Passing Your CCNA and CCNP: Configuring And Troubleshooting
Router-On-A-Stick
For CCNA and CCNP candidates, it's hard not to laugh the first
time you hear the phrase "router on a stick". Let's face it,
that's a pretty silly term. But as those who have passed the
CCNA and CCNP exams know, this is a vital exam topic that you
must know how to configure and troubleshoot.
Basic Cisco theory states that for hosts in different VLANs to
communicate, a Layer 3 device must be involved to handle the
routing between the VLANs. That device is a router, and there
are special considerations that must be taken into account for
both the physical router itself and the configuration you'll be
writing.
The router will be connected to a switch via a FastEthernet port
(or higher). The router port cannot be a regular Ethernet port,
since the router port will need the ability to send and receive
data at the same time. The configuration of the interface is
where things get interesting. Let's say we have two VLANs that
will be using router-on-a-stick to communicate. Here is the VLAN
information:
VLAN 20: 20.20.20.0 /24
VLAN 40: 40.40.40.0 /24
The port on the switch that will be connected to the router's
FastEthernet port must be in trunking mode, and you must know
the trunking protocol in use. We'll go with the
Cisco-proprietary ISL here.
The physical FE port on the router will not have an IP address.
The use of router-on-a-stick mandates the use of logical
subinterfaces. While we don't have to use the VLAN numbers for
the subinterface numbers, I've found this helps you keep the
interfaces straight. One subinterface must be given an IP
address in VLAN 20, and the other will have an IP address in
VLAN 40.
After creating subinterfaces fast 0.20 and fast 0.40, the config
looks like this:
interface fastethernet0
no ip address
interface FastEthernet 0.20
ip address 20.20.20.1 255.255.255.0
interface FastEthernet 0.40
ip address 40.40.40.1 255.255.255.0
Believe it or not, you're almost done! Now we need the
encapsulation statement under each subinterface. The
subinterface statement must reflect both the VLAN number and the
encapsulation type being used. When we're finished, the config
would look like this:
interface fastethernet0
no ip address
interface FastEthernet 0.20
ip address 20.20.20.1 255.255.255.0
encapsulation isl 20
interface FastEthernet 0.40
ip address 40.40.40.1 255.255.255.0
encapsulation isl 40
And that's it! Your hosts in VLAN 20 should now be able to
communicate with hosts in VLAN 40, and vice versa.
A couple of final troubleshooting points - the most common error
with router-on-a-stick is to put the wrong vlan number in the
encapsulation statement. Also, make sure you have configured the
router's IP address in VLAN 20 as the default gateway for hosts
in VLAN 20, and do the same for VLAN 40.
I hope you've enjoyed this look at router-on-a-stick. While the
name may get a chuckle out of you, it's still used in quite a
few networks out there, and knowing how to configure and
troubleshoot it will get you that much closer to earning your
CCNA and CCNP.