Answer to the ROAS Question

By certskills May 13, 2013 09:05

ROAS is on the new ICND1 and CCNA exams. Check. The previous post posed a question about ROAS. Check. Today’s post wraps the topic, showing the answer. Check out the question before flipping the page!

Link to the question
Link to the brief topic introduction

The Answer

(Wendell’s note to self: internal question number 111.)

Answer(s): D

The Explanation

This question basically requires you to be able to do the following: given the scenario as described in the question stem, what are the possible correct configuration options on both the router and switch? Then, which answers fit within the possible correct configuration?

Note that the new ICND1 Official Cert Guide shows a very similar example on page 441 with Example 16-2.

Figure 3: ROAS: One Router LAN Interface, >1 VLAN and Subnet

First, focus on router B1. To give the router a presence in VLAN 10, which is not the native VLAN, the router MUST use a subinterface of its G0/0 interface, and the router must have an IP address in the match subnet. Likewise, router B1 must use a subinterface to connect to VLAN 20, also not the native VLAN, with a different subinterface, and with an IP address in subnet

Also, note that B1’s configuration does NOT have to use a subinterface number that matches the VLAN. EG, B1 does not have to use subinterface G0/0.10 to connect to VLAN 10 on this trunk. B1 could use interface G0/0.10, but the answer that says that B1 “must” use that subinterface is incorrect because of the “must”. The encapsulation dot1q subcommand must use the correct VLAN number.

As a result, of the three answers that mention router B1, only answer D is correct.

The two answers that mention SW1 are both incorrect, but for different reasons. First, the answer that mentions the encapsulation dot1q 20 subcommand: that subcommand is part of a valid router configuration for VLAN trunking, but not part of a switch configuration for VLAN trunking. The other answer lists two valid switch subcommands for VLAN trunking when dynamically forming VLAN trunks between two switches. However, routers do not dynamically negotiate trunking, so the switch must be statically configured to trunk (switchport mode trunk).

More Practice Questions:

This question is like those you get if when you buy the ICND1 100-101 Official Cert Guide. This blog also lists various practice questions as well. For more questions on a large variety of topics:

#CCENT and #CCNA Fast Start: a ROAS Question
#CCENT and #CCNA Fast Start: IPv6 Addressing
By certskills May 13, 2013 09:05
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March 20, 2014 11:56 am


just a question on this one: why should we assume that neither vlans 10 or 20 may be the native vlan? in which case there isnt a single valid answer on the options you gave? or did i misunderstood something?

March 20, 2014 12:27 pm

Hi Ruissalo,
I think you missed it – the question states that the switch on the other end of the link uses the default native VLAN. It’s up to you to then know that the default native VLAN is VLAN 1.