Every IOS version has quirks. For 15.4(1)T in particular:
This is a community-added tag. It usually implies that the image has been patched or optimized to bypass certain license checks or bugs specifically for use in GNS3 or other simulators. Why Use This Image? i86bi-linux-adventerprisek9-ms.154-1.t-antigns3.bin
Deep Dive into i86bi-linux-adventerprisek9-ms.154-1.t-antigns3.bin for GNS3 Labs Every IOS version has quirks
The naming structure breaks down as follows: i86bi-linux-adventerprisek9-ms.154-1.t-antigns3.bin
: This refers to the IOS version. The "T" train is the technology release, often containing newer features compared to the standard mainline releases.