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Cisco Bug: CSCvv14885 - Enable Ethernet slow-drain feature on 4G Fabric Interconnects

Last Modified

Sep 02, 2020

Products (1)

  • Cisco Unified Computing System

Known Affected Releases

4.1(1c)A

Description (partial)

Symptom:
Large amount of PAUSE frames being sent from any IOM to Fabric Interconnect can affect uplinks due to all buffers being taken by PAUSE frames.

In this case there is no no-drop QoS class configured, there are no Fiber Channel uplinks, however due to high pause frame traffic from any IOM was affecting incoming LACP PDUs from upstream N9k being dropped by Fabric Interconnect. In this case UCS was only processing Best Effort Ethernet traffic.

Conditions:
on affected Fabric Interconnect , 

  show int priority-flow-control
============================================================
Port               Mode Oper(VL bmap)  RxPPP      TxPPP
============================================================

 
Ethernet1/19       Auto Off           501096684      0  <<=== High PAUSE frame traffic coming to Fabric Interconnect


 show int po17
port-channel17 is down (No operational members)
  30 seconds input rate 1408 bits/sec, 176 bytes/sec, 2 packets/sec
  30 seconds output rate 296 bits/sec, 37 bytes/sec, 2 packets/sec
  Load-Interval #2: 5 minute (300 seconds)
    input rate 416 bps, 2 pps; output rate 344 bps, 2 pps  <<===== v. little incoming traffic to FI port-channel
  RX
    0 unicast packets  2925 multicast packets  0 broadcast packets  <<===== LACP PDUs
    2925 input packets  502713 bytes
    0 jumbo packets  0 storm suppression bytes
    0 runts  0 giants  0 CRC  0 no buffer
    0 input error  0 short frame  0 overrun   0 underrun  0 ignored
    0 watchdog  0 bad etype drop  0 bad proto drop  0 if down drop
    0 input with dribble  2321 input discard <==== despite v. little incoming traffic, LACP PDUs are being dropped

Incoming LACP PDUs not being seen as they are dropped as seen above, thus uplink Port channel would stay down
 
show lacp counters interface po17
                    LACPDUs         Marker      Marker Response    LACPDUs
Port              Sent   Recv     Sent   Recv     Sent   Recv      Pkts Err
---------------------------------------------------------------------
port-channel17
Ethernet1/39       5324   56       0      0        0      0        0
Ethernet1/40       4782   35       0      0        0      0        0
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