[c-nsp] Best practice for CAM and ARP aging timers

[george at dalyshome.co.uk]

 I'm trying to establish consensus on best practice CAM and ARP aging
timers for Cat6500 12.2(33)SXI5.
 Various cisco docs state these should be synched to minimise unknown
unicast flooding.  I'm looking into modifying them from the default
values (ARP timer 14400 sec, MAC aging time 300 sec) to minimise
excessive unknown unicast flooding which I'm seeing in the L2 network
(~400 downstream 3560 switches with ~8000 downstream hosts) which
aggregates at these 6500s.  This topology does not have asymmetric
traffic flows - have others observed unicast flooding in topologies
without asymmetric traffic flows but with mismatched ARP/CAM timers?
 I don't see the same problem with excessive unknown unicast flooding
in a similar network which aggregates at a pair of Nexus 7000.  Cisco
modified the default values for ARP and MAC aging in NX-OS to 1500
sec and 1800 sec respectively.  Downstream access switches are still
using the IOS defaults, I've not seen a negative impact from the
mismatched MAC timers but would be keen to hear if others have
experienced issues with mismatched MAC timers between collapsed cores
and access switches.
 The quickest option I can see would be to reduce the ARP timer to
300sec as I would only need to touch the SVIs on the 6500s but I'm
concerned that 300sec feels too low for an ARP timer in a relatively
large L2 domain and could have CPU impact.  Alternatively I could
increase the MAC aging timer to 14400sec to synch with the ARP timer,
or choose some new value altogether (perhaps the NX-OS defaults?).  If
I did change the MAC aging timer is it strictly necessary to roll that
configuration out throughout the L2 domain?  Is there any particular
risk due to temporary CAM timer mismatch during the transition period
while the configuration is being rolled out?
 Any comments/advice much appreciated!
 Thanks,
 George