Loading a New Path or a Tertiary Path
Once a migration has occurred, a QP's or EEC's primary and alternate path information are the same. A new alternate path needs to be specified before re-enabling migration.
First, the alternate path variables are reloaded:
On a local HCA, the variables are reloaded using the Modify QP or Modify EEC verb.
On the remote QP or EEC, the local CA's CM sends the remote CA's CM a LAP (Load Alternate Path) communications message with new alternate path information).
The HCA QP or EEC state can then be transitioned from Migrated to ReArm via the Modify QP or Modify EEC verb. This causes the BTH:MigReq bit in all subsequently transmitted outbound packets to be cleared to zero. Upon receiving an inbound packet with BTH:MigReq = 0, a QP's or EEC's APM state is set to Arm. Migration is then re-enabled.
Two CM messages are used to change the alternate path information in a remote QP or EEC:
LAP. The Load Alternate Path message provides new alternate path information to the remote QP or EEC. For more information on the LAP MAD, refer to “LAP (Load Alternate Path) MAD” on page 1104.
APR. The Alternate Path Response message is returned by the remote CA's CM in response to the receipt of the LAP message. It tells the CM that sent the LAP message whether or not the new alternate path information was accepted by the remote CA's CM. If it wasn't accepted, the reason is specified in the APR message's AP Status field (see Table 23-3 on page 587). For more information on the APR MAD, refer to “APR (Alternate Path Response) MAD” on page 1107.
The CM that receives the LAP message must return a failure in the APR if the LAP request specifies an alternate path that is identical to the QP's or EEC's current primary path information.
The LAP message may be sent for one of the following reasons:
To update the alternate path information when fabric changes cause the QP's or EEC's current alternate path information to become invalid.
To load new alternate path information into a remote QP or EEC after a path migration has occurred. Loading new alternate path information into the remote QP or EEC does not initiate the migration process for automatic failover; it just specifies which path is to be used when the path migration occurs.
| Value | Meaning |
|---|---|
| 0 | Alternate path information accepted and loaded. |
| 1 | Invalid Communication Instance tuple (i.e., the combination of the Local Communication ID, Remote Communication ID, and QPN or EECN). |
| 2 | APM not supported. Alternate path parameters were not checked. |
| 3 | APM supported and all alternate path parameters are valid, but the failover port was rejected for some other reason. |
| 4 | Alternate path information rejected—redirect. The remote CA's CM has rejected the proposed alternate path information and wants the CM that sent the LAP message to resend the LAP using the alternate address information supplied in the APR message's Additional Information field. This data structure adheres to the format of the ClassPortInfo attribute. |
| 5 | The proposed alternate path matches the current primary path. |
For a detailed description of the LAP and APR communications messages, refer to “LAP (Load Alternate Path) MAD” on page 1104 and “APR (Alternate Path Response) MAD” on page 1107.