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Once a UDSP policy is instantiated on an LNet construct, the UDSP action becomes part of the LNet construct. When the selection algorithm is running on per message send case, the policy action takes effect. To give a more concrete example, let's say the admin applies a policy which gives the highest Selection Priority for o2ib0 local network. This policy action translates to the local network selection priority field set to 0, which is the highest priority. When the selection algorithm runs it checks the priorities of all local networks and selects o2ib0 since it has the highest Selection priority.
Policy Actions however, do not exist in a vacuum and must integrate with the rest of the Selection Criteria used by the selection algorithm. For more details on the selection algorithm, please refer to the initial Multi-Rail HLD.
There is one particular rule which all UDSPs will adhere to, : Health will always trump trumps UDSPs. The selection algorithm will always select selects the healthiest interface to send from or to send to irregardless regardless if there is another network and/or interface which has a higher user assigned Selection Priority. The following diagram attempts to give an gives a simplified overview of the selection algorithm, but the . The HLD linked above is the best place to get more details, as this HLD concentrates on the UDSP design and does not intend to rehash all the details of the selection algorithm. Only the details needed to explain the UDSP functionality.
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