Passing Your CCNA and CCNP: Configuring and Troubleshooting Router-on-a-Stick

For CCNA and CCNP candidates, it is hard not for the first time you hear the term "router on a stick to laugh." Let's face it, is a concept, rather ridiculous. But to know from those who have the CCNA and CCNP exams, is a fundamental consideration that you need to know how to configure and troubleshoot problems.

From hosts other than the basic theory Cisco VLANs to communicate, a Layer 3 device must be combined to handle the routing between VLANs. The device is a router, and there are considerations, special attention to the physical router itself and the configuration that you will need to write.

The router is connected to a switch via a FastEthernet port (or higher). The router port can not be a regular Ethernet connection, since the router port is the ability to send and receive data simultaneously need.

The configuration interface is very interesting. To say that we communicate with two VLANs that will be used router-on-a-stick at.The following VLAN information:

VLAN 20: 20.20.20.0 / 24VLAN 40: 40.40.40.0 / 24

The switch port that the router's FastEthernet is connected, must be in the mode of network connectivity, and you must know the protocol on the network service. We go with the Cisco-proprietary ISL here.

The physical FE port on the router an IP address. The use of router-on-a-stick requires the use of logical subinterfaces. Although we are not the VLAN numbers for the number of sub-interface, I found this helps the right line interfaces. A sub-interface must be provided an IP address in VLAN 20 and the other has an IP address in VLAN 40th

After creating subinterfaces fast 0.20 and fast 0.40, the configuration is similar to the following:

interface FastEthernet0No IP addressInterface FastEthernet 0.20IP address 20.20.20.1 255.255.255.0Interface FastEthernet 0.40IP address 40.40.40.1 255.255.255.0

Believe it or not is almost done! Now we need the encapsulation statement under each subinterface. The explanation of sub-interface is taken into account both the number and type of VLAN encapsulation used. When you are finished, the configuration should look like this:

interface FastEthernet0No IP addressInterface FastEthernet 0.20IP address 20.20.20.1 255.255.255.0Encapsulation ISL 20Interface FastEthernet 0.40IP address 40.40.40.1 255.255.255.0Encapsulation ISL 40

And that's it! Your hosts in VLAN 20 should now be able to communicate with the hosts in 40 VLAN, and vice versa.

A couple of final troubleshooting points - the most common mistakes in router-on-a-stick is the wrong number in the state of VLAN encapsulation provided. Also, make sure you configure the router's IP address in VLAN 20 as do the default gateway for hosts in VLAN 20, and the same for VLAN 40.

I hope you enjoy this look at router-on-a-stick. While the name may make people laugh, is is still widely used in some networks out there, and how to configure and earn even more difficult to troubleshoot, the CCNA and CCNP.