Cisco Cisco ASR 5500 トラブルシューティングガイド
ASR 5000 Series ICMP Packet Generation from the
CLI and Fragmentation Identification
CLI and Fragmentation Identification
Document ID: 119210
Contributed by Dave Damerjian, Cisco TAC Engineer.
Jul 27, 2015
Contents
Introduction
Problem
Solution
IP Fragmentation
Problem
Solution
IP Fragmentation
Introduction
This document describes the byte counts reported by the show port commands and the ping command when
pings are executed in the CLI on the Aggregation Services Router (ASR) 5000 Series platform. It also
demonstrates the effects of fragmentation when the packets sent are greater than the configured maximum
transmission unit (MTU) in the interface. This is good background information to have when you troubleshoot
user-plane issues with ping or even in general for packets that pass through the chassis. Also, experimentation
with pings on a node is a great way to confirm the concepts explained in this document.
pings are executed in the CLI on the Aggregation Services Router (ASR) 5000 Series platform. It also
demonstrates the effects of fragmentation when the packets sent are greater than the configured maximum
transmission unit (MTU) in the interface. This is good background information to have when you troubleshoot
user-plane issues with ping or even in general for packets that pass through the chassis. Also, experimentation
with pings on a node is a great way to confirm the concepts explained in this document.
Problem
When you specify the size of an Internet Control Message Protocol (ICMP) packet, the size refers to the raw
payload that gets wrapped up into the packet. It does NOT include the ICMP header (8 bytes) OR the IP
header (20 bytes). Also notable is the Ethernet header (14 bytes = destination MAC (6) + source MAC (6) +
Ethernet frame type (2)), the VLAN tag (4 bytes), and the trailing Ethernet Frame Check Sequence (FCS, 4
bytes), the later of which is NOT to be displayed in a Wireshark trace.
payload that gets wrapped up into the packet. It does NOT include the ICMP header (8 bytes) OR the IP
header (20 bytes). Also notable is the Ethernet header (14 bytes = destination MAC (6) + source MAC (6) +
Ethernet frame type (2)), the VLAN tag (4 bytes), and the trailing Ethernet Frame Check Sequence (FCS, 4
bytes), the later of which is NOT to be displayed in a Wireshark trace.
Solution
When you view the output from the show port [npu | datalink] counters command, the math that can be
applied is shown in this example. Match the colors in order to obtain a straightforward understanding. This
exercise works on a port with little to no traffic as it allows a ping to be sent without any other traffic to cloud
the resultant output.
applied is shown in this example. Match the colors in order to obtain a straightforward understanding. This
exercise works on a port with little to no traffic as it allows a ping to be sent without any other traffic to cloud
the resultant output.
Payload size:
56
bytes (which is also the default for this command)
Payload + ICMP header:
64
Payload + ICMP header + IP header:
84
Payload + ICMP header + IP header + Ethernet header + VLAN tag:
102
Payload (56) + ICMP header (8) + IP header (20) + Ethernet header (14) + VLAN tag (4) + FCS (4):
106