About fast path

What fast path is

Fast path is an alternate routing mechanism available in Data ONTAP. Instead of using the routing table of the filer to route, this mechanism uses

The source Media Access Control (MAC) address of the incoming packet as the destination MAC address of the outgoing packet for NFS-over-UDP and all TCP traffic transmitted from the filer
The same interface for incoming and outgoing traffic

Using this mechanism provides the following advantages:

Load balancing between multiple filer interfaces on the same subnet
The load balancing is achieved by sending responses on the same interface of the filer as incoming requests.
Increasing filer performance
The increase in filer performance is achieved by skipping routing table lookups.

Fast path is enabled automatically on the filer; however, you can disable it.

NFS-over-UDP

The NFS-over-UDP traffic uses fast path only when sending a reply to a request. The reply packet is sent out on the same interface that the request packet came in on. For example, a filer named toaster uses the toaster-e1 interface to send reply packets in response to NFS-over-UDP requests received on the toaster-e1 interface.

TCP

Because TCP is connection-oriented and because data is acknowledged as part of the TCP protocol, Data ONTAP can use fast path on every TCP packet transmitted except the very first SYN packet (if Data ONTAP initiates a connection). For fast path, the interface used to transmit a packet is the same interface the last packet was received on.

For TCP connections, Data ONTAP automatically turns off fast path if it detects that using fast path in a network setup is not optimal.

Effect of fast path: Telnet works, but ping fails

If fast path is enabled and the default router stops working, you might notice that Telnet sessions to a filer can still be established from a non-local subnet, even though you cannot use the ping utility to communicate with the filer's interfaces. This happens because the fast path mechanism skips routing table lookups. As a result, the routing table is not used to respond to any NFS-over-UDP or TCP connection requests. Therefore, Telnet requests (which use the TCP protocol) succeed, while ping requests--which use the ICMP (Internet Control Message Protocol) protocol--fail.

Effect of fast path on asymmetric routing

If fast path is enabled on a filer in an asymmetric network, the destination MAC address of the response packet will be that of the router that forwarded the incoming packet. However, in asymmetric networks the router forwarding packets to the filer is not the one forwarding the packets that the filer sends back. In this case, you must disable fast path.