Guest

Cisco Port Adapters

When Does the no buffer Error Counter Increment on the PA-A3?

Document ID: 6189

Updated: Nov 15, 2007

   Print

Introduction

This document explains when the no buffer counter increments in the output of the show interface atm command. No buffer defines an output counter.

atm-1# show interface atm 4/0

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 136/255, rxload 1/255 
Encapsulation ATM, loopback not set 
Encapsulation(s): AAL5 
4095 maximum active VCs, 5 current VCCs 
VC idle disconnect time: 300 seconds 
Signalling vc = 4, vpi = 0, vci = 5 
UNI Version = 3.0, Link Side = user 
4 carrier transitions 
Last input 00:02:30, output 00:00:00, output hang never 
Last clearing of "show interface" counters never 
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 103197668 
Queueing strategy: Per VC Queueing 
30 second input rate 0 bits/sec, 0 packets/sec 
30 second output rate 80210000 bits/sec, 6650 packets/sec 
308 packets input, 9856 bytes, 4138 no buffer 
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort    
338179038 packets output, 3163620726 bytes, 0 underruns 
0 output errors, 0 collisions, 0 interface resets 
0 output buffer failures, 0 output buffers swapped out 
atm-1#

Note: The no buffer counter shown by the show interface atm command differs from the rx_no_buffer counter in the output of show controller atm. Refer to Input Errors Using show controllers atm Command.

To ensure high forwarding performance, the PA-A3 port adapter uses separate segmentation and reassembly (SAR) chips for receive and for transmit. Each SAR is supported by its own subsystem of local memory to store packets and key data structures like the virtual channel (VC) table. This memory specifically includes 4 MB of synchronous DRAM (SDRAM) on the PA-A3.

The PA-A3 driver starts incrementing the no buffer counter when the interface exhausts its supply of local particle buffers. These buffers are tracked with the Tx-BFD number, which you can view in the output of the show controller atm command.

BFD Cache status:
     base=0x62931AA0, size=6144, read=143
Rx Cache status:

Packet buffer descriptor (BFD) describes the index used by the PA-A3 driver to access a specific packet buffer. The BFD cache size indicates the total number of buffers in the local port adapter (PA) memory. The current number of free particles is given by the read value.

On the transmit path, the local memory consists of 6144 particles of 576 bytes (or 580 bytes, with an internal 4-byte header that travels with the packet inside the router). Of these, the PA-A3 reserves 144 particles for system packets like operations, administration, and maintenance (OAM) cells. When the read value reaches 144, the PA-A3 driver starts dropping packets until a sufficient number of local memory particles becomes available.

No buffer packet drops are different from the OutPktDrops counted in the output of the show atm vc vcd command. The PA-A3 driver increments the OutPktDrops counter when a VC fills its individual transmit buffer quota. The purpose of the quota is to prevent a consistently over-subscribed VC from grabbing all of the packet buffer resources and hindering other VCs from transmitting normal traffic within their traffic contracts.

An incrementing no buffer counter indicates that the local memory on the PA-A3 simply is exhausted and does not imply that the VCs have exhausted their per-VC transmit credit.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Buffer-Shortage Drops on the VIP: show queueing interface atm

In some exceptional situations, the output Versatile Interface Processor (VIP) could have no buffers left to store a packet that is switched to this output VIP from the Route/Switch Processor (RSP) or from an input VIP. Consequently, the VIP will need to indiscriminately drop that packet regardless of its precedence.

Such an exceptional situation could occur as a result of heavy congestion combined with misconfiguration of the weighted random early detection (WRED) parameters. As an example, if the exponential weighting constant has been reconfigured from the default value to an excessively large value, then the WRED algorithm is slow to react to congestion (because the moving average increases only slowly as the instantaneous queue fills up). Thus, WRED may not start its intelligent discard early enough and the bursts keep filling the buffers.

You should avoid these situations, because these drops indiscriminately affect high precedence traffic.

Drops on the VIP due to buffer shortage can be monitored through the show queueing interface atm command through the nobuffer drops counter.

7513-1-31# show queueing interface atm 11/0/0.103

VC 5/103 -
 ATM11/0/0.103 queue size 46
        packets output 1346100, drops 134315, nobuffer drops 0
 WRED: queue average 44
       weight 1/512, max available buffers 1021
 Precedence 0: 40 min threshold, 81 max threshold, 1/10 mark weight
               1344366 packets output, drops: 134304 random, 10 threshold
 Precedence 1: 45 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)
 Precedence 2: 50 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)
 Precedence 3: 55 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)
 Precedence 4: 60 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)
 Precedence 5: 65 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)
 Precedence 6: 70 min threshold, 81 max threshold, 1/10 mark weight
               1734 packets output, drops: 0 random, 1 threshold
 Precedence 7: 75 min threshold, 81 max threshold, 1/10 mark weight
               (no traffic)

The nobuffer drops counter indicate how many packets have been dropped indiscriminately by the VIP, because no buffer was available at that time to accept the packet when it was handed over to the output VIP by the RSP or by the VIP that received the packet. Because the VIP drops the packet without being able to run the IP to ATM class of service (CoS) feature—and, in fact, without even looking at the packet at all—such packets are dropped irrespective of the moving average queue occupancy for the particular VC and irrespective of the packet precedence.

In addition to the show queueing interface command, you can use the show vip hqf command to display the number of nobuffers on a PA-A3 in a Cisco 7500 series router.

VIP-Slot0# show vip hqf

!--- Output suppressed.

qsize 1525 txcount 46810 drops 0 qdrops 0 nobuffers 0 
aggregate limit 2628 individual limit 657 availbuffers 2628 
weight 1 perc 0 ready 1 shape_ready 1 wfq_clitype 0

For more detailed information about Buffer-Shortage Drops on the VIP, refer to IP to ATM Class of Service Phase 1 Design Guide.

Related Information

Updated: Nov 15, 2007
Document ID: 6189