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How does IP fast path function?

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Applies to

  • Data ONTAP 7
  • Data ONTAP 7-Mode 
  • Data ONTAP 8
  • ONTAP 9.0 - 9.1

Answer

How does routing work in Data ONTAP?
You can have Data ONTAP route its own outbound packets to network interfaces. Although your storage system can have multiple network interfaces, it does not function as a router. However, it can route its outbound packets.

Data ONTAP uses two routing mechanisms:

  • Fast path: Data ONTAP uses this mechanism to route NFS packets over UDP and to route all TCP traffic.
  • Routing table: To route IP traffic that does not use fast path, Data ONTAP uses the information available in the local routing table.
    • The routing table contains the routes, that have been established and are currently in use, as well as the default route specification.

What is fast path?
Fast path is an alternative routing mechanism to the routing table, in which 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 shorter route for data access. Fast path is used in all TCP and NFS/UDP protocols.
By default, fast path is enabled on the storage system; however, you can disable it.

  • Fast path is used for all TCP and NFS/ UDP protocols.
    Note: Fast path is used only in the NFS/UDP protocols. However, fast path is not used in other UDP-based NFS services such as port mapper, mountd, and nlm.
  • In a TCP connection, fast path is disabled after three consecutive retransmissions of the same data packet.
  • Fast path should be disabled in asymmetric networks with lousy data paths.
  • Using fast path has the following advantages:
    • Load balancing between multiple network interfaces on the same subnet.
      Load balancing is achieved by sending responses on the same interface of your storage system that receives the incoming requests.
      If fast path is enabled on an interface group and a physical interface in that group receives an incoming request, fast path does not govern which physical interface group member port sends a response to the request (this is specifically related to LACP or static multimode interface group). In an interface group, the interface selected when transmitting a packet is determined based upon the configured load balancing policy, and incoming traffic will arrive based upon how the neighboring switch utilizes its own load balancing policy to select ports when transmitting traffic.

How fast path works with TCP?
Data ONTAP can use fast path on every TCP packet transmitted except the first SYN packet (if Data ONTAP initiates a connection). The network interface that is used to transmit a packet is the same interface that received the last packet.

  • Fast path is not compatible with asymmetric routing.
  • If fast path is enabled on your storage system 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 that forwards packets to the storage system is not the router that forwards packets sent by the storage system. In such scenarios, disable fast path.

Additional Information

KB Article: How to disable ip.fastpath in Clustered Data ONTAP - NetApp Knowledgebase

 

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