5.2.7 Packet Tracer – Configure and Modify Standard IPv4 ACLs Answers

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Packet Tracer – Configure and Modify Standard IPv4 ACLs (Answers Version)

Answers Note: Red font color or gray highlights indicate text that appears in the instructor copy only.

Addressing Table

Objectives

Part 1: Verify Connectivity

Part 2: Configure and Verify Standard Numbered and Named ACLs

Part 3: Modify a Standard ACL

Background / Scenario

Network security and traffic flow control are important issues when designing and managing IP networks. The ability to configure proper rules to filter packets, based on established security policies, is a valuable skill.

In this lab, you will set up filtering rules for two business locations that are represented by R1 and R3. Management has established some access policies between the LANs located at R1 and R3, which you must implement. The Edge router sitting between R1 and R3 has been provided by the ISP will not have any ACLs placed on it. You would not be allowed any administrative access to the Edge router because you can only control and manage your own equipment.

Instructions

Part 1:  Verify Connectivity

In Part 1, you verify connectivity between devices.

Note: It is very important to test whether connectivity is working before you configure and apply access lists. You want to be sure that your network is properly functioning before you start to filter traffic.

Questions:

From PC-A, ping PC-C and PC-D. Were your pings successful?

Type your answers here.

Yes

From R1, ping PC-C and PC-D. Were your pings successful?

Type your answers here.

Yes

From PC-C, ping PC-A and PC-B. Were your pings successful?

Type your answers here.

Yes

From R3, ping PC-A and PC-B. Were your pings successful?

Type your answers here.

Yes

Can all of the PCs ping the server at 209.165.200.254?

Type your answers here.

Yes

Part 2:  Configure and Verify Standard Numbered and Named ACLs

Step 1:  Configure a numbered standard ACL.

Standard ACLs filter traffic based on the source IP address only. A typical best practice for standard ACLs is to configure and apply the ACL as close to the destination as possible. For the first access list in this activity, create a standard numbered ACL that allows traffic from all hosts on the 192.168.10.0/24 network and all hosts on the 192.168.20.0/24 network to access all hosts on the 192.168.30.0/24 network. The security policy also states that an explicit deny any access control entry (ACE), also referred to as an ACL statement, should be present at the end of all ACLs.

Questions:

What wildcard mask would you use to allow all hosts on the 192.168.10.0/24 network to access the 192.168.30.0/24 network?

Type your answers here.

0.0.0.255

Following Cisco’s recommended best practices, on which router would you place this ACL?

Type your answers here.

R3

On which interface would you place this ACL? In what direction would you apply it?

Type your answers here.

G0/0/0. The ACL should be applied going out. Students may answer with placing the ACL on the S0/1/1 interface on R3 going in. Emphasize to them that this would effectively block the LANs on R1 from getting to the 192.168.40.0/24 network as well!

1. Configure the ACL on R3. Use 1 for the access list number.

Open configuration window

R3(config)# access-list 1 remark Allow R1 LANs Access

R3(config)# access-list 1 permit 192.168.10.0 0.0.0.255

R3(config)# access-list 1 permit 192.168.20.0 0.0.0.255

R3(config)# access-list 1 deny any

2. Apply the ACL to the appropriate interface in the proper direction.

R3(config)# interface g0/0/0

R3(config-if)# ip access-group 1 out

3. Verify a numbered ACL.

The use of various show commands can help you to verify both the syntax and placement of your ACLs in your router.

Questions:

To see access list 1 in its entirety with all ACEs, which command would you use?

Type your answers here.

R3# show access-lists 1

or

R3# show access-lists

What command would you use to see where the access list was applied and in what direction?

Type your answers here.

R3# show ip interface g0/0/0

or

R3# show ip interface

1)      On R3, issue the show access-lists 1 command.

R3# show access-list 1

Standard IP access list 1

    permit 192.168.10.0, wildcard bits 0.0.0.255

    permit 192.168.20.0, wildcard bits 0.0.0.255

    deny any

2)      On R3, issue the show ip interface g0/0/0 command.

R3# show ip interface g0/0/0

GigabitEthernet0/0/0 is up, line protocol is up (connected)

  Internet address is 192.168.30.1/24

  Broadcast address is 255.255.255.255

  Address determined by setup command

  MTU is 1500 bytes

  Helper address is not set

  Directed broadcast forwarding is disabled

  Outgoing access list is 1

  Inbound access list is not set

   <Output omitted>

Questions:

3)      Test the ACL to see if it allows traffic from the 192.168.10.0/24 network to access the 192.168.30.0/24 network.

From the PC-A command prompt, ping the PC-C IP address. Were the pings successful?

Type your answers here.

Yes

4)      Test the ACL to see if it allows traffic from the 192.168.20.0/24 network access to the 192.168.30.0/24 network.

From the PC-B command prompt, ping the PC-C IP address. Were the pings successful?

Type your answers here.

Yes

5)      Should pings from PC-D to PC-C be successful? Ping from PC-D to PC-C to verify your answer.

Type your answers here.

No, the pings should not be successful. Trying the ping verifies that the ACL is working as intended.

4. From the R1 prompt, ping PC-C’s IP address again.

R1# ping 192.168.30.3

Question:

Was the ping successful? Explain.

Type your answers here.

No, the pings failed. When you ping from the router, it uses the closest interface to the destination as its source address. The pings had a source address of 10.1.1.1. The access list on R3 only allows the 192.168.10.0/24 and the 192.168.20.0/24 networks access.

5. Issue the show access-lists 1 command again. Note that the command output displays information for the number of times each ACE was matched by traffic that reached interface Gigabit Ethernet 0/0/0.

R3# show access-lists 1

Standard IP access list 1

    permit 192.168.10.0 0.0.0.255 (4 match(es))

    permit 192.168.20.0 0.0.0.255 (4 match(es))

    deny any (4 match(es))

Step 2:  Configure a named standard ACL.

Create a named standard ACL that conforms to the following policy: allow traffic from all hosts on the 192.168.40.0/24 network access to all hosts on the 192.168.10.0/24 network. Also, only allow host PC-C access to the 192.168.10.0/24 network. The name of this access list should be called BRANCH-OFFICE-POLICY.

Questions:

Following Cisco’s recommended best practices, on which router would you place this ACL?

Type your answers here.

R1

On which interface would you place this ACL? In what direction would you apply it?

Type your answers here.

G0/0/0. The ACL should be applied going out. Students may answer with placing the ACL on the S0/0/0 interface on R1 going in. Emphasize to them that this would effectively block all traffic from the LANs on R3 from getting to the 192.168.20.0/24 network.

1. Create the standard named ACL BRANCH-OFFICE-POLICY on R1.

R1(config)# ip access-list standard BRANCH-OFFICE-POLICY

R1(config-std-nacl)# permit host 192.168.30.3

R1(config-std-nacl)# permit 192.168.40.0 0.0.0.255

R1(config-std-nacl)# end

R1#

*Feb 15 15:56:55.707: %SYS-5-CONFIG_I: Configured from console by console

Question:

Look at the first ACE in the access list. What is another way to write this?

Type your answers here.

permit 192.168.30.3 0.0.0.0

2. Apply the ACL to the appropriate interface in the proper direction.

R1# config t

R1(config)# interface g0/0/0

R1(config-if)# ip access-group BRANCH-OFFICE-POLICY out

3. Verify a named ACL.

1)      On R1, issue the show access-lists command.

R1# show access-lists

Standard IP access list BRANCH-OFFICE-POLICY

    10 permit host 192.168.30.3

    20 permit 192.168.40.0 0.0.0.255

Question:

Is there any difference between this ACL on R1 and the ACL on R3? If so, what is it?

Type your answers here.

Although there is no line 30 with a deny any on R1, it is implied. You may wish to emphasize this to your students. Having them explicitly configure the deny any ACE is a good practice and reinforces the concept because it shows up in the output of the show access-lists command. It is easy to forget the implicit deny any when troubleshooting ACLs. This could easily result in traffic being denied that should have been allowed. In addition, if the explicit deny any ACE is present, it can be logged, and the number of matches for the ACE condition can be viewed with show access-lists.

2)      On R1, issue the show ip interface g0/0/0 command to verify that the ACL is configured on the interface.

R1# show ip interface g0/0/0

GigabitEthernet0/0/0 is up, line protocol is up (connected)

  Internet address is 192.168.10.1/24

  Broadcast address is 255.255.255.255

  Address determined by setup command

  MTU is 1500 bytes

  Helper address is not set

  Directed broadcast forwarding is disabled

  Outgoing access list is BRANCH-OFFICE-POLICY

  Inbound  access list is not set

<Output omitted>

Question:

Test the ACL. From the command prompt on PC-C, ping the IP address of PC-A. Were the pings successful?

Type your answers here.

Yes

3)      Test the ACL to ensure that only the PC-C host is allowed access to the 192.168.10.0/24 network. You must do an extended ping and use the G0/0/0 address on R3 as your source. Ping PC-A’s IP address.

R3# ping

Protocol [ip]:

Target IP address: 192.168.10.3

Repeat count [5]:

Datagram size [100]:

Timeout in seconds [2]:

Extended commands [n]: y

Source address or interface: 192.168.30.1

Type of service [0]:

Set DF bit in IP header? [no]:

Validate reply data? [no]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.10.3, timeout is 2 seconds:

Packet sent with a source address of 192.168.30.1

U.U.U

Question:

Were the pings successful?

Type your answers here.

No

4)      Test the ACL to see if it allows traffic from the 192.168.40.0/24 network access to the 192.168.10.0/24 network. From the PC-D command prompt, ping the PC-A IP address.

Question:

Were the pings successful?

Type your answers here.

Yes

Close configuration window

Part 3:  Modify a Standard ACL

It is common in business for security policies to change. For this reason, ACLs may need to be modified. In Part 3, you will change one of the ACLs you configured previously to match a new management policy that is being put in place.

Attempt to ping the server at 209.165.200.254 from PC-A. Notice that the ping is not successful. The ACL on R1 is blocking internet traffic from returning to PC-A. This is because the source address in the packets that are returned is not in the range of permitted addresses.

Management has decided that traffic that is returning from the 209.165.200.224/27 network should be allowed full access to the 192.168.10.0/24 network. Management also wants ACLs on all routers to follow consistent rules. A deny any ACE should be placed at the end of all ACLs. You must modify the BRANCH-OFFICE-POLICY ACL.

You will add two additional lines to this ACL. There are two ways you could do this:

OPTION 1: Issue a no ip access-list standard BRANCH-OFFICE-POLICY command in global configuration mode. This would remove the ACL from the router. Depending upon the router IOS, one of the following scenarios would occur: all filtering of packets would be cancelled, and all packets would be allowed through the router; or, because you did not remove the ip access-group command from the G0/1 interface, filtering is still in place. Regardless, when the ACL is gone, you could retype the whole ACL, or cut and paste it in from a text editor.

OPTION 2: You can modify ACLs in place by adding or deleting specific lines within the ACL itself. This can come in handy, especially with ACLs that are long. The retyping of the whole ACL or cutting and pasting can easily lead to errors. Modifying specific lines within the ACL is easily accomplished.

For this activity, use Option 2.

Step 1:  Modify a named standard ACL.

1. From R1, issue the show access-lists command.

Open configuration window

R1# show access-lists

Standard IP access list BRANCH-OFFICE-POLICY

    10 permit 192.168.30.3 (8 matches)

    20 permit 192.168.40.0 0.0.0.255 (5 matches)

2. Add two additional lines at the end of the ACL. From global config mode, modify the ACL, BRANCH-OFFICE-POLICY.

R1#(config)# ip access-list standard BRANCH-OFFICE-POLICY

R1(config-std-nacl)# 30 permit 209.165.200.224 0.0.0.31

R1(config-std-nacl)# 40 deny any

R1(config-std-nacl)# end

3. Verify the ACL.

1)      On R1, issue the show access-lists command.

R1# show access-lists

Standard IP access list BRANCH-OFFICE-POLICY

    10 permit 192.168.30.3 (8 matches)

    20 permit 192.168.40.0, wildcard bits 0.0.0.255 (5 matches)

    30 permit 209.165.200.224, wildcard bits 0.0.0.31

    40 deny any

Question:

Do you have to apply the BRANCH-OFFICE-POLICY to the G0/1 interface on R1?

Type your answers here.

No, the ip access-group BRANCH-OFFICE-POLICY out command is still in place on G0/1.

2)      Test the ACL to see if it allows traffic from the 209.165.200.224/27 network access to return to the 192.168.10.0/24 network. From PC-A, ping the server at 209.165.200.254.

Question:

Were the pings successful?

Type your answers here.

Yes

Close configuration window

Reflection Questions

1. As you can see, standard ACLs are very powerful and work quite well. Why would you ever have the need for using extended ACLs?

Type your answers here.

Standard ACLs can only filter based on the source address. Also, they are not granular. They allow or deny everything (all protocols and services). Extended ACLs, while harder to write, are well-suited for complex networks where you may need to allow traffic for only certain Layer 4 ports to have access to networks while denying others. In addition, standard ACLs must be applied as close to the destination as possible. This allows unnecessary traffic to use network bandwidth. Extended ACLs can block traffic close to the source. This prevents unnecessary traffic from traveling to the destination where it is blocked.

2. More typing is typically required when using a named ACL as opposed to a numbered ACL. Why would you choose named ACLs over numbered?

Type your answers here.

Students could list two reasons here. The first reason is that using named ACLs gives you the ability to modify specific lines within the ACL itself, without retyping the entire list. NOTE: Newer versions of the IOS allows numbered ACLs to be modified just liked named ACLs. Secondly, having a named ACL is a good best practice as it helps to document the purpose of the ACL with a descriptive name.

End of document

Answer Scripts

Router R1

enable

configure terminal

ip access-list standard BRANCH-OFFICE-POLICY

 permit host 192.168.30.3

 permit 192.168.40.0 0.0.0.255

interface g0/0/0

 ip access-group BRANCH-OFFICE-POLICY out

ip access-list standard BRANCH-OFFICE-POLICY

 30 permit 209.165.200.224 0.0.0.31

 40 deny any

end

Router R3

enable

configure terminal

access-list 1 remark Allow R1 LANs Access

access-list 1 permit 192.168.10.0 0.0.0.255

access-list 1 permit 192.168.20.0 0.0.0.255

access-list 1 deny any

interface g0/0/0

 ip access-group 1 out

end

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