Router(config-if)#int fa 0/0.1
Router(config-subif)#encap dot1q 10
Router(config-subif)#ip address 192.168.10.1 255.255.255.0
Router(config)#int fa 0/0.2
Router(config-subif)#encap dot1q 20
Router(config-subif)#ip address 192.168.20.1 255.255.255.0
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.
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:
Benefits of Cisco EtherChannel:
Flexible incremental bandwidth
Resiliency and fast convergence
Ease of management
Transparent to network applications
Supports 100 Megabit, 1 Gigabit and 10 Gigabit Ethernet
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.
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.
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.
The show platform tcam utilization command can be used to see how much the tcam is being stressed.
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.
The show processes cpu command can be utilized to display CPU load.
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.
The show spanning-tree vlan
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.
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.
Spanning Tree problems occur at either Layer 1 or 2 of the OSI Reference Model.
Accurate / up-to-date network diagrams are key to troubleshooting.
Use debug commands with caution as they can be very CPU intensive and can potentially cause problems in an operation environment.