HP Brocade 8/12c Brocade Fabric OS Troubleshooting and Diagnostics Guide - Sup - Page 60
In-Order Delivery, Port mirroring considerations
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3 Port mirroring Port mirroring captures traffic between two devices. It mirrors only the frames containing the SID/DID to the mirror port. Because of the way it handles mirroring, a single mirror port can mirror multiple mirror connections. This also means that the port cannot exceed the maximum bandwidth of the mirror port. Attempts to mirror more traffic than what available bandwidth allows results in the port mirror throttling the SID/DID traffic so that traffic does not exceed the maximum available bandwidth. The bandwidth of the mirror port is unidirectional. In general, a host (SID) talks to multiple storage devices (DIDs). Thus, a host does not send full line rate to a single target. A mirror port configured at 4 Gbps can only support up to 4 Gbps of traffic. A normal 4 Gbps F_Port is bi-directional and can support up to 8 Gbps (4 Gbps transmit and 4 Gbps receive) of traffic. If the mirror port bandwidth is exceeded, no credits are returned to the receiver port and thus those devices involved in mirror connection see a degraded level of performance. Use port mirroring to detect missing frames, which may occur with zoning issues or hold timeouts, capture protocol errors, and capture ULP traffic (SCSI/FICON). This feature cannot be used on embedded switch traffic. In-Order Delivery If In-Order Delivery (IOD) is enabled, adding or deleting a port mirror connection causes a frame drop. Port mirroring basically reroutes a given connection to the mirror port. The mirror traffic takes an extra route to the mirror port. When the extra route is removed, the frames between the two ports go directly to the destination port. The frames at the mirror port could be queued at the destination port behind those frames that went directly to the destination port. To prevent this IOD issue, port mirroring drops those frames from the mirror port when a connection is disabled. If IOD has been disabled, port mirroring does not drop any frames, but does have an IOD error. Port mirroring considerations Before creating port mirror connections, consider the following limitations: • A mirror port can be any port on the same switch as the source identifier port. • If FCR is enabled, do not enable port mirroring. • Only one domain can be mirrored. After a domain is defined, only mirror ports on the defined domain can be used. The first connection defines the restriction on the domain, which can be either the local domain or a remote domain. • A switch that is capable of port mirroring can support a minimum of one and a maximum of three mirror connections.Refer to Table 9 on page 42 to determine the number of mirror connections your switch or blade can support. • Mirror port bandwidth limits mirror connections. • Deleting a port mirroring connection with IOD enabled causes frame drop between two endpoints. • Using the firmware download procedure to downgrade to previous Fabric OS releases that do not support port mirroring requires that you remove all the port mirroring connections. If you downgrade to a previous versions of Fabric OS, you cannot proceed until the mirroring connections are removed. • Port mirroring is supported with Virtual Fabrics with the limitation that you cannot have FCR enabled within the same 8-port group. 40 Fabric OS Troubleshooting and Diagnostics Guide 53-1002150-02