WHY SSM

Benefis of PIM SSM over PIM SM
------------------------------

- SSM consumes minimum link bandwidth
- SSM simplifies multicast address management (specially important for
  inter-domain multicast)
  - SSM (S,G) channels easily provide unique per-application addressing
  - SSM does not require MSDP between PIM domains
- SSM does not suffer instabilities from traffic-driven SPT switchover
- SSM is not suscetible to DoS attack from unwanted sources
- SSM does not use RP. Some RP issues:
  - RP is possible point of failure
  - RP demands redundancy management
  - RP may require PIM dense mode support for RP election
  - RP is possible performance bottleneck
  - RP may demand lots of extra management
- SSM can be deployed in an existing PIM SM network (only the last hop
  routers need to support IGMPv3)
- SSM is easier to deploy and maintain

PIM-SSM drawbacks
-----------------

- SSM requires IGMPv3 support on both receivers and last-hop routers
- SSM may be memory intensive when managing (S,G) states for
  many-to-many multicast distribution
- SSM will keep (S,G) state as long as there are subscriptions from
  receivers, even if the source is not actually sending traffic

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