Cisco 10005 Guia de Instalação descarregar pdf (Página 35)

Cisco 10000 Series Router Performance Routing Engine Installation

OL-3971-03

Analyzing and Troubleshooting Packets

For example, the router processes the following ACL from top to bottom as follows:

access-list 108 permit udp any host 10.68.1.10 range 0 5000 log

access-list 108 permit udp host 10.1.1.10 range 0 5000 any log

• Statement 1—Allows any UDP packet to access host 10.68.1.10 if the UDP destination port of the

packet is between 0 and 5000. The router logs packet information to the console if a match is made.

• Statement 2—Allows any UDP packet from host 10.1.1.10 with a source port between 0 and 5000

to be permitted. The router logs packet information to the console if a match is made.

• Implicit Deny—Denies all remaining packets and does not log the packet information to the console.

Packet Statistics and PXF Counters

The Cisco 10000 router Performance Routing Engine (PRE) provides high performance Layer 3

processing using its Parallel eXpress Forwarding (PXF) engine and Route Processor (RP). The PRE

installed in the chassis can be a ESR-PRE, ESR-PRE1, or ESR-PRE2.

As the PXF engine processes packets, counters such as the following reflect the internal operation of the

PRE:

• IP forwarding

• ICMP created

• Feedback

The statistical information that the PXF counters provide is useful in analyzing and troubleshooting

denied and logged packets. Because the internal operation of the ESR-PRE/ESR-PRE1 and ESR-PRE2

differs for ACLs, the PXF counters are inconsistent between the PREs. However, system-wide router

behavior is consistent for ESR-PRE1 and ESR-PRE2 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 a FIB lookup successfully

occurs.

ICMP Created Counters

Some FIB lookup operations can cause 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 a 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.

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Cisco 10005 Guia de Instalação Página 35