Fast path is an alternative routing mechanism to the routing table. In fast path, the responses to incoming network traffic are sent back by using the same interface as the incoming traffic. By avoiding the routing table lookup, fast path provides a quick access to data.
If fast path is enabled on an interface group and a physical interface in that group receives an incoming request, the same physical interface might not send a response to the request. Instead, any other physical interface in an interface group can send the response.
NFS/UDP traffic uses fast path only when sending a reply to a request. The reply packet is sent out on the same network interface that received the request packet.
For example, a storage system named toaster uses the toaster-e1 interface to send reply packets in response to NFS/UDP requests received on the toaster-e1 interface.
Fast path is used only in NFS/UDP. However, fast path is not used in other UDP-based NFS services such as portmapper, mountd, and nlm.
In a TCP connection, fast path is disabled on the third retransmission and the consecutive retransmissions of the same data packet. If Data ONTAP initiates a connection, Data ONTAP can use fast path on every TCP packet transmitted, except the first SYN packet. The network interface that is used to transmit a packet is the same interface that received the last packet.
In a symmetric network, the destination MAC address of the response packet is that of the router that forwarded the incoming packet. However, in asymmetric networks, the router that forwards packets to your storage system is not the router that forwards packets sent by the storage system. Therefore, in asymmetric networks, you must disable fast path.