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Cisco 4500 output drops

GoncaloContente
Level 1
Level 1

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

1 Accepted Solution

Accepted Solutions

Joseph W. Doherty
Hall of Fame
Hall of Fame

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.

View solution in original post

3 Replies 3

Joseph W. Doherty
Hall of Fame
Hall of Fame

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.

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. 

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

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