#CCENT and #CCNA Fast Start: Router on a Stick (ROAS) Overview

By certskills May 8, 2013 09:05

To help get you started with the new #CCENT and #CCNA exams, I’ve planned a series of blog posts on some of the new and more prominent exam topics for the new exams. By now, anyone beginning CCENT or CCNA study should be headed for the new exams. This series will explore several of the new topics, and topics that now have more emphasis, to help you get off to a great start with the new exams.

Today’s topic: ROAS. Cool acronym, and was in #CCNA in the old days (before the March 2013 changes), but it gets a little more focus in the new exams. Today’s post hits the highlights of Router on a Stick, now squarely in the ICND1 and CCENT part of the Cisco cert equation.

The Exam Topics: Old and New

Cisco reveals what’s on each certification exam through their published exam topics. Router on a Stick, or ROAS, was in the ICND2 half of the CCNA Routing and Switching breakdown, in the ICND2 half of the mix, as:

Configure, verify, and troubleshoot interVLAN routing

As worded, you could argue whether or not “interVLAN routing” included ROAS or not. It is in my books related to the old exams.

Cisco’s new exam topics starting in March 2013 clearly state that ROAS exists in the new ICND1 100-101 exam and CCNA 200-120 exam. Cisco lists about three times the detail in their exam topics for the new exams versus the old. (The new exam does not have three times the topics; Cisco just gives us more information about the details.) In this case, the new ICND1 100-101 exam topics lists:

Configure and verify interVLAN routing (Router on a stick)

    • sub interfaces
    • upstream routing
    • encapsulation

Clearly, the new exam includes ROAS, and several specific features. It also exists in the CCNA exam topics.

Figure 1: ROAS Location in the ICND1 & ICND2 Topic Breakdown

ROAS Overview

ROAS refers to a router feature with which the router can connect to multiple LAN-based subnets using a single physical interface. A router with one LAN interface, connected to a LAN switch, acts to support the multiple subnets associated with the multiple VLANs defined by that LAN switch. The router must route packets between multiple subnets, over one physical interface. The router is “on a stick” because the drawing looks like a router with one link to the LAN switch, like a stick, as shown in Figure 2.

Figure 2: ROAS: One Router LAN Interface, >1 VLAN and Subnet

ROAS can be a challenge for several reasons. First, the router and switch must both use VLAN trunking, and it must be statically defined. The static definition means that the configuration on both the router and switch must match, and if not, problems can occur.

The router uses subinterfaces: logical interfaces that subdivide the router’s logic as applies to what happens on a physical interface. ROAS is the first feature that most people see in their Cisco journey that happens to use subinterfaces.

ROAS can also be a challenge because it combines both layer 2 and layer 3 concepts. ROAS exists to route IP packets (a layer 3 concept). However, the means to connect to these layer 3 subnets requires VLAN trunking (a layer 2 concept). That combination can cause a little heartburn.

More on ROAS

The new ICND1 Official Cert Guide gets into some depth on ROAS, for concepts, configuration, and verification. Note that the book also gets into layer 3 switch concepts and configuration as well.

Next post: a sample question that uses ROAS.

What People are Thinking About the New #CCENT and #CCNA (Polls)
#CCENT and #CCNA Fast Start: a ROAS Question
By certskills May 8, 2013 09:05
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May 10, 2013 9:06 am

[…] Introduction to this topic as it exists in the new CCENT and CCNA exams: […]

November 25, 2013 9:15 am

[…] Link to the question Link to the brief topic introduction […]

Router Guy
December 22, 2013 10:58 pm

I’m an IT guy, been working with computers for over 15 years and have had a number of routers. I always fell back on Linksys routers, I’ve tried Buffalo brand and others but Linksys was always the reliable go to brand. Well, I’ve had 2 linksys routers in the past year that had to be replaced. The first one died after many solid months of performance. I reached out for support and got no response. I couldn’t go without a router so I bought a buffalo router, it worked sort of but was overly complex, it went back and I bought a new Linksys (Cisco) N router, it’s been working fine for a few months and suddenly my wireless devices started to disconnect randomly. Long story short after a lot of troubleshooting and attempts to get service with no response from Linksys/Cisco I gave in and decided to get a replacement.