Andrew
In my early days of working with Cisco equipment I tended to believe that the default behavior of enabling auto-summary was a good thing and was the norm. (and in those days most of the networks that I dealt with were contiguous) As I gained more experience I began to see more and more networks that were not contiguous and for which the default of auto-summary poses a problem. Increasingly I regard no auto-summary as the default posture.
I agree with Sundar that if we knew more about your network and its topology and its addressing scheme then we could give you better advice. So tell us a bit more about your network environment and its addressing plan.
Sundar discusses the issue as the possibility of sub-optimal routing. I will go a step farther and say that if there are parts of your network that are discontiguous and that have auto-summary enabled, then you have parts of your network that are broken and that have lost connectivity to some other parts of your network.
The problem will be made worse if you have the other old default enabled of no ip classless. In some networks with discontiguous networks, configured with ip classless and a good default route they can overcome the problem of discontiguous networks.
HTH
Rick
HTH
Rick
