Following the Cisco IOS 12.4(24)T release, the next release of Cisco IOS Software was 15.0.

IOS 15.0 provides several enhancements to the operating system including:

As shown in the figure, Cisco IOS 15.0 uses a different release model from the traditional separate mainline and T trains of 12.4. Instead of diverging into separate trains, Cisco IOS Software 15 mainline and T will have extended maintenance release (EM release) and standard maintenance release (T release). With the new IOS release model, Cisco IOS 15 mainline releases are referred to as M trains.

Beginning with 15.0, new releases in the form of a T train are available approximately two to three times per year. EM releases are available approximately every 16 to 20 months. T releases enable faster Cisco feature delivery before the next EM release becomes available.

An EM release incorporates the features and hardware support of all the previous T releases. This makes newer EM releases available that contain the full functionality of the train at the time of release.

In summary, the benefits of the new Cisco IOS release model include: