This was a post from Petr Lapukhov from INE on a question about the 2600’s and the 12.4T code being used for the 4.0 hardware revision.

You may give a try to the good old IOS 12.3(26), TELCO or ENTERPRISE PLUS feature set. While this is much behind 12.4T, many core features remain the same. Combining 2600XMs running 12.3(26) with 2600XMs running 12.4T (e.g. in3+3 proportion) you may obtain a mix of features enough to practice almost any relevant technology. Remember though, that 12.4T images are hungry for memory(both flash and DRAM), but you may save on the boxes running 12.3(26). Back in days I was doing all sort of crazy things to keep my rack price within the tight budget 🙂

Petr Lapukhov,
CCIE #16379 (R&S/Security/SP/Voice)

Internetwork Expert, Inc.
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We should start to see more configuration options in the coming weeks I would think from all the vendors.