Understanding OxID Hash Control for FCoE Traffic Load Balancing on Standalone Switches
The originator exchange identifier (OxID) field is one of several fields that the switch can use in its hash function computation for FCoE traffic load balancing over multiple outgoing links in an Ethernet link aggregation group (LAG) on ports that face an FCoE forwarder (FCF). The originator of an exchange between a pair of Fibre Channel (FC) endpoints (such as an FCoE host and an FC storage device) uses the OxID field as an identifier for that exchange. The originator also uses the OxID field to track the progress of the series of sequences that comprise the exchange.
When FCoE traffic traverses a LAG that faces an FCF, it can take multiple different links between the source and destination endpoints. The idea is to distribute the FCoE traffic across the FCF-facing LAG links, thus balancing the link load. The switch creates a hash value from some of the packet header fields, and uses the hash value to assign each packet to one of the LAG links. The switch always uses five packet header fields to compute the hash value:
Source ID (SID)
Destination ID (DID)
Fabric ID (FID)
Source Port ID (SPID)
Source Module ID (SMID)
In addition, the OxID field is included by default in the FCoE
load-balancing hash computation. However, if you do not want to use
the OxID field in the FCoE load-balancing hash computation, you can
remove it from the computation by using the set forwarding-options
hash-key family fcoe oxid disable command.
Including the OxID field in the load-balancing hash computation allows different exchanges between a pair of Fibre Channel (FC) endpoints (such as an FCoE host and an FC storage device) to take different paths across the network, thus improving the aggregate network throughput.
However, if the paths between different sets of FC endpoints have common links, congestion on one set of FC endpoints can affect the other set of endpoints. Such congestion can happen if the FCoE traffic on the two sets of endpoints uses the same priority (IEEE 802.1p code point). It is common for networks to use priority 3 (IEEE 802.1p code point 011) for FCoE traffic. However, you can assign different IEEE priorities to different lossless FCoE flows as described in Understanding CoS IEEE 802.1p Priorities for Lossless Traffic Flows to further separate the traffic flows.