Config Lab: Standard Numbered ACL 1

Wendell Odom
By Wendell Odom October 1, 2021 09:05

Write a 1-line ACL to match something? Easy. Write a several line ACL with a bunch of requirements? A little harder. Choosing where to put that multi-line ACL, when the ACL requirements span multiple subnets? Yet another bump. Combining all that with Router-as-a-Stick (ROAS) config, choosing the interface can be a challenge. Today’s lab gives you all that! Check it out.

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


This lab gives you a set of relatively straightforward ACL requirements, but with enough flexibility to make you think beyond just making this an access-list command syntax 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 numbered ACL that performs the following functions:
    • Block all traffic from the network to all of the subnets displayed in the figure
    • Block all traffic from host to all of the subnets displayed in the figure
    • Block all traffic from host to all of the subnets displayed in the figure
    • Permit all other traffic
  • You choose the device on which to enable the ACL, the interface, and the direction
  • You may enable the ACL in one place only, in one direction only
  • As seen in the initial configurations:
    • Assume all router interfaces shown in the lab are up and working, plus have correct IP addresses assigned
    • Assume routing between all devices is configured and operational
    • Assume that at least one host 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 initial 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)


Figure 1: Two  Router ROAS Topology


Example 5: R1 Config

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


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 one and two different specific hosts for subnets connected to router R2. The solution I chose matches a source IP address of the subnets or addresses that begin with 20, located in connected subnets off router R2. That leaves the choice of where to place the ACL in the network.

As for where to place the ACL:

The lab said all switches were layer 2 switches, so a layer 3 ACL could not be applied on the two switches. So the ACL would need to be applied on routers R1 or R2.

For the direction from R2’s LAN subnets towards R1’s LAN subnets, the options for interfaces were:

  • R2’s G0/2 (and/or subinterfaces) inbound
  • R2’s G0/1 outbound
  • R1’s G0/1 inbound
  • R2’s G0/2 (and/or subinterfaces) outbound

The lab exercise asks for a standard ACL. Standard ACLs should be applied as close to the destination as possible – in this case,  router R1. However, the lab requirements made an extra requirement: you can only enable the ACL on one interface and one direction. This begs the question: could you apply the ACL outbound on R1’s G0/2 interface and have it filter traffic on both that physical interface and all the subinterfaces? The answer is no. IP ACLs must be applied on the interface where the IP addresses are configured, so to place them to filter outbound traffic on R1’s G0/2 interface, you would actually need to enable them on G0/2.1, G0/2.2, and G0/2.3. So that one requirement to enable the ACL in one location only disallows the use of the ACL on R1’s subinterfaces.

The sample answer uses the close-to-the-destination (R1) solution but is inbound on R1’s G0/1 interface. The actual access-list commands are relatively straightforward: access-list 1 deny, access-list 1 deny, access-list 1 deny, and access-list 1 permit any. (Note: In older versions of IOS, it is required to use the host parameter in front of single matching addresses).

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.
  3. Add some hosts to the topology and use some ping and traceroute commands to generate traffic and test the ACLs. Because all the requirements mention IP packets only and not specific applications, you can use any command to drive traffic to test the ACL.

More Labs with Related Content!

Config Lab: IPv6 Special Addresses 2
Config Lab: Standard Named ACL 1
Wendell Odom
By Wendell Odom October 1, 2021 09:05
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