Cisco 4500 output drops

[GoncaloContente]

Hi,

I have a access switch with a 10gb upstream interface and a 1gb downstream interface, i can see from the show interfaces [int] that traffic is being dropped on the upstream downstream direction.

TenGigabitEthernet1/1 is up, line protocol is up (connected)
Hardware is Ten Gigabit Ethernet Port, address is f4cf.e213.7e30 (bia f4cf.e213.7e30)
Description: A01
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
 reliability 255/255, txload 5/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseSX
input flow-control is on, output flow-control is on
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:45, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 7556661
Queueing strategy: Class-based queueing
Output queue: 0/40 (size/max)
5 minute input rate 5364000 bits/sec, 1098 packets/sec
5 minute output rate 22402000 bits/sec, 2980 packets/sec
 863690200 packets input, 401065551603 bytes, 0 no buffer
 Received 6568826 broadcasts (1428513 multicasts)
 0 runts, 0 giants, 0 throttles
 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
 0 input packets with dribble condition detected
 2787171938 packets output, 1962145408543 bytes, 0 underruns
 0 output errors, 0 collisions, 4 interface resets
 0 unknown protocol drops
 0 babbles, 0 late collision, 0 deferred
 0 lost carrier, 0 no carrier
 0 output buffer failures, 0 output buffers swapped out

Most probably the cause of these drops are due to the fact that the traffic is comming from a higher bandwitdh interface to a lower bandwith one. the buffer gets full quickly and the switch as to discard some packets.

Is there a command where i can see the packet drops at a buffer level? Are there more causes for output drops? how can i troubleshoot these output drops more in detail?

Thanks

[Joseph W. Doherty · Accepted solution]

I'm unaware of any 4500 commands to check for different kinds of packets drops at the buffer level.

Overflowing a queue's buffers is the common cause for output drops.  Depending on the platform you could also have drops due to hitting some logical limits before you actually overflow buffers (e.g. on a 4500 using it's FRED or DBL features).

As to troubleshooting, if you split your traffic into classes directed to a 4500's different egress queues, I believe you can obtain drop statistics per queue.

BTW, if there are specific commands for analysis of output drops, they might vary per supervisor model and/or IOS version being used.  It might be helpful if you mentioned both.

[GoncaloContente]

Hi, thanks for the reply. i totaly forgot that i have applied class-based queueing on that interface and i can see that the class-default is droping belligerent flows.

Class-map: class-default (match-any)
19731402087 packets
Match: any
Queueing
queue limit 1504 packets
(queue depth/total drops) 0/0
(bytes output) 6794488568175
bandwidth remaining 25%
dbl
Probabilistic Drops: 117901 Packets
Belligerent Flow Drops: 7785763 Packets

Apparently this is a congestion avoidance technique used by the 4500 to prevent those buffer overflows. 

[Joseph W. Doherty]

Yes, and note especially your other policy has DBL within it.