


	More Testimonials >

Visit my blog for free daily Cisco CCNA and CCNP certification questions, my latest free articles and tutorials, and more!

Cisco CCNA 640-802 And CCENT Certification Exam Training :

Troubleshooting "Router On A Stick"

By Chris Bryant, CCIE #12933

In the first two parts of this CCNA 640-802 certification exam tutorial on when to configure router-on-a-stick, we talked about the reasons why we'll use ROAS along with a basic configuration. Here's the network we're using:

Router On A Stick CCNA 640-802 Lab

I think you'll agree with me that the ROAS config is very straightforward, but it is commonly misconfigured. Since there's not much to configure in the first place, the misconfiguration is pretty easy to spot!

Since we perform most of the ROAS config on the router, we tend to concentrate on the router config when we have a problem. What we have to keep in mind with ROAS troubleshooting is that the problem might not be on the router - it might be on the hosts, or even the switch! Take the following configuration - what's the problem here?

Frequent ROAS Misconfig #1: Wrong Default Gateway Settings

If you spotted that right away, nice work! The default gateway settings on the hosts are backwards. The default gateway address must always be in the same subnet as the host's IP address.

What about the following network?

More CCNA Router On A Stick Troubleshooting

R1 Config:

interface FastEthernet0/0
no ip address
duplex auto
speed auto
!
interface FastEthernet0/0.2
encapsulation isl 2
ip address 172.12.4.1 255.255.255.0
no ip redirects
no snmp trap link-status
!
interface FastEthernet0/0.4
encapsulation isl 4
ip address 172.12.2.1 255.255.255.0
no ip redirects
no snmp trap link-status

This is one of the two most common ROAS configs. An IP address from VLAN 2's subnet has been applied to the subinterface with the VLAN 4 ID, and vice versa. With that config, neither host will even be able to ping its own default gateway.

Host4#ping 172.12.4.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.4.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

Host2#ping 172.12.2.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.2.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

Router-on-a-stick troubleshooting can be a little tricky without a structured approach, so I suggest the approach I've always used:

Always check the default gateway settings on the hosts first.
Make sure the port leading to the router is trunking.
On the router, make sure the IP address assigned to each subinterface is from the subnet assigned to the VLAN that's assigned to that subinterface.

Follow those three tips and you'll configure and troubleshoot ROAS successfully every time!

For more free Cisco CCNA and CCENT certification exam training, visit my exclusive Cisco CCENT / CCNA certification exam tutorial page!

Five Minutes From Now, You Can Be Studying For CCNA Exam Success Just As Thousands Of Other CCNA Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!