Flushes

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Sep 26th, 2008
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Should I be concerned with three of my interfaces on my core router (atm, GB, and Fast Eth.) flushing on the input Queue. The GBit interface has flushed 230,000 times in 7 days, atm a little over 10,000. No errors listed.

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Joseph W. Doherty Sun, 09/28/2008 - 11:52
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Beside absolute flush counts, how do they compare to total inbound packets? (In other words, what's the percentage of flushes over all traffic?)


What type of router is this, and how is it being used?

babuffalo8 Mon, 09/29/2008 - 07:06
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Here is the results from show int. It is a core router being fed through 2 LS1010's for the ATM Fabric. 20 + sites, GB handles a lot of server traffic.


GigabitEthernet0/0 is up, line protocol is up

Hardware is i82543 (Livengood),

MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex mode, link type is autonegotiation, media type is SX

output flow-control is off, input flow-control is off

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters 1w2d

Input queue: 8/4096/0/263927 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/4096 (size/max)

5 minute input rate 11981000 bits/sec, 1795 packets/sec

5 minute output rate 5195000 bits/sec, 1442 packets/sec

692249830 packets input, 3233036850 bytes, 195 no buffer

Received 6684413 broadcasts, 0 runts, 0 giants, 147 throttles

10 input errors, 0 CRC, 0 frame, 0 overrun, 10 ignored

0 watchdog, 0 multicast, 0 pause input

0 input packets with dribble condition detected

621049642 packets output, 849153723 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier, 0 pause output

0 output buffer failures, 0 output buffers swapped out

ATM4/0 is up, line protocol is up

Hardware is ENHANCED ATM PA


MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit, DLY 80 usec,

reliability 255/255, txload 15/255, rxload 1/255

NSAP address:


Encapsulation ATM, loopback not set

Encapsulation(s): AAL5

4095 maximum active VCs, 22 current VCCs

VC idle disconnect time: 300 seconds

Signalling vc = 1, vpi = 0, vci = 5

UNI Version = 4.0, Link Side = user

0 carrier transitions

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters 1w2d

Input queue: 1/150/0/10510 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: Per VC Queueing

5 minute input rate 1023000 bits/sec, 862 packets/sec

5 minute output rate 9343000 bits/sec, 1217 packets/sec

214964311 packets input, 1776724838 bytes, 5 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

1 input errors, 1 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

277701037 packets output, 1337394842 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out


Joseph W. Doherty Mon, 09/29/2008 - 09:04
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For the gig interface, we see 263,927 flushes and 692,249,830 packets input which is only about .04%. The percentage, itself, doesn't appear overly alarming with regard to its traffic impact. However, when we see the input queue is 4,096 deep, there are, with this stats snapshot, 8 packets in the input queue, and the 5 minute average rate is about 12 Mbps, makes one wonder whether there is an underlying issue.


Likewise for the ATM stats we see 10,510 flushes and 214,964,311 packets input provides about .005% drops, but again this is with an input queue of 150, with 1 packet in the queue, and a 5 minute input rate of about 1 Mbps.


See if the following two links help:


http://www.cisco.com/en/US/products/hw/modules/ps2033/products_tech_note09186a00800ac5a8.shtml

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml


I've never worked with a LS1010, and the above links are not for the LS1010 device, but sometimes issues/solutions for one devices extends to others.


PS:

Cisco's Output Interpreter notes this (it also provides links to the same two references but provides a few other suggestions):


"WARNING: 5 packets have been dropped because there were no free buffers to copy

the packet.

TRY THIS: If this is incrementing, paste the output from the 'show buffers'

command into Output Interpreter to see if the buffers can be tuned. Also compare

with the 'ignored' counter and input/output queue drops. Broadcast storms and

bursts of noise on the line are often responsible for no buffer events.

REFERENCE: For more information see:

Buffer Tuning

Troubleshooting Input and Output Queue Drops

INFO: There have been 10510 flushes on this interface.

Flushes are similar to input drops, but are proactively forced by the router before

the input queue is full. Selective Packet Discard (SPD) is the congestion avoidance

mechanism used, where non-control packets are dropped in preference to control

packets (i.e. routing updates). Monitor the number of flushes. If they continue

to increment, consider increasing the size of the input queue and/or improving

the switching mechanism used by this interface (e.g. change process switching

to fast switching). Excessive flushes may indicate too much process switched

traffic.

You may use the 'show ip interface atm' command to see what switching mechanism

is in use on this interface.

REFERENCE: Troubleshooting Input Drops on ATM Router Interfaces

REFERENCE: Command Reference - 'show interface atm'"

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