This document explains when the no
buffer counter increments in the output of the
show interface atm command. No buffer defines an
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
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
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
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
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
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
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.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
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.
For more information on document conventions, refer to the
Cisco Technical Tips
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
Precedence 2: 50 min threshold, 81 max threshold, 1/10 mark weight
Precedence 3: 55 min threshold, 81 max threshold, 1/10 mark weight
Precedence 4: 60 min threshold, 81 max threshold, 1/10 mark weight
Precedence 5: 65 min threshold, 81 max threshold, 1/10 mark weight
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
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,
ATM Class of Service Phase 1 Design Guide.