ROAS Basics 1

By certskills March 10, 2016 09:05

#ROAS can be one of those topics that seemed to be clear when you read about it, but when you see a sample test question, you just sit there and scratch your head wondering exactly what to do. ROAS can require you to connect several small pieces of information to complete a configuration. Today’s lab gives you practice at connecting the ideas.


Layer 2 switches forward Ethernet frames using Layer 2 logic, so that frames sent inside one VLAN flow only to devices in that same VLAN. To allow traffic between devices in different VLANs, a Layer 3 routing service must be used, typically in the form of a Layer 3 switch or a router. This lab asks you to configure routing between VLANs with one router, connected to one switch with an 802.1Q trunk, with the router using a Router on a Stick (ROAS) configuration.

Configure both router R1 and switch SW1 (as a Layer 2 switch only) so that R1 will route IP packets between the three subnets shown at the bottom of the figure. The specific rules for this lab are:

  • On the router, configure ROAS using only subinterfaces.
  • On the router, use a subinterface number to match the VLAN ID for subnet
  • On the router, for the other subinterfaces, choose the next two numbers as subinterface numbers. For example, if you picked subinterface 101 for the first subinterface, use 102 and 103 for the next two.
  • On the router, assign the IP addresses used on the trunk by referring to the figure and then choosing the numerically-lowest IP address in the respective subnets.
  • To determine the VLAN IDs to use, refer on the initial configuration of the switch plus the figure.
  • Assume all router interfaces shown in the lab are physically working
  • Assume all PCs have been configured with an address on the correct IPv4 subnet and are working.


Figure 1: ROAS with three VLAN’s

Initial Configuration

Example 1 and 2 show the beginning configuration state of R1 and SW1.

Example 1: R1 Config

Example 2: SW1 Config


Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear or other tools, configure the lab using them.

To test your solution if you happen to try it with CML/VIRL or real gear, you can check by going to each of the PCs and attempt to ping the other PCs. To verify the configuration on R1 you could either check the configuration directly with the show running-config command or by displaying the current ROAS configuration on the device by using the show vlans command.


Do this Lab with Cisco’s CML (Formerly VIRL)

You can do these labs on paper and still get a lot out of the lab. As an extra help, we have added files for Cisco Modeling Lab – Personal (CML-P). CML-P replaces Cisco Virtual Internet Routing Lab (VIRL) software, in effect serving as VIRL Version 2.

Below, find two files: a file useful with CML-P and another useful with VIRL. (Note that the CML-P file has a .yaml filetype, while the older VIRL file has a VIRL filetype.) Once the file is loaded, CML-P or VIRL will create a lab topology similar to this lab’s topology, with the initial configuration shown in the lab as well.

This lab’s CML file!

This lab’s VIRL file!

All interfaces in topology match the lab figure.

Host device info:

 This table lists host information pre-configured in CML/VIRL, information that might not be required by the lab but may be useful to you.


IP Address










Handy Host Commands:

To see PC IP address: ifconfig eth1

Ping example: ping -c 4

Trace example: tracepath

To connect to another node within the topology: telnet

Answers: IPv4 Static Routes 2
Answers: ROAS Basics 1
By certskills March 10, 2016 09:05
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