Lab – Configuring and Verifying VTY Restrictions

Topology

Addressing Table

Device Interface IP Address Subnet Mask Default Gateway

R1 G0/0 192.168.0.1 255.255.255.0 N/A

G0/1 192.168.1.1 255.255.255.0 N/A
S1 VLAN 1 192.168.1.2 255.255.255.0 192.168.1.1
PC-A NIC 192.168.1.3 255.255.255.0 192.168.1.1
PC-B NIC 192.168.0.3 255.255.255.0 192.168.0.1

Objectives
Part 1: Configure Basic Device Settings
Part 2: Configure and Apply the Access Control List on R1
Part 3: Verify the Access Control List Using Telnet
Part 4: Challenge - Configure and Apply the Access Control List on S1

Background / Scenario
It is a good practice to restrict access to the router management interfaces, such as the console and vty lines.
An access control list (ACL) can be used to allow access for specific IP addresses, ensuring that only the
administrator PCs have permission to telnet or SSH into the router.
Note: In the Cisco device outputs, ACL is abbreviated as access-list.
In this lab, you will create and apply a named standard ACL to restrict remote access to the router vty lines.
After the ACL has been created and applied, you will test and verify the ACL by accessing the router from
different IP addresses using Telnet.
This lab will provide the commands necessary for creating and applying the ACL.
Note: The routers used with CCNA hands-on labs are Cisco 1941 Integrated Services Routers (ISRs) with
Cisco IOS Release 15.2(4)M3 (universalk9 image). The switches used are Cisco Catalyst 2960s with Cisco
IOS Release 15.0(2) (lanbasek9 image). Other routers, switches, and Cisco IOS versions can be used.
Depending on the model and Cisco IOS version, the commands available and output produced might vary
from what is shown in the labs. Refer to the Router Interface Summary Table at the end of the lab for the
correct interface identifiers.
Note: Make sure that the routers and switches have been erased and have no startup configurations. If you
are unsure, contact your instructor.

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Lab – Configuring and Verifying VTY Restrictions

Required Resources
 1 Router (Cisco 1941 with Cisco IOS Release 15.2(4)M3 universal image or comparable)
 1 Switch (Cisco 2960 with Cisco IOS Release 15.0(2) lanbasek9 image or comparable)
 2 PCs (Windows 7, Vista, or XP with terminal emulation program, such as Tera Term)
 Console cables to configure the Cisco IOS devices via the console ports
 Ethernet cables as shown in the topology
Note: The Gigabit Ethernet interfaces on Cisco 1941 routers are autosensing and an Ethernet straight-
through cable may be used between the router and PC-B. If using another model Cisco router, it may be
necessary to use an Ethernet crossover cable.

Part 1: Configure Basic Device Settings

In Part 1, you will set up the network topology and configure the interface IP addresses, device access, and
passwords on the router.

Step 1: Cable the network as shown in the topology diagram.

Step 2: Configure the PC-A and PC-B network settings according to the Addressing Table.

Step 3: Initialize and reload the router and switch.

a. Console into the router and enter global configuration mode.
b. Copy the following basic configuration and paste it to the running-configuration on the router.
no ip domain-lookup
hostname R1
service password-encryption
enable secret class
banner motd #
Unauthorized access is strictly prohibited. #
Line con 0
password cisco
login
logging synchronous
line vty 0 4
password cisco
login
c. Configure IP addresses on the interfaces listed in the Addressing Table.
d. Save the running configuration to the startup configuration file.
e. Console into the switch and enter global configuration mode.
f. Copy the following basic configuration and paste it to the running-configuration on the switch.
no ip domain-lookup
hostname S1
service password-encryption
enable secret class

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Lab – Configuring and Verifying VTY Restrictions

banner motd #
Unauthorized access is strictly prohibited. #
Line con 0
password cisco
login
logging synchronous
line vty 0 15
password cisco
login
exit
g. Configure IP address on VLAN1 interface listed in the Addressing Table.
h. Configure the default gateway for the switch.
i. Save the running configuration to the startup configuration file.

Part 2: Configure and Apply the Access Control List on R1

In Part 2, you will configure a named standard ACL and apply it to the router virtual terminal lines to restrict
remote access to the router.

Step 1: Configure and apply a standard named ACL.

a. Console into the router R1 and enable privileged EXEC mode.
b. From global configuration mode, view the command options under ip access-list by using a space and a
question mark.
R1(config)# ip access-list ?

c. View the command options under ip access-list standard by using a space and a question mark.
R1(config)# ip access-list standard ?

d. Add ADMIN-MGT to the end of the ip access-list standard command and press Enter. You are now in
the standard named access-list configuration mode (config-std-nacl).
R1(config)# ip access-list standard ADMIN-MGT
e. Enter your ACL permit or deny access control entry (ACE), also known as an ACL statement, one line at
a time. Remember that there is an implicit deny any at the end of the ACL, which effectively denies all
traffic. Enter a question mark to view your command options.
R1(config-std-nacl)# ?

f. Create a permit ACE for Administrator PC-A at 192.168.1.3 and an additional permit ACE to allow other
reserved administrative IP addresses from 192.168.1.4 to 192.168.1.7. Notice how the first permit ACE
signifies a single host by using the host keyword. The ACE permit 192.168.1.3 0.0.0.0 could have been
used instead. The second permit ACE allows hosts 192.168.1.4 through 192.168.1.7, by using the 0.0.0.3
wildcard, which is the inverse of a 255.255.255.252 subnet mask.
R1(config-std-nacl)# permit host 192.168.1.3
R1(config-std-nacl)# permit 192.168.1.4 0.0.0.3
R1(config-std-nacl)# exit

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Lab – Configuring and Verifying VTY Restrictions

You do not need to enter a deny ACE because there is an implicit deny any ACE at the end of the ACL.
g. Now that the named ACL is created, apply it to the vty lines.
R1(config)# line vty 0 15
R1(config-line)# access-class ADMIN-MGT in
R1(config-line)# exit

Part 3: Verify the Access Control List Using Telnet

In Part 3, you will use Telnet to access the router, verifying that the named ACL is functioning correctly.
Note: SSH is more secure than Telnet; however, SSH requires that the network device be configured to
accept SSH connections. Telnet is used with this lab for convenience.
a. Open a command prompt on PC-A and verify that you can communicate with the router by issuing a ping
command.
C:\Users\user1> ping 192.168.1.1

b. Using the command prompt on PC-A, launch the Telnet client program to telnet into the router. Enter the
login and then the enable passwords. You should be successfully logged in, see the banner message,
and receive an R1 router command prompt.
C:\Users\user1> telnet 192.168.1.1

Was the Telnet connection successful? ____________________________________________________

c. Type exit at the command prompt and press Enter to exit the Telnet session.
d. Change your IP address to test if the named ACL blocks non-permitted IP addresses. Change the IPv4
address to 192.168.1.100 on PC-A.
e. Attempt to telnet into R1 at 192.168.1.1 again. Was the Telnet session successful?
____________________________________________________________________________________
What message was received? ___________________________________________________________
f. Change the IP address on PC-A to test if the named ACL permits a host with an IP address from the
192.168.1.4 to 192.168.1.7 range to telnet into the router. After changing the IP address on PC-A, open a
Windows command prompt and attempt to telnet into router R1.
Was the Telnet session successful?
____________________________________________________________________________________
____________________________________________________________________________________
g. From privileged EXEC mode on R1, type the show ip access-lists command and press Enter. From the
command output, notice how the Cisco IOS automatically assigns line numbers to the ACL ACEs in
increments of 10 and shows the number of times each permit ACE has been successfully matched (in
parenthesis).
R1# show ip access-lists

Because two successful Telnet connections to the router were established, and each Telnet session was
initiated from an IP address that matches one of the permit ACEs, there are matches for each permit
ACE.

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Lab – Configuring and Verifying VTY Restrictions

Why do you think that there are two matches for each permit ACE when only one connection from each
IP address was initiated?
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
How would you determine at what point the Telnet protocol causes the two matches during the Telnet
connection?
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
h. On R1, enter into global configuration mode.
i. Enter into access-list configuration mode for the ADMIN-MGT named access list and add a deny any
ACE to the end of the access list.
R1(config)# ip access-list standard ADMIN-MGT
R1(config-std-nacl)# deny any
R1(config-std-nacl)# exit
Note: Because there is an implicit deny any ACE at the end of all ACLs, adding an explicit deny any
ACE is unnecessary. However; the explicit deny any at the end of the ACLis can still be useful to the
network administrator to log or simply know how many times the deny any access-list ACE was matched.
j. Try to telnet from PC-B to R1. This creates a match to the deny any ACE in the ADMIN-MGT named
access list.
k. From privileged EXEC mode, type show ip access-lists command and press Enter. You should now see
multiple matches to the deny any ACE.
R1# show ip access-lists

The failed Telnet connection produces more matches to the explicit deny ACE than a successful one.
Why do you think this happens?
____________________________________________________________________________________
____________________________________________________________________________________

Part 4: Challenge - Configure and Apply the Access Control List on S1

Step 1: Configure and apply a standard named ACL for the vty lines on S1.
a. Without referring back to the R1 configuration commands, try to configure the ACL on S1, allowing only
the PC-A IP address.
b. Apply the ACL to the S1 vty lines. Remember that there are more vty lines on a switch than a router.

Step 2: Test the vty ACL on S1.

Telnet from each of the PCs to verify that the vty ACL is working properly. You should be able to telnet to S1
from PC-A, but not from PC-B.

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Lab – Configuring and Verifying VTY Restrictions

Reflection
1. As evidenced by the remote vty access, ACLs are powerful content filters that can be applied to more than
just inbound and outbound network interfaces. What other ways might ACLs be applied?
_______________________________________________________________________________________
_______________________________________________________________________________________
2. Does an ACL applied to a vty remote management interface improve the security of Telnet connection? Does
this make Telnet a more viable remote access management tool?
_______________________________________________________________________________________
_______________________________________________________________________________________
_______________________________________________________________________________________
3. Why does it make sense to apply an ACL to vty lines instead of specific interfaces?
_______________________________________________________________________________________
_______________________________________________________________________________________
_______________________________________________________________________________________

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