Cisco CCNA TShoot Config


Router(config)#int fa0/0

Router(config-if)#no shutdown

Router(config-if)#int fa 0/0.1

Router(config-subif)#encap dot1q 10

Router(config-subif)#ip address


Router(config)#int fa 0/0.2

Router(config-subif)#encap dot1q 20

Router(config-subif)#ip address


Switch1(config)#int fa 0/1

Switch1(config-if)#switchport mode trunk


Remember, router ports are administratively down by default so the “no shutdown” command is needed on the router but by default switch ports are administratively up so the command is not needed on the switch.

Cisco CCNA TShoot Communications

The show interface command is your first line of defense when troubleshooting why one device cannot communicate with another.  The output of the show interface command list things such as interface speed / duplex, interface statistics and other information.

Use the show interface command:

•Make sure the VLAN membership of the switch port is correct.
•If the host is in the same subnet as the switch interface, make sure the switch interface and the switch port to which the host is connected are assigned to the same VLAN.

Cisco CCNA Etherchannel

Benefits of Cisco EtherChannel:

Standards Based

Multiple Platforms

Flexible incremental bandwidth

Load balancing

Resiliency and fast convergence

Ease of management

Transparent to network applications

Supports 100 Megabit, 1 Gigabit and 10 Gigabit Ethernet


•EtherChannel provides incremental trunk speeds between Fast Ethernet, Gigabit Ethernet and 10 Gigabit Ethernet
•EtherChannel combines up to eight physical Fast Ethernet, Gigabit Ethernet or 10 Gigabit Ethernet links into a single logical bundle and distributes the traffic across those links
•EtherChannel requires the following
•Same configuration (compatibility) between physical links
•Consistent channel definitions between switches
•Use of correct load-balancing hash



Cisco CCNA Tshoot Performance

First determine if the problem is real or is there a perceived problem.  If determined to be real (e.g. performance is not as expected), start troubleshooting.  Try and isolate the problem to a particular link, device or component.  After you have narrowed down the problem to a specific area, propose a solution and test it.

Cisoc CCNA Layer 2 Switching

The control plane is separate from the data plane.  The control plane is responsible for updating the information in the forwarding hardware.  Also, the control plane handles any traffic that cannot be handled by the forwarding hardware. Hence, a high load on the control plane hardware could be an indication the forwarding hardware has either reached its capacity or is not handling traffic properly.

The data plane is responsible for switching traffic from ingress interface to egress interface.

Cisco CCNA Tshoot CAM

How a packet is processed can have a big impact on performance.  The TCAM utilizes a specialized high performance memory lookup which speeds up performance.  Control plane information that is necessary to make packet forwarding decisions (e.g. MAC address tables, routing information, etc) are programmed into the TCAM for fast lookup.

Cisco CCNA Tshoot TCAM

The show platform tcam utilization command can be used to see how much the tcam is being stressed.

Cisco CCNA Tshoot Traffic

If subsequent outputs of the show controllers cpu-interface command has a rapidly increasing number for “sw forwarding” that is an indication that traffic is being forwarded to the CPU and is being switched in software in which case you will typically see high CPU utilization.

Cisco CCNA TShoot CPU Load

The show processes cpu command can be utilized to display CPU load.

Cisco CCNA Tshoot Load Problems

Remember that having debugging turned on will increase CPU utilization so only use when necessary and verify it is not enabled if you encounter a switch with high CPU utilization.

Cisco CCNA Tshoot Verify STP

The show spanning-tree vlan command displays spanning tree information for the specified vlan.

Cisco CCNA Tshoot PortFast

Portfast is used to minimize server or workstation downtime.  Portfast is configured on a port to port basis. The Portfast feature causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

Cisco CCNA Tshoot PortFast

The spanning-tree portfast command should only be utilized on switch ports connected to end hosts, not to other network switches/routers.  This is because PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

Cisco CCNA Tshoot STP Problems

Spanning Tree problems occur at either Layer 1 or 2 of the OSI Reference Model.


•Duplex mismatch
•Unidirectional link failure
•Frame corruption
•Resource errors
•PortFast configuration error
•Inappropriate STP parameter tuning and diameter issues


Cisco CCNA Tshoot STP

Accurate / up-to-date network diagrams are key to troubleshooting.


•Use your network diagram
•Identify a bridging loop
•Restore connectivity
•Check ports
•Look for resource errors
•Disable unneeded features


Cisco CCNA Tshoot STP Debug

Use debug commands with caution as they can be very CPU intensive and can potentially cause problems in an operation environment.

Cisco CCNA Tshoot Summary


•We learned basic troubleshooting at Layer 1, 2 and 3 of the OSI Reference Model
•We examined numerous Cisco IOS show commands to both configure and debug spanning-tree in an operational network
•We reviewed typical symptoms associated with spanning-tree problems and discussed how to resolve them
•We learned about EtherChannel and discussed common causes for EtherChannel failures
•We learned how to isolate and resolve performance problems caused by duplex mismatches or other physical problems on switch ports



Cisco CCNA Tshoot Summary


•We discussed the difference between switching packets in TCAM versus CPU and how both affect switch performance
•We learned how to view CPU usage on a switch and diagnose and resolve problems caused by high CPU utilization
•We learned about verifying and debugging Spanning Tree Protocol which is used to create a loop-free Layer 2 topology