How does a multimode vif/ifgrp interact with ip.fastpath?
The main difference between a load balancing "multimode VIF" (now called an ifgrp starting with Data ONTAP 8.0) and the function of the 'ip.fastpath' feature lies in their relationship to the members of the VIF/ifgrp.
- vifs/ifgrps work on Layer 2 (Data-Layer, usually Ethernet)
- ip.fastpath is enabled or disabled relative to Layer 4 (Transport Layer, TCP), but makes its' determinations relative to physical/virtual/logical interfaces (also Layer 2).
Therefore the fastpath-function will see the vif/ifgrp as a single interface. 'ip.fastpath' will not be aware of the individual physical interfaces or 1st level vifs/ifgrps that are members of the top level vif/ifgrp.
Enabling fastpath causes ONTAP to record the incoming MAC address, and the physical interface (if not a member of a vif or ifgrp) or virtual interface (configured in a vif or ifgrp) which received it. Replies to that MAC address will attempt to use the interface that is associated with the next hop MAC address, *without* consulting the IP routing table. The impact of this can be a complete change in the route a packet traverses through the user's infrastructure, as this can impact which gateway device is used.
Vif/ifgrp load balancing, however, will only determine which vif/ifgrp member (physical interface or lower level vif/ifgrp) to use when transmitting the next packet. This will only ultimately affect which physical interface on the switch finally receives that packet. The IP route (routers, gateways, etc.) will not change based on VIF load balancing decisions.