Week 1: Troubleshooting approaches Flashcards
What is the troubleshooting process?
1 - Defining the problem
2 - Gathering information
3 - Analyzing information
4 - Eliminating possible causes
5 - Formulating / proposing a hypothesis
6 - Testing hypothesis
Is the shoot-from-the-hip a good approach?
Quite ineffective for inexperienced troubleshooters and huge set backs when it does not yield immediate results.
What are all the troubleshooting approaches?
- Top-down
- Bottom-up
- Divide-and-conquer
- Follow-the-path
- Spot-the-difference
- Move-the-problem
What are the logging severity levels?
(0) Emergency
(1) Alert
(2) Critical
(3) Error
(4) Warning
(5) Notification
(6) Informational
(7) Debugging
What do these commands mean?
logging buffered 16384
logging console warnings
logging 10.1.152.1
- logging buffered 16384
Messages are logged to a circular buffer in RAM, that is limited to 16384 bytes - logging console warnings
logging messages are limited to level 4 (warnings) and lower
By default every logging message is enabled
logging 10.1.152.1
messages are logged to a syslog server at IP
What are the functions of the control plane?
Responsible for processing Layer 2 and Layer 3 control packets. (e.g. OSPF, ISIS, STP, CDP)
Responsible for procession packets that cannot be CEF switched
What are the protection mechanism of the control plane?
- Control Plane Policing (CoPP)
- Access lists to control routing updates
What are the key show commands for troubleshooting the control plane?
- show processes cpu
- show ip arp
- show ip route
What are the functions of the management plane?
Responsible for management functions for a network device.
What are the protection mechanism for the management plane?
- Management plane protection
- Access lists to limit users access to Telnet
What are the key show commands for troubleshooting the management plane?
- show snmp user
- show snmp view
- show ip ssh
What are the functions of the data plane?
Responsible for the switching / forwarding of data packets through the network device using specialized hardware
What are the protection mechanism of the data plane?
Access lists to block unwanted users or traffic
What are the key show commands for troubleshooting the data plane?
- show ip cef
- show adjacency table
- show cef not-cef switched
What are CEF’s data structures?
Forwarding Information Base (FIB):
- reflects the routing table with all recursive lookups resolved
- a lookup up in the FIB results in a pointer to an adjacency entry in the adjacency table
CEF Adjacency Table
- consists of egress interface only for a point-to-point interface or an egress interface and next-hop IP address for a multipoint interface
Which commands do you use to verify CEF’s FIB and Adjacency table?
- show ip cef “ip-address”
- show ip cef “network-mask”
- show adjacency detail
How should you use the ip debug command?
- Always use an access-list
access-list 101 permit tcp any any eq telnet
debug ip packet detail 101
What do the numbers mean in “show processes cpu”?
CPU utilization for five seconds: 30% / 26% ; one minute: 31% ; five minutes: 14%
- 30%, 31%, 14% : Total CPU spent on processes and interrupts
- 26% CPU spent on interrupts
- Total time spent on processes: 30% - 26%
What are the DHCP messages?
Discover
Offer
Request
Acknowledge
What is a DHCP relay agent?
DISCOVER messages are sent as broadcast, but cannot leave the router boundary. You can use a boundary router to send DISCOVER messages further into the network towards the actual server
What are the commands for troubleshooting DHCP?
show ip dhcp conflict
show ip dhcp binding
debug ip dhcp server packet
What is SLAAC?
StateLess Address Auto Configuration: preferred method of assigning IP addresses in an IPv6 network
The device sends the router a request for the network prefix, and then uses this prefix with its own MAC address to create an IP address.
How do you troubleshoot SLAAC?
- show ipv6 interface g0/0
-> check if RAs are suppressed - only works if route is using a /64 prefix
- check if IPv6 unicast routing is enabled
What is stateful DHCPv6?
It provides an IP address to the device as well as other information such as NTP server, DNS, etc.
