A lot of the regional networks have more than one attachment point to ANSnet. Some use one as primary and one as backup. Most of the regional networks announce some of their networks as primary at one attachment point and other networks at another attachment point. This means that that regional could not aggregate before reaching ANSnet because to do so would mean losing the load balancing. One longer term way around this is to have ANS accept more specific routes plus an aggregate and propogate the more specific routes into it's IBGP but not propogate them further (just the aggregate).
Is this really a common case? BARRNet has two attachments to ANS/NSFNET, but they are strictly primary/secondary, so the same aggregates will be advertised at both locations.
PREPnet is planning to put such a scheme into place in the near future.
Any network provider which is using multiple ANS/NSFNET connections to split load should have an addressing plan that assigns a different aggregate to each exit point. The basic rule is: for each routing policy you have, you should have different CIDR block. I thought we'd been over this in at least the Regional Techs forum before.
Unfortunately, PREPnet already has a CIDR block half allocated, and it doesn't match the policy needed to do traffic splitting into ANSnet. It would be wise to do this, but only when you can foresee what the policies will be in advance. If you can't guess what your policy will be in the future... I'm sure Noel would have inspiring comments to make about re-addressing at this point. --Jamshid