Config Lab: Standard Named ACL 1

Wendell Odom
By Wendell Odom October 1, 2021 11:05

Standard named ACLs follow a nice simple format, which is great for getting started with ACLs. This next lab gives you some exercise on the basic syntax, while throwing in a few issues related to the application of the ACL. Where should you put it? How does Router-on-a-Stick config (router trunking) impact that choice? And how could you match two consecutive subnets with one deny command? Check out this latest lab to exercise your skills and answer these questions.

All about Config Labs

The blog has a series of lab exercises called “Config Labs.” Each lab presents a topology with the relevant initial configuration for each device. The lab also lists new requirements, after which you should create the additional configuration to meet those requirements. You can do the lab on paper, in a text editor, or use software tools like Cisco Packet Tracer or Cisco Modeling Labs.

Once you have created your answer, you can click various tabs at the bottom of this post to see the lab answers, comments about the lab, and other helpful information.

The Lab Exercise

Requirements

This lab gives you a set of relatively straightforward ACL requirements, but with enough flexibility to make you think beyond just making this a configuration exercise. You will also need to review a pretty detailed initial configuration to get your bearings first. Then you have to think about where to put the ACL, on which interface, and in which direction. So it’s a good thinking lab.

The specific rules for this lab are:

  • Create a standard named ACL named “ThisACL” which performs the following functions:
    1. Block all traffic from the 10.0.1.0/26 and 10.0.1.64/26 subnets to all of the 20.0.0.0/8 subnets networks displayed in the figure, using a single command
    2. Block all traffic from the 10.0.1.254 host to all of the 20.0.0.0/8 networks displayed in the figure
    3. Permit all other traffic
  • You choose the device on which to enable the ACL, the interface, and the direction
  • You may enable the ACL on one router only, but on multiple interfaces and directions as desired
  • As seen in the initial configurations:
    1. Assume all router interfaces shown in the lab are up, working, and have correct IP addresses assigned
    2. Assume routing between all devices is configured and operational
    3. Assume that at least one device exists on each VLAN with an IP address ending in .100 with correct gateways configured.

 

Figure 1: Two Router ROAS Topology

Initial Configuration

Examples 1, 2, 3, and 4 show the beginning configuration state of R1, R2, SW1, and SW2.

Example 1: R1 Config

 

Example 2: R2 Config

 

Example 3: SW1 Config

 

Example 4: SW2 Config

Answer Options - Click Tabs to Reveal

You can learn a lot and strengthen real learning of the topics by creating the configuration – even without a router or switch CLI. In fact, these labs were originally built to be used solely as a paper exercise!

To answer, just think about the lab. Refer to your primary learning material for CCNA, your notes, and create the configuration on paper or in a text editor. Then check your answer versus the answer post, which is linked at the bottom of the lab, just above the comments section.

You can also implement the lab using the Cisco Packet Tracer network simulator. With this option, you use Cisco’s free Packet Tracer simulator. You open a file that begins with the initial configuration already loaded. Then you implement your configuration and test to determine if it met the requirements of the lab.

(Use this link for more information about Cisco Packet Tracer.)

Use this workflow to do the labs in Cisco Packet Tracer:

  1. Download the .pkt file linked below.
  2. Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
  3. Add your planned configuration to the lab.
  4. Test the configuration using some of the suggestions below.

Download this lab’s Packet Tracer File

You can also implement the lab using Cisco Modeling Labs – Personal (CML-P). CML-P (or simply CML) replaced Cisco Virtual Internet Routing Lab (VIRL) software in 2020, in effect serving as VIRL Version 2.

If you prefer to use CML, use a similar workflow as you would use if using Cisco Packet Tracer, as follows:

  1. Download the CML file (filetype .yaml) linked below.
  2. Import the lab’s CML file into CML and then start the lab.
  3. Compare the lab topology and interface IDs to this lab, as they may differ (more detail below).
  4. Add your planned configuration to the lab.
  5. Test the configuration using some of the suggestions below.

Download this lab’s CML file!

 

Network Device Info:

This table lists the interfaces used in the lab exercise documentation that differ from those used in the sample CML file.

Device Lab Port  CML Port
SW1 G1/0/1 G0/1
SW1 G1/0/2 G0/2
SW1 G1/0/3 G0/3
SW1 G1/0/4 G1/0
SW2 G1/0/1 G0/1
SW2 G1/0/2 G0/2
SW2 G1/0/3 G0/3
SW2 G1/0/4 G1/0

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers

Figure 1: Two Router ROAS Topology

 

Example 5: R2 Config

Commentary, Issues, and Verification Tips (Click Tabs to Reveal)

Commentary

Engineers use router IP ACLS for several useful purposes. Those purposes range from filtering packets as they pass through the router, based on the values in the packet header, to using those same header fields to determine which packets need to have an IP service applied – and which do not. This is why it is good to get comfortable with how they are configured, processed, and applied.

With this lab, you were tasked with configuring a standard ACL to block the traffic from two different networks and a specific host. Cisco suggests that we place standard ACLs as close to the destination as possible. In this case, the closest location is on R2. However, the requirements allow us to choose any interfaces on that one router. So, the solution shown here puts the ACL on router R2 and enables the ACL outbound on the three ROAS subinterfaces on R2’s G0/2 interface.

The one challenging matching action per the requirements is to block the traffic from the 10.0.1.0/26 and 10.0.1.64/26 subnets, but with a single command. These two named IPv4 ACL commands could be used to match and deny packets from those subnets separately:

deny 10.0.1.0 0.0.0.63

deny 10.0.1.64 0.0.0.63

To match them with one command, consider these two subnets as a single range of addresses, including the numbers from 10.0.1.0 – 10.0.1.127. That happens to be the same numbers as in subnet 10.0.1.0/25, which can be matched with the deny 10.0.1.0 0.0.0.127 command, as seen in the answer.

The command to match the single host is: deny 10.0.1.254. Note that in older versions of IOS host parameter is required in front of single matching addresses, but not today.

This lab also might have made you wonder if the ACL could have been applied to the G0/2 physical interface, in this case, filtering all IP traffic exiting the interface, and the answer is no. An ACL applied under interface G0/2 – not one of its subinterfaces – would be considered for packets routed out G0/2, but not for packets routed out of its subinterfaces. So, as shown in the answer, the ip access-group ThisACL out command is used as a subcommand on all three subinterfaces.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

# Summary Detail
1 None No known issues related to this lab.

 

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

  1. Issue the show ip access-lists and show access-lists commands to display the access-lists.
  2. Issue the show ip interfaces commands and look for the lines on each interface that identify if any ACLs are enabled, and if so, which ACLs and in what direction.

More Labs with Related Content!

Config Lab: Standard Numbered ACL 1
Config Lab: Extended Numbered ACL 1
Wendell Odom
By Wendell Odom October 1, 2021 11:05
Write a comment

3 Comments

  1. almeidajoaodealmeida October 25, 13:01

    Hi Mr. Odom,
    What do u think about the approach of configuring the ACL, on the G0/2 of R2(outbound) ?

    Reply to this comment
    • certskills October 25, 13:56

      Hello,
      Thanks for the question. Using R2’s G0/2 interface would have no effect, however, as the interface has not been configured with an IP address. Whereever you choose, the ACL must be enabled on an interface that has IP enabled by virtue of being configured with an IP address. (See the last paragraph of the commentary section for a little more about that particular subject.)
      Hope this helps,
      Wendell

      Reply to this comment
View comments

Write a comment

Comment; Identify w/ Social Media or Email

Subscribe

Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

Thank you for subscribing.

Something went wrong.

Search

Categories