Packet capture on Cisco ISR/CSR

Learned this from my friend Ricardo today, no longer running blind trying to figure out if a specific packet is received and what the router did with it.

To check if packet capture is already enabled:

show debugging

To enable capture from specific IP

debug platform condition ipv4 <ip/cidr> both

To start the capture and tell it the max packet to be captured

debug platform condition start
debug platform packet-trace packet 1024

To check the capture result

show platform packet-trace summary

Example output of above command

Pkt   Input             Output            State  Reason
0     Gi0/0/0.703       Gi0/0/1.32        FWD    
1     Gi0/0/0.703       Gi0/0/1.32        FWD    
2     Gi0/0/0.703       Gi0/0/1.32        FWD    
3     Gi0/0/0.703       Gi0/0/1.32        FWD    
4     Gi0/0/0.703       Gi0/0/1.32        FWD   

To check the detail of specific packet, eg packet 3

show platform packet-trace packet 3

Example of above command:

Packet: 3           CBUG ID: 18
  Input     : GigabitEthernet0/0/0.703
  Output    : GigabitEthernet0/0/1.32
  State     : FWD 
    Start   : 90348958754799 ns (09/22/2022 18:40:07.21077 UTC)
    Stop    : 90348958764809 ns (09/22/2022 18:40:07.21087 UTC)
Path Trace
  Feature: IPV4(Input)
    Input       : GigabitEthernet0/0/0.703
    Output      : <unknown>
    Source      :
    Destination :
    Protocol    : 1 (ICMP)

To reset

clear platform condition all
clear platform packet-trace statistics

Another method

Create access list named CAPTURE

conf t
ip access-list extended CAPTURE
permit ip any any

Enable and setup capture name ‘CAP’ using access list named CAPTURE, add interface g1, specify both directions, and running circular buffer (new override old when filled up)

monitor capture CAP access-list CAPTURE interface g1 both buffer circular

It’s possible to add additional interfaces to be captured, eg following adds Tunnel 1 to be captured

monitor cap CAP interface Tunnel 1 both

To check current capture setting

show monitor cap CAP

Start/Stop/Clear capture

monitor cap CAP start
monitor cap CAP stop
monitor cap CAP clear

To view the capture

show monitor cap CAP buffer brief

To view the capture for specific IP:

show monitor cap CAP buffer brief | inc

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