When Packet Input FIFO (PIF) overflows occur because of excessively
high credit maximum (Cmax) settings on Cisco
Internetwork Packet Exchange (IPX) and IGX platforms, isolation and diagnosis
can be difficult. Trickling discards in conjunction with high
Cmax settings and large port rates are typical
signs of an IPX or IGX network with PIF overflow problems.
These PIF overflows are the result of an inability of the FIFO buffer
to meet the instantaneous demand to process packets. Overflows that high
Cmax settings cause can occur in networks with
both ForeSight and non-ForeSight permanent virtual circuits (PVCs). Networks
that use ForeSight in combination with high Cmax
and high port rate settings are especially vulnerable to PIF overflows. In
addition, this combination can hinder the ability of ForeSight to effectively
manage PVC rates. To avoid these complications, use a
Cmax setting of 10.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
Trickling egress errors is a term that refers to a
low rate of consistent errors or frame discards. The cause of these discards
can be difficult to determine because many possibilities exist.
is the number of
packets a connection can burst into the network at port speed without a
throttle to minimum information rates (MIR) or quiescent information rates
(QIR). Generally, configure Cmax to approximate
the size of the average frame you expect from user equipment.
ForeSight is a proprietary, dynamic closed-loop,
rate-based, congestion management feature that yields bandwidth savings at the
transmit of bursty data across cell-based networks. ForeSight ensures that
connections get the maximum available throughput and that no connection takes
the capacity that other connections require.
Trickling egress frame discards can occur on connections with high
Cmax values and high port rates.
Cmax is a configurable parameter for Frame Relay
fixed rate and ForeSight connections. The Cmax
value is the maximum number of credits a Frame Relay connection accrues during
idle time. Each credit equals one FastPacket.
These credits allow connections to burst a set quantity of FastPackets,
with no interruption, from the IPX or IGX Frame Relay card. When a connection
is able to burst with use of its credits, there is no mechanism, including
ForeSight, that can throttle the burst of packets regardless of congestion
along the PVC path. Therefore, on Frame Relay networks that use ForeSight, a
Cmax value of 10 gives the best overall
performance. Though this parameter is configurable, do not change the default
setting to a value greater than 10.
If you set the Cmax value above 10, you
can obstruct the effective management by ForeSight of the network PVCs. In
addition, packet discards can occur on trunks, even during times of moderate
trunk congestion. Networks with high Cmax values
in conjunction with large port rates have a higher probability of packet
discards because of PIF overflows. Trickling egress discards are typical
symptoms of PIF overflows. Sometimes, you can attribute trickling discards to
ingress discards or trunk errors. But, in the case of PIF overflows, there is
no obvious explanation for the occurrence.
To identify PIF overflows, you must perform Cbus traces on all through
and terminating nodes for a specific PVC. The PIF on the trunk card can hold 64
FastPackets and has an approximate throughput rate of 4 Mbps. When overflows
occur, the FIFO buffer is unable to pass the packets to the trunk rapidly
enough to meet the instantaneous demand of packets input from the bus.
This event can occur so rapidly that, when you use the
dsptrkutl command to average the event over the
one-second update, the event does not have a significant impact on the
statistics that display.