Use IOS Troubleshooting Tools
Multiple troubleshooting tools are built into IOS.
- show – monitor normal behavior and isolate problems
- version – system hardware, software version, uptime, boot image
- running-config – current configuration
- startup-config – config stored in NVRAM
- interface – interface statistics, bandwidth, errors
- debug – assist in isolating a protocol and configuration problem
- ping – determine connectivity
- trace – show the path packets are taking
debug, conditional debug
Debugs must be turned on using the debug command. To show running debugs – show debug
Debugs are sent to console by default (no logging console – to turn off). User terminal monitor if you are remotely connected into the device.
Turn off debug – R1#undebug all
Conditional debug – add parameters around what debugs you want displayed to the console.
Stacking multiple debug conditions will generate output if at least 1 condition is met.
R1#debug condition ?
called called number
callingcalling
cplCisco Provisioning Language debugging
glbp interface group
interfaceinterface
ip IP address
mac-addressMAC address
match-list apply the match-list
profileMedia Services Profile
standbyinterface group
username username
vcid VC ID
vrfVirtual Routing and Forwarding
xconnect Xconnect conditional debugging on segment pair
R1#debug condition ip 1.1.1.1
Condition 1 set
R1#debug condition interface gi0/0
Condition 2 set
R1#
R1#
R1#sho debug
Condition 1: ip 1.1.1.1 (0 flags triggered)
Condition 2: interface Gi0/0 (1 flags triggered)
Flags: Gi0/0
R1#
Above condition will generate debugs messages for anything containing the ip 1.1.1.1 or interface gi0/0
ping, traceroute with extended options
Ping – common method for troubleshooting accessibility to a device
- Uses ICMP echo
- Tells if host is active / inactive
- RTD to host
- Packet Loss
Issues if cannot ping
- Routing issue
- Interface down
- ACL
- ARP issue
- Delay
- Source Address
- High Input Queue drops
Traceroute – Discover the routers a packet takes to a destination
- Sequence of UDP datagrams on an invalid port
- 3 datagrams sent with TTL of 1
- TTL of 1 causes datagram to timeout and first hop responds with ICMP “Time Exceeded Message (TEM)
- Process continues increasing TTL by 1 each step until packets reach the destination
- Destination responds with ICMP Port Unreachable message, indicates traceroute is finished
Traceroute Text Characters
Embedded packet capture
- Onboard packet capture facility
- Consumes CPU and memory resources during its operation
- Export captures via TFTP, FTP and local disk
- Define a buffer size and type (circular or linear) and max number of bytes of each packet capture
- Capture can be throttled using admin controls
- Filter packets with ACL
- Specify max packet capture rate or specify sampling interval
- Benefits
- Ability to capture IPv4 and IPv6 in CEF path
- Flexible method to specify capture buffer parameters
- Filter captured packets
- Method to decode data packets
- Facilty to export capture (PCAP)
- Extensible infrastructure for enabling capture points
Performance monitor
Enables to be able to monitor the flow of traffic in the network. Similar to netflow.
Pre-req for configuration
- IPv4
- routing and CEF must be configured / enabled
- IPv6
Can monitor a long list of traffic – https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/media_monitoring/configuration/15-mt/mm-15-mt-book/mm-pasv-mon.html
Configuration Components
- Interface – attach performance monitor to interface – service-policy type performance-monitor
- Policy – Associate with flow monitor – policy-map type performance-monitor
- Class – filtering criteria – class-map
- Flow Monitor – Associated with flow record and optional flow monitor – flow monitor performance-monitor
- Flow Record – Specify match and collect – flow record type performance-monitor
- Flow Exporter – Specify the destination for exporting traffic
show performance monitor status
Router#show policy-map type performance-monitor
Policy Map type performance-monitor PM_FLOW_MONITOR
Class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
react status: inactive
Router#
Router#sh run | s flow
flow record type performance-monitor FLOW_RECORD
match ipv4 destination address
match transport destination-port
collect application media event
collect counter bytes long
collect ipv4 dscp
collect monitor event
collect routing forwarding-status
collect timestamp interval
collect transport packets expected counter
collect flow direction
flow exporter FLOW_EXPORT
description *** Export Flows ***
destination 2.2.2.2
source GigabitEthernet0/0
output-features
dscp 46
transport udp 650
flow monitor type performance-monitor FLOW_MONITOR
description *** FLOW MONITOR ***
exporter FLOW_EXPORT
flow monitor FLOW_MONITOR
Router#
Router#sh run | s class
class-map match-all CM_FLOW_MONITOR
match any
class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
Router#
Router#
Router#sh run | s policy
policy-map type performance-monitor PM_FLOW_MONITOR
class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
Router#
Apply troubleshooting methodologies
Diagnose the root cause of networking issue (analyze symptoms, identify and describe root cause)
Design and implement valid solutions according to constraints
Verify and monitor resolution
Interpret packet capture
Using Wireshark trace analyzer
https://www.wireshark.org/docs/wsug_html/
Personal writing from experience – you need to understand traffic flows. Knowing protocol basics and using different filters in wireshark to remove the noise in the capture. Would love feedback for this section on your experience with wireshark.
Using IOS embedded packet capture
Router#monitor capture buffer CAPTURE size 256 max-size 100 circular
Router#monitor capture point ip cef CAPTURE_POINT gi0/0 ?
bothcapture ingress and egress
incapture on ingress
out capture on egress
removeremove capture point
Router#monitor capture point ip cef CAPTURE_POINT gi0/0 both
Router#monitor capture point associate CAPTURE_POINT CAPTURE
*Aug 18 04:12:11.060: %BUFCAP-6-CREATE: Capture Point CAPTURE_POINT created.
Router#monitor capture point associate CAPTURE_POINT CAPTURE
Router#
Router#monitor capture point start CAPTURE_POINT
Router#
Router#
Router#
Router#
*Aug 18 04:12:29.789: %BUFCAP-6-ENABLE: Capture Point CAPTURE_POINT enabled.
Router#show monitor capture buffer CAPTURE dump
Router#monitor capture point stop all
Router#
*Aug 18 04:15:23.960: %BUFCAP-6-DISABLE: Capture Point CAPTURE_POINT disabled.
Router#
Packet capture can be exported to TFTP server to look at the capture in wireshark
Router#monitor capture buffer CAPTURE export ?
flash0: Location to dump buffer
flash1: Location to dump buffer
flash2: Location to dump buffer
flash3: Location to dump buffer
flash: Location to dump buffer
ftp: Location to dump buffer
http: Location to dump buffer
https: Location to dump buffer
pram: Location to dump buffer
rcp: Location to dump buffer
scp: Location to dump buffer
snmp: Location to dump buffer
tftp: Location to dump buffer
Router#monitor capture buffer CAPTURE export
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