On multiple occasions some members of IETF expressed concern about the migration path from classful protocols to classless protocols such as BGP-4.
BGP-4 was rushed into production use on the Internet because of the exponential growth of routing tables and the increase of memory and CPU utilization required by BGP. As such, migration issues that normally would have stalled deployment were cast aside in favor of pragmatic and intelligent deployment of BGP-4 by network operators.
There was much discussion about creating "route exploders" which would enumerate individual class-based networks of CIDR allocations to BGP-3 speaking routers, however a cursory examination showed that this would vastly hasten the requirement for more CPU and memory resources for these older implementations. There would be no way internal to BGP to differentiate between known used networks and the unused portions of the CIDR allocation.
The migration path chosen by the majority of the operators was known as "CIDR, default, or die!" To test BGP-4 operation, a virtual "shadow" Internet was created by linking Alternet, Ebone, ICM, and cisco over GRE based tunnels. Experimentation was done with actual live routing information by establishing BGP version 3 connections with the production networks at those sites. This allowed extensive regression testing before deploying BGP-4 on production equipment.
After testing on the shadow network, BGP-4 implementations were deployed on the production equipment at those sites. BGP-4 capable routers negotiated BGP-4 connections and interoperated with other sites by speaking BGP-3. Several test aggregate routes were injected into this network in addition to class-based networks for compatibility with BGP-3 speakers.
At this point, the shadow-Internet was re-chartered as an "operational experience" network. tunnel connections were established with most major transit service operators so that operators could gain some understanding of how the introduction of aggregate networks would affect routing.
After being satisfied with the initial deployment of BGP-4, a number of sites chose to withdraw their class-based advertisements and rely only on their CIDR aggregate advertisements. This provided motivation for transit providers who had not migrated to either do so, accept a default route, or lose connectivity to several popular destinations.