[c-nsp] Multicast group filtering

[ML]

On a typical day my network can have ~500Mbps of multicast traffic
flowing across a GigE cross country long haul circuit.  I wanted some
redundancy and I am only able to afford another 100M circuit for backup.

When our primary circuit goes down I can afford to live without some of
the multicast groups I normally carry.  My Google-fu has turned up:
"ip multicast boundary" and a standard ACL to deny certain groups from
crossing a specific interface.

What effect will this have on the CPU for routers that can't build SPF
trees for the groups I deny?  I've seen my router CPUs spike to 99% when
the RP is unreachable.