[c-nsp] max-reserved-bandwidth question

[James Brown]

All,

I realise there are more posts out there about max-reserved-bandwidth 
than you can shake a stick at, but could anyone help me with this please...?

Does the 25% only kick in during times of congestion, and which traffic 
does it really contain? I'm using 12.4 Mainline and if several sources 
are to be believed I have reason to think that 25% of the bandwidth is 
hived off all the time, never to be seen again unless you set 
max-reserved-bandwidth to 100%. Can that really be true? I would have 
expected the 25% 'hidden' queue to only be serviced when there are heaps 
of routing updates waiting to use a congested interface.

What I'm trying to achieve is to set all DSCP values to zero, but not 
starve routing updates in the process. In which case, how well would 
this policy work?

!
class-map match-all CM-ALL
  match any
!
service-policy PM-BLEACH
  class CM-ALL
   set ip dscp default
!
inter serial1/0
  max-reserved-bandwidth 100
  service-policy out PM-BLEACH
!

Have I just un-prioritised routing updates by removing the 25% 'hidden' 
queue? Do I need to then create a "class-default" within PM-BLEACH and 
allocate bandwidth 25?

Any advice on this would be really appreciated and help answer a 
question I just can't seem to find and answer to online.

Many thanks

James.