AR1921 serial interface lose packets due to the port queue size is too small.

Publication Date:  2014-10-31 Views:  238 Downloads:  0
Issue Description
 AR19-21 serial interface connects the carrier network, the packets loss on the interface increase rapidly

Serial5/0 current state: UP 
Line protocol current state: UP
Description: ENLACE VIA E1 A CTL_ANAHUAC_DI_1 (FMD, 05/08/10, REF: D32-1006-1386)
The Maximum Transmit Unit is 1500, Hold timer is 10(sec)
Physical layer is E1-F, baudrate is 1984000 bps
fe1 timeslot-list 1-31
Internet Address is 10.79.*.* /30 Primary
Link layer protocol is PPP
LCP opened, IPCP opened
Output queue : (Urgent queuing : Size/Length/Discards)  0/100/0
Output queue : (Protocol queuing : Size/Length/Discards)  0/500/0
Output queue : (FIFO queuing : Size/Length/Discards)  0/75/14942460
Last clearing of counters: 15:19:45 MEXICO Mon 05/19/2014
    Last 300 seconds input rate 53379.64 bytes/sec, 427037 bits/sec, 970.98 packets/sec
    Last 300 seconds output rate 189931.78 bytes/sec, 1519454 bits/sec, 1697.80 packets/sec
    Input: 274176668 packets, 3729560312 bytes
           0 broadcasts, 0 multicasts
           0 errors, 0 runts, 0 giants
           0 CRC, 0 align errors, 0 overruns
           0 dribbles, 0 aborts, 0 no buffers
           0 frame errors
    Output:474734230 packets, 3529192820 bytes
           0 errors, 0 underruns, 0 collisions
           0 deferred
Alarm Information

when  ping the public ip, there are some  packets loss.
Target IP address: 106.*.*.*
Repeat count [5]: 5000
Type escape sequence to abort.
Sending 5000, 100-byte ICMP Echos to 106.*.*.*, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 99 percent (4993/5000), round-trip min/avg/max = 1/6/124 ms
Handling Process
1. Maybe the physical link have problem.
2. The bandwidth of this interface is 2M,maybe the burst traffic cause the packets loss.
3. the queue size is too small causing this issue.

according to the above possibilities ,we check as this:

a. Customer said he changed another serial cable but the issue persists ,so the cable problem is impossible .
b.. According to the interface information ,the interface bandwidth is 2M,
                   Physical layer is E1-F, baudrate is 1984000 bps
c. according to the past 300 seconds traffic statistic ,the average traffic is 1.5M,the burst traffic could be a cause.
      Last 300 seconds output rate 189931.78 bytes/sec, 1519454 bits/sec, 1697.80 packets/sec
    we configured  the Qos CAR cir 1980,and check by display qos car interface serial 5/0,there is no packets dropped counts ,
    that is the traffic is stable ,there is no burst traffic.
Root Cause
As we can see the packets loss in FIFO queue  ,the queue size is too small causing the traffic loss 
Solution
.

 I changed the parameter like this :

Qos FIFO queue-size 200

After this the packets loss is slower ,we adjust  it to 256 and there is no packets loss statistic .

After test  there is no packets lost and the link is stable .
Target IP address: 106.*.*.*
Repeat count [5]: 3000
Type escape sequence to abort.
Sending 3000, 100-byte ICMP Echos to 106.*.*.*, timeout is 3  seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 99 percent (3000/3000) , round-trip min/avg/max = 1/20/173 ms
Suggestions
For interface packets loss ,first please check if there are some physical link problem ,then check if  there exist burst traffic  causing the congestion ,or catch packets to analyze it .

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