Answers: ROAS Basics 1

By certskills March 12, 2016 09:05

ROAS pulls many concepts together: IP addressing, VLANs, and VLAN trunking. Plus ROAS requires configuration on both a router and switch. This post lists the answers to the previous lab – check out the lab, and check here for the answers.



Figure 1: ROAS with three VLAN’s

Example 3: R1 Config

Example 4: SW1 Config



The configuration of ROAS can often be confusing for those new to routers and VLANs on switches. ROAS configuration allows a router to be used to route between multiple VLANs using a single trunk interface which is connected into the switched network. Using one router interface has an obvious advantage versus having to use a different physical router interface per VLAN. Having to use one physical router interface per VLAN would cause us to quickly run out of physical interfaces on all but the smallest networks.

For this lab, you were tasked with configuring a ROAS configuration on R1, with matching configuration on switch SW1. The lab requirements asked you to use subinterfaces only, and to use a subinterface number equal to the VLAN ID for subnet (the subnet on the left). These instructions require several steps of analysis, as follows:

  1. Per the figure, subnet is listed with port G0/1.
  2. Per the initial configuration of switch SW1, G0/1 is an access port, with no access VLAN defined, so it must be in VLAN 1.
  3. Per the requirements, the R1 configuration should use subinterface .1 for subnet
  4. VLAN 1 is the default VLAN to be used as the native VLAN.

All of these facts combine to result in the first group of three commands in Example 3. Note that because the native VLAN is used with a subinterface, the command encapsulation dot1q 1 native needed to include the native keyword.

The requirements asked that you use the next two subinterface numbers, so Example 3 shows subinterfaces .2 and .3. Note that the subinterface numbers do not have to match the VLAN IDs. To connect to VLAN 20, R1 uses the encapsulation dot1q 20 subcommand under subinterface .2. Similarly, R1 uses the encapsulation dot1q 30 subcommand under the .3 subinterface to connect to VLAN 30.

Finally, note that ROAS would not work without switch configuration as well. Example 4 shows the required configured on SW4’s G1/0 interface, switchport mode trunk, to manually enable 802.1Q trunking on the link connected to R1.

ROAS Basics 1
VLAN Basics 3
By certskills March 12, 2016 09:05
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  1. Amine November 22, 19:11

    Hi Wendell,
    Hope you doing well,
    Thank you very much for the blog and the book
    I have great respect/inspiration/admiration for you, All the best.

    Reply to this comment
  2. Harold_G February 19, 11:45

    Hi Wendell,

    Thanks for all this material that allow us to improve day by day. Just a question, for this example will be just fine if I set up the no in a sub-interface but instead in the G0/1 on R1? Thinking that traffic on vlan1 shouldn’t use any tag.

    Reply to this comment
    • CCENTSkills February 28, 15:17

      Hey Harold,
      Sure, not a problem. But note:
      The key is “native VLAN”, not “VLAN 1” as the VLAN that doesn’t use a tag. In this lab, VLAN 1 is the native VLAN. Just saw your comment and wanted to make sure.
      Yes, your suggested config will work. It won’t match the literal lab requirements, but that’s ok with me – the labs are just to give us something to work towards. For real life, I’m good with your alternative.

      Reply to this comment
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