Powerconnect Switches: Oversized Packet Counter, bcmRX task

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In a blade chassis implementation some time ago, some concerns were raised on the value of packet counters, and an inquiry on the bcmRX task. the notes i compiled as part of my reply, are as below:

The Oversized Packets counter can be safely ignored, as it is not indicative of a problem. What happens is that the statistics block inside the chip uses a different threshold to count oversized packets (MAC_CNTMAXSZr) than what is set by the “mtu xxxx” command (EGR_MTU_SIZEr). Even for normal 1518 byte frames, every frame that goes across a HiGig interface gets a few bytes of meta-data tagged to the front of the packet. This meta-data too is accounted by the MAC counters. The counter itself is not indicative of frame errors, but simply type (by size) frames logged. These are registered even on the internal link between the two chips on, for e.g. PC 6248.

A quick sample from Chassis blade switch B2:

8024-B2#show interfaces counters tengigabitethernet 1/1/1

FCS Errors: …………………………….. 0

Single Collision Frames: …………………. 0

Late Collisions: ………………………… 0

Excessive Collisions: ……………………. 0

Multiple Collisions: …………………….. 0

Oversize Packets: ……………………….. 4292719    

Internal MAC Rx Errors: ………………….. 0

Received Pause Frames: …………………… 0

Transmitted Pause Frames: ………………… 0

 

8024-B2#show statistics tengigabitethernet 1/1/1

Total Packets Received (Octets)……………. 29168671778

Packets Received > 1518 Octets…………….. 4292719       

Total Packets Received with MAC Errors……… 0

Jabbers Received…………………………. 0

Fragments/Undersize Received………………. 0

Alignment Errors…………………………. 0

FCS Errors………………………………. 0

Overruns………………………………… 0

Total Received Packets Not Forwarded……….. 0

802.3x Pause Frames Received………………. 0

Unacceptable Frame Type…………………… 0

And a few other interfaces:

8024-B2# show interfaces counters tengigabitethernet 1/1/2

Oversize Packets: ……………………….. 32536146   

          

8024-B2# show statistics tengigabitethernet 1/1/2

Packets Received > 1518 Octets…………….. 32536146      

 

8024-B2# show interfaces counters tengigabitethernet 1/1/3

Oversize Packets: ……………………….. 24291926   

 

8024-B2# show statistics tengigabitethernet 1/1/3

Packets Received > 1518 Octets…………….. 24291926      

 

8024-B2#show interfaces counters tengigabitethernet 1/1/4

Oversize Packets: ……………………….. 22580781   

 

8024-B2# show statistics tengigabitethernet 1/1/4

Packets Received > 1518 Octets…………….. 22580781       

 

8024-B2# show interfaces counters port-channel 1

Oversize Packets: ……………………….. 83708464   

 

8024-B2# show statistics port-channel 1

Packets Received > 1518 Octets…………….. 83708482      

 

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As we can see, the Oversize packet number reported in the counters, corresponds very closely, if not exactly, to the statistics output for 1518+ byte packets received, which is only a counter for packets classed on their size. Further, there are no other L2 errors of any kind being reported.

The IP traffic output on the switch gives:

8024-B2#show ip traff

IpInReceives…………………………….. 305912

IpInDiscards…………………………….. 0

IpInDelivers…………………………….. 107060

IpOutRequests……………………………. 119279

IpOutDiscards……………………………. 0

 

The CPU Utilization gives:

8024-B2#show process cpu

..

PID      Name                    5 Secs     60 Secs    300 Secs

—————————————————————–

5144f70 bcmRX                      8.41%       8.60%       8.45% 

570d8e0 MAC Send Task              0.80%       0.66%       0.66%

—————————————————————–

Total CPU Utilization             30.06%      31.03%      31.35%

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I will give details on a few of the tasks given as an output when this command is executed:

bcmL2X.0            Driver scan task looking for new units (stacking) (unit 0)
bcmCNTR.0         Driver statistics task (unit 0)
bcmLINK.0           Driver link task (unit 0))
bcmRX                Driver frame receive task
RMONTask          RMON Task
dot1s_timer_task  Spanning Tree Task
unitMgrTask        System stacking manager task – looking for new units

BcmRX process controls the Control plane packet dispatcher. This is control for other process running on the switch, for example input and output traffic passing in and out of the switch. It handles and pre-processes all the packets that hit the CPU.  This includes a multitude of processes including (but not limited to) dot1x, STP, mcast, unicast, bcast, LACP, the IP stack.  In addition to pre-processing and forwarding PDUs to existing tasks, it also copies packets from DMA buffers to CPU buffers, and many other data processing tasks. This task and the osapiTimer task gets busier and busier with each protocol in use. BcmRX can consume more than 10% with no ill effects.