Description

ADSL2+ interface showing excessive amount of queue-dropped packets. This is normal to see when one interface (ethernet) receives much higher packet-rate than the other interface can send (ADSL). However, the default egress buffer size can be increased to store more packets.

Symptoms

DSL interface showing packet drops:

user@srx340> show chassis hardware
FPC 1            REV 09   750-064612   XXXXXXXX          FPC
  PIC 0                                                  1x VDSL2 mPIM (RoHS)

user@srx340> show interfaces at-1/0/0 extensive
Physical interface: at-1/0/0, Enabled, Physical link is Up
...
  Link-level type: ATM-PVC, MTU: 1514, Clocking: Internal, ADSL mode, Speed: ADSL2+, Loopback: None
...
  ADSL status:
    Modem status  : Showtime (Adsl2plus)
    DSL mode      :     Auto    Annex A
...
  Egress queues: 8 supported, 4 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                        356785893            154148423            202637470
    1                                0                    0                    0
    2                                0                    0                    0
    3                          5977842              5977842                    0


Same amount of packets shown as tail-dropped on at-1/0/0 and Queue buffer is 18384B:

user@srx340> show interfaces queue at-1/0/0
Physical interface: at-1/0/0, Enabled, Physical link is Up
  Interface index: 165, SNMP ifIndex: 556
Forwarding classes: 8 supported, 4 in use
Egress queues: 8 supported, 4 in use
Queue: 0, Forwarding classes: Q0-class
  Queued:
    Packets              :             356787531                     0 pps
    Bytes                :          314362100441                     0 bps
  Transmitted:
    Packets              :             154149093                     0 pps
    Bytes                :           71704989168                     0 bps
    Tail-dropped packets :             202638434                     0 pps
...
  Queue Buffer Usage:
    Reserved buffer      :                 18384 bytes

Solution

One solution for such issue is to increase the egress queue size using q-pic-large-buffer command:

set chassis fpc 1 pic 0 q-pic-large-buffer


This increases the buffer size depending on the interface speed. In this case it increased the buffer size from 18384B to 95000B:
 

user@srx340> show interfaces queue at-1/0/0
Physical interface: at-1/0/0, Enabled, Physical link is Up
  Interface index: 165, SNMP ifIndex: 556
Forwarding classes: 8 supported, 4 in use
Egress queues: 8 supported, 4 in use
Queue: 0, Forwarding classes: Q0-class
...
  Queue Buffer Usage:
    Reserved buffer      :                 95000 bytes
This does not guarantee that there will be no packet drops, but can help to reduce tail-drops. Additional CoS techniques might need to be applied to fine-tune egress packet handling.

Modification History

2023-10-31 : Article Created