Fabric Interconnect Operation Modes

Fabric Interconnect Modes of Operation

There is no local switching on IOM/FEX ever, FI switches veth's traffic which are locally present on it and utilizes an uplink switch to switch traffic to the veths defined on the corresponding FI for inter-host (within chassis) communication.

End Host Mode (EHM): - 

1. Default mode for FI. FI doesn't learn any MACs from upstream ports. Only learns MACs southbound from connected blades/racks (server ports). In this mode the switch is exposed to upstream switches as a host and not as a switch. 
2. RPF check. It ensures that traffic ingress from northbound switch to FI is coming to the correct FI uplink interface to which the downstream blade vifs are bound to. else, it's dropped. This ensures again that there are no duplicate packets sent back to the blade's vifs from the remote host.
3. Deja vu check. Looped packet back from another adjacent port/s within the same FI ? If yes, this will be dropped at the FI itself. For instance, a broadcast packet from the blade might loop back to another uplink port into the chassis again and can result in creating an unwanted broadcast storm. Since the FI has track of the packet being seen on a different port earlier, it will promptly drop it.
4. No STP and hence no blocked ports. All ports can forward traffic.
5. Designated Broadcast/Multicast up-link port. for receiving traffic (G pin port).