Cisco 10008 - Manual (Page 27)

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Cisco 10008 Router Performance Routing Engine 3 Installation
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Analyzing and Troubleshooting Packets
The statistical information that the PXF counters provide is useful in analyzing and troubleshooting
denied and logged packets. Because the internal operation of the PRE3 differs for ACLs, the PXF
counters are inconsistent between the PREs. However, system-wide router behavior is consistent for
PREs despite the differences in counters.
The following sections describe the PXF counters and the way in which they increment.
IP Forwarding Counter
A Forwarding Information Base (FIB) lookup is one of the initial steps in forwarding a packet. When
the router forwarding processor needs information to forward a packet, it performs a lookup operation
on the FIB table. The IP forwarding counter reflects the state of that lookup operation. It does not reflect
whether or not the packet was forwarded. This counter increments each time an FIB lookup successfully
occurs.
ICMP Created Counters
Some FIB lookup operations can cause Internet Control Message Protocol (ICMP) messages to be
generated. For example, if a packet’s time-to-live (TTL) expires, an address is unreachable, or an
ACL-denied packet is dropped, an ICMP message is generated. The ICMP created counters reflect the
number of ICMP packets created. The counters increment each time an FIB lookup results in the
generation of an ICMP message.
Feedback Counter
Sometimes the PXF engine cannot complete the processing of a packet before the packet completes a
single pass through the PXF; the packet requires additional processing. As a result, the packet is fed back
through the PXF and processing continues. This is referred to as a feedback operation.
The following are examples of packets that can cause feedbacks to occur:
• Packets that are forwarded and logged to the console
• ICMP packets that are sent
• Packets that require both input and output quality of service (QoS)
The feedback counter reflects the total number of feedbacks through the PXF by all packets. The counter
increments one time for each additional pass a packet makes.
When a packet is denied because of an ACL deny statement, the router drops the packet. Dropped packets
do not need further processing and, therefore, are not fed back through the PXF. In this case, the feedback
counter does not increment.
Displaying Packet Statistics
The Cisco 10008 router supports show pxf cpu commands that allow you to determine the following
information:
• Forwarding engine traffic load
• Forwarding engine actions on the traffic
• Traffic load from the PXF to the RP
• Status of output packet buffers for the queuing system