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Passing Your CCNA and CCNP: Configuring And Troubleshooting Router-On-A-Stick |
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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.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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