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Cisco Bug: CSCun30488 - Nexus 55xx/56xx/600x supports maximum of 255 Tx B2B credits on native FC interfaces

Last Modified

Aug 18, 2019

Products (5)

  • Cisco Nexus 5000 Series Switches
  • Cisco Nexus 5596UP Switch
  • Cisco Nexus 5548P Switch
  • Cisco Nexus 5548UP Switch
  • Cisco Nexus 5596T Switch

Known Affected Releases

5.2(1)N1(7)

Description (partial)

Symptom:
Buffer-to-buffer credits (B2B) are a flow-control mechanism to ensure that Fibre Channel switches do not run out of buffers, so that switches do not drop frames. Buffer-to-buffer credits are set on a per-hop basis.

When a device connected on a native FC interface is configured with Buffer-to-Buffer (B2B) credits higher than 255 the Nexus 55xx/56xx/600x switch (except N5K-C5672UP-16G) will only utilize some of the B2B credits received from the adjacent device due to a hardware limitation where only the rightmost (least significant) byte of the B2B credits is used.

For example, if the adjacent device sends 300 B2B credits this is 0x012c. Since only the rightmost byte is used this is 0x2c or 44 in decimal. These are the Tx B2B credits that will be used:

    Transmit B2B Credit is 300
      44 transmit B2B credit remaining <<<

This could lead to lower than expected performance especially on longer distance connections.

Conditions:
Nexus 55xx/56xx/600x switch (except N5K-C5672UP-16G) connected to a device which is configured with Rx B2B credits higher than 255. This is most likely to occur on an Interswitch Link (ISL) since the default B2B credits are higher on ISLs. However, it could occur on an F port as well.
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