Guys,
Please note that this is a very interesting question. Now the point of decreasing timers for IGPs is to ensure convergnce is faster. Where this may work well in some scenarios, is may cause what we call "postive routing feedback" and this has to be taken into account. I currently do not have the awnser to your questions, just things to watch out for.
The way the IGP timers are chosen are to stop route flapping causing postive feedback, and things such as bandwdith link util, cpu etc etc may come into play when changing timers.
There are certain other techniques for faster convergence in the market place today, and where these timers for IGPs seem OK for a data only network, I am not sure they are OK for a fully converged network.
If you consider that EIGRP is 5 and 15 and ospf is 10 and 40, and then you factor in such protocols as TCP, pls refer to post
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dde9768
you will see that with some timers in some networks for TCP, this may break you users TCP sessions.
Some of the new technologies for convergnce include :
SSO (Stateful switchover) with NSF (Non-Stop Forwarding) in redundant platform linecards separating the control-plane from the data-plane in distributed platform architectures.
Graceful Restarts of routing protocol adjacencies, a method for not always reporting network failures should the failure be short lived and recovering the adjacencies routing information without having to reset the neighbor as traditional routing protocols do.
Fast Hellos. This is where you simply cut down the timers between adjacencies in the network to milliseconds, rather than seconds, but this must be used cautiously.
MARP (Multiaccess Reachability Protocol). This is a useful technique for allowing layer 3 routing protocols to use Layer 2 switch MAC address to signal of a routing adjacency is reachable.
Link State Expediential Backoff. This is where certain routing protocols can delay sending of updates if they see that a link in the network is flapping.
IP Event Dampening. This is very similar to the previous bullet, with the difference that it will stop advertising network reachability information for periods of calculated time depending on how often the link flaps.
EIGRP Feasible Successor tweaking. If a design has two paths to a network, and the reported distance is equal to the reported distance, convergence has to take place. Using metrics to make feasible successors is an option.
BFD (Bidirectional Forwarding Detection). This is a routing protocol independent hello mechanism for detecting link failure in the sub-millisecond range, which can enhance the routing protocol convergence significantly.
Please let me know if this makes sense or helps.
Regards,
Ken
