Port Mirroring; In-Order Delivery; Port Mirroring Considerations - Brocade Communications Systems StoreFabric SN6500B Troubleshooting Manual

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Port mirroring

With port mirroring, you can configure a switch port to mirror the traffic between a specific source
and destination port. This is only supported between F_Ports. This is a useful way to troubleshoot a
problem port without bringing down the host and destination links to insert an inline analyzer.
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:
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Port mirroring

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
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.
Table 9 on page 42 to determine the number of mirror
Fabric OS Troubleshooting and Diagnostics Guide
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