With IOS 15.1(3)T for ISR G2 comes quite a change when in comes how communication between SM, ISM and SM-ES modules occurs. Without Multi Gigabit Fabric (MGF) all communication between devices within a router involves the CPU. With MGF modules can communicate with each other without involving the router CPU.
This is how communication looks like in pre-15.1(3)T IOS version.
High-Speed Intrachassis Module Interconnect (HIMI) connections or VLAN connections could be used to connect different modules via the MGF. With new software MGF acts like a switch and traffic flow is slightly different. CPU is not used anymore and can be used for other purposes. All traffic between supported modules comes through MGF only.
HIMI connections are not supported on the MGF interfaces anymore with new software unless legacy modules are installed in chassis.
The HIMI interconnection to MGF in earlier releases were configured using special Layer 3 virtual interface. With new release this interface is Layer 2 only and should be configured as trunk. Appropriate IP addressing have to be configured as SVI’s on modules. Therefor upgrading might require reconfiguration.
As mentioned before MGF usage is limited only to SM, ISM and SM-ES modules. There are also few limitations mentioned in documentation, but two are worth mentioning – Online Insertion and Removal (OIR) is not supported with the MGF (router have to be rebooted) and using legacy VLAN management (vlan database) is the same as having a legacy switch module in chassis. Many users still prefer to use vlan database unfortunately.
Note: All images taken from Cisco documentation