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Using DA-only hashing generally reduces the overall number of forwarding entries, but it can cause performance issues if used when an ACL is enabled. These performance issues are magnified when the ACL uses protocol and port identifiers.
An ACL that specifies a source address, protocol ID, or port ID requires closer analysis of packets than just the destination address. Every flow to the destination needs its own forwarding cache entry based on the ACL criteria, and all of the entries hash to the same value. In this scenario, the switch must sequentially search every entry in the forwarding cache that has the same DA (thus hash-location).
When you set the hash mode to SA-DA, each different source-destination combination hashes to a different value. Thus the number of entries hashed to a single value significantly decreases. However, SA-DA can also cause performance issues in some situations. If many entries that do not match the ACL have similar hash values to those that do, DA-only hashing provides more efficient usage of the forwarding memory.
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