specific Multicast (SSM): This is the multicast service
model defined in [5]. An IP datagram is transmitted by a source S
to an SSM destination address G, and receivers can receive this
datagram by subscribing to channel (S,G). SSM provides host
applications with a “channel” abstraction, in which each channel
has exactly one source and any number of receivers.”
There is no control of sources in ASM (Any source multicast)/ISM (Internet standard
multicast/RFC 1112 multicast. But in RFC 3569, it addresses this short-coming with the
following statement:
” Access Control: SSM lends itself to an elegant solution to the
access control problem. When a receiver subscribes to an (S,G)
channel, it receives data sent only by the source S. In
contrast, any host can transmit to an ASM host group. At the
same time, when a sender picks a channel (S,G) to transmit on,
it is automatically ensured that no other sender will be
transmitting on the same channel (except in the case of
malicious acts such as address spoofing). This makes it much
harder to “spam” an SSM channel than an ASM multicast group.”
So it seems that indeed, SSM is designed for 1-to-many deployments. Thus A,B & D seem
correct to me.
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfssm.pdf
“SSM is a datagram delivery model that best supports one-to-many applications, also known as broadcast” – Cisco
so, ABD