CCNP Switch Lab 6-1 Securing Layer 2 Switches

CCNP Switch Lab 6-1 Securing Layer 2 Switches



  • Secure the Layer 2 network against MAC flood attacks.
  • Prevent DHCP spoofing attacks.
  • Prevent unauthorized access to the network using AAA and 802.1X.

A fellow network engineer that you have known and trusted for many years has invited you to lunch this week. At lunch, he brings up the subject of network security and how two of his former co-workers had been arrested for using different Layer 2 attack techniques to gather data from other users in the office for their own personal gain in their careers and finances. The story shocks you because you have always known your friend to be very cautious with security on his network. His story makes you realize that your business

network has been cautious with external threats, Layer 3–7 security, firewalls at the borders, and so on, but insufficient at Layer 2 security and protection inside the local network.

When you get back to the office, you meet with your boss to discuss your concerns. After reviewing the company’s security policies, you begin to work on a Layer 2 security policy.

First, you establish which network threats you are concerned about and then put together an action plan to mitigate these threats. While researching these threats, you learn about other potential threats to Layer 2 switches that might not be malicious but could threaten network stability. You decide to include these threats in the policies as well. Other security measures need to be put in place to further secure the network, but you begin with configuring the switches against a few specific types of attacks, including MAC flood attacks, DHCP spoofing attacks, and unauthorized access to the local network. You plan to test the configurations in a lab environment before placing them into production.

Note: This lab uses Cisco WS-C2960-24TT-L switches with the Cisco IOS image c2960-lanbasek9-mz.122- 46.SE.bin, and Catalyst 3560-24PS with the Cisco IOS image c3560-advipservicesk9-mz.122-46.SE.bin. You can use other switches (such as 2950 or 3550) and Cisco IOS Software versions if they have comparable capabilities and features. Depending on the switch model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.

Required Resources

  • 2 switches (Cisco 2960 with the Cisco IOS Release 12.2(46)SE C2960-LANBASEK9-M image or comparable)
  • 2 switches (Cisco 3560 with the Cisco IOS Release 12.2(46)SE C3560-ADVIPSERVICESK9-mz image or comparable)
  • Ethernet and console cables

Note: Be sure to save your final device configurations to use with the next lab. Because the VLAN and VTP commands do not display in the configs, you must re-enter them in the next lab.

Step 1: Prepare the switches for the lab.

Erase the startup config, delete the vlan.dat file, and reload the switches. Refer to Lab 1 -1, “Clearing a Switch” and Lab 1 -2, “Clearing a Switch Connected to a Larger Network” to prepare the switches for this lab. Cable the equipment as shown.

Step 2: Configure the basic switch parameters and trunking.

a. Configure the management IP addresses in VLAN 1. Configure the hostname, password, and Telnet access on all four switches. HSRP will be used later in the lab, so set up the IP addressing for VLAN 1 on DLS1 and DLS2. Because will be the virtual default gateway for VLAN 1, use .3 and .4 for the IP addresses on DLS1 and DLS2, respectively.

b. Configure a default gateway on the access layer switches. The distribution layer switches are Layer 3 devices and do not need default gateways.

c. Configure 802.1q trunking between the switches according to the diagram. On the 2960 switches, only dot1q is supported, therefore the switchport trunk encapsulation command is unavailable.

d. Verify trunking and spanning-tree operations using the show interfaces trunk and show spanning-tree commands.

Which switch is the root bridge?Answers will vary depending on the switches used. With this equipment, the root bridge is DLS2. Refer to the output of the show spanning-tree command below.

For ALS1 and ALS2, which trunks have a role of designated (Desg), Alternate (Altn), and Root?
Answers will vary depending on the switches used. Refer to the output of the show spanning-tree command below.ALS1#

Is trunk negotiation being used here? Which mode are the trunks in?
No. The trunks are being statically set to 802.1q trunks. Refer to the output of the show interfaces trunk command below.

Step 3: Configure VTP on ALS1 and ALS2.

Set up the VLANs according to the diagram. Two VLANs are in use at this time: one for students, and one for faculty and staff. These VLANs will be created on DLS1, which is set up as a VTP server. DLS2 also remains in its default VTP mode and acts as a server as well. ALS1 and ALS2 are configured as VTP clients. The user access ports for these VLANs also need to be configured on ALS1 and ALS2. Set up these ports as static access ports and activate spanning-tree PortFast. Configure these ports according to the diagram.

a. Configure ALS1 for the VTP client changes.

%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION %Portfast will be configured in 10 interfaces due to the range command but will only have effect when the interfaces are in a non-trunking mode.

b. Configure ALS2 for the VTP client changes.

%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION
%Portfast will be configured in 10 interfaces due to the range command but will only have effect when the interfaces are in a non-trunking mode.

Step 4: Configure IP routing, the VLANs, VLAN SVIs, and HSRP on DLS1 and DLS2.

HSRP is a requirement for the network, and VLANs 100 and 200 are configured to use HSRP to provide redundancy at Layer 3. Use the priority command to make DLS1 the active router for VLANs 1 and 100, and DLS2 the active router for VLAN 200.

a. Configure VTP, VLANs, and IP routing on DLS1.

b. Configure switch virtual interfaces (SVIs) and HSRP on DLS1.

c. Configure IP routing, VLAN SVIs, and HSRP on DLS2.

d. Verify your configurations using the show vlan brief, show vtp status, show standby brief, and show

How many VLANs are active in the VTP domain?
There should be seven active VLANs in this VTP domain: five built-in VLANs, and two new VLANs that were created earlier.

What is the active router for VLANs 1 and 100? What is the active router for VLAN 200?
DLS1 is the active router for VLANs 1 and 100. DLS2 is the active router for VLAN 200.

The status of SVI VLAN 100 would be up, but the line protocol would be down. Also, the directlyconnected network would not be present in the routing table.

Step 5: Specify verification methods and mitigation techniques for attack types.

Complete the following table with the appropriate verification methods and mitigation approaches for the attack types specified in the left column.

Attack Type

Verification Mitigation
MAC address spoofing or
show mac-address
Configure port security
Configure DHCP snooping
DHCP spoofing View DHCP leases for
Configure DHCP snooping
Unauthorized LAN access Verification is very difficult
for this type of attack

Configure authentication using

Step 6: Configure port security.

To protect against MAC flooding or spoofing attacks, configure port security on the VLAN 100 and 200 access ports. Because the two VLANs serve different purposes—one for staff and one for students—configure the ports to meet the different requirements.

The student VLAN must allow MAC addresses assigned to a port to change, because most of the students use laptops and move around within the network. Set up port security so that only one MAC address is allowed on a port at a given time. This type of configuration does not work on ports that need to service IP phones with PCs attached. In this case, there would be two allowed MAC addresses. To enable security on a port, you must first issue the switchport port-security command by itself.The staff MAC addresses do not change often, because the staff uses desktop workstations provided by the IT department. In this case, you can configure the staff VLAN so that the MAC address learned on a port is added to the configuration on the switch as if the MAC address were configured using the switchport portsecurity mac-address command. This feature, which is called sticky learning, is available on some switch platforms. It combines the features of dynamically learned and statically configured addresses. The staff ports also allow for a maximum of two MAC addresses to be dynamically learned per port.

a. Enter the configuration for the student access ports on ALS2. To enable basic port security, issue the switchport port-security command.

Note: By default, issuing the switchport port-security command by itself sets the maximum number of MAC addresses to 1 , and the violation mode to shutdown. It is not necessary to specify the maximum number of addresses, unless it is greater than 1.

b. Verify the configuration for ALS2 using the show port-security interface command.

c. Enter the configuration of the staff ports on ALS1. First, enable port security with the switchport portsecurity command. Use the switchport port-security maximum #_of_MAC_addresses command to change the maximum number of MAC addresses to 2, and use the switchport port-security macaddress sticky command to allow the two addresses to be learned dynamically.

This time two MAC addresses are allowed. Both will be dynamically learned and then added to the running configuration.

d. Verify the configuration using the show port-security interface command.

Step 7: Configure DHCP snooping.

DHCP spoofing is a type of attack primarily used to assign IP addressing and configuration information by an unauthorized device. This can lead to a denial of service or traffic interception. The attacker replies to a DHCP request, claiming to have valid gateway and DNS information. A valid DHCP server might also reply to the request, but if the attacker’s reply reaches the requestor first, the invalid information from the attacker is used.

To help protect the network from such an attack, you can use DHCP snooping. DHCP snooping is a Cisco Catalyst feature that determines which switch ports are allowed to respond to DHCP requests. Ports are identified as trusted or untrusted. Trusted ports permit all DHCP messages, while untrusted ports permit (ingress) DHCP requests only. Trusted ports can host a DHCP server or can be an uplink toward a DHCP server. If a rogue device on an untrusted port attempts to send a DHCP response packet into the network, the port is disabled. From a DHCP snooping perspective, untrusted access ports should not send any DHCP server responses, such as a DHCPOFFER, DHCPACK, or DHCPNAK.

a. Enable DLS1 and DLS2 to trust DHCP relay information from ALS1 and ALS2 so that the DHCP server can respond to the ALS1 and ALS2 trusted port requests. This is accomplished using the ip dhcp relay information trust-all command.

Note: It is not necessary to enable DHCP snooping on the distribution layer switches, although this would allow DLS1 and DLS2 to trust ALS1 and ALS2 as relay agents.

b. Configure ALS1 and ALS2 to trust DHCP information on the trunk ports only, and limit the rate that requests are received on the access ports. Configuring DHCP snooping on the access layer switches involves the following process:

  •  Turn snooping on globally using the ip dhcp snooping command.
  • Configure the trusted interfaces with the ip dhcp snooping trust command. By default, all ports are considered untrusted unless statically configured to be trusted.
  • Configure a DHCP request rate limit on the user access ports to limit the number of DHCP requests that are allowed per second. This is configured using the ip dhcp snooping limit rate rate_in_pps. This command prevents DHCP starvation attacks by limiting the rate of the DHCP requests on untrusted ports.
  • Configure the VLANs that will use DHCP snooping. In this scenario, DHCP snooping will be used on both the student and staff VLANs.
c. Verify the configurations on ALS1 and ALS2 using the show ip dhcp snooping command.
Will DHCP replies be allowed on access ports assigned to VLAN 200?
No. The access ports assigned to VLAN 200 are untrusted for DHCP snooping. Only DHCP requests can be sent out of these ports, not replies.

How many DHCP packets will be allowed on Fast Ethernet 0/16 per second?
It will be limited to 20 packets per second.

Step 8: Configure AAA.

The authentication portion of AAA requires a user to be identified before being allowed access to the network. Authentication is configured by defining a list of methods for authentication and applying that list to specific interfaces. If lists are not defined, a default list is used. For this network, it has been decided that AAA using 802.1X will be used to control user access for the staff
VLAN using a local list of usernames and passwords. When a radius server is added to the network, all user ports, including the student VLAN, will also be added to the configuration.

The IEEE 802.1X standard defines a port-based access control and authentication protocol that restricts unauthorized workstations from connecting to a LAN through publicly accessible switch ports. The authentication server authenticates each workstation that is connected to a switch port before making any services that are offered by the switch or the LAN available.

Until the workstation is authenticated, 802.1X access control allows only Extensible Authentication Protocol over LAN (EAPOL) traffic through the port to which the workstation is connected. After authentication succeeds, normal traffic can pass through the port.

a. Enter the configuration for ALS1.

Use the aaa new-model command to turn on AAA authentication on ALS1. The aaa authentication dot1x default local command tells the switch to use a local database of usernames and passwords to authenticate the users. Users are assigned to the database using the username username password password command. The dot1x system-auth-control command activates global support for 802.1X authentication.

The Fast Ethernet interfaces used for VLAN 100 staff access are configured using the dot1x port-control auto command. The auto keyword allows the switch port to begin in the unauthorized state, and allows the negotiation between the client and server to authenticate the user. Once authenticated, the user is allowed access to the network resources.

Note: For switches running Cisco IOS version 12.2(50)SE or later, the dot1x port-control auto command is replaced with the following interface-level commands: authentication port-control auto dot1x pae authenticator

b. Verify the AAA configuration using the show dot1x interface command.

If a user with a username frankadams attempts to connect to the staff VLAN access ports, will the user be allowed access? Will the user be allowed access to the student VLAN ports?
A user with a username frankadams will not be able to get onto the staff VLAN ports because dot1x authentication is set up on them and that username is not listed in the local usernames. A user will be able to get onto the student VLAN ports because dot1x authentication is not configured on the student VLAN ports.

Note: Save your final device configurations for use with the next lab.

How will the configuration need to be changed when a radius server is added to the network?
First, the radius server is configured with the radius-server host command. Next, the aaa authentication dot1x default local command is replaced with either the aaa authentication dot1x default group radius command or the aaa authentication dot1x default group radius local command. The first replacement command replaces local authentication with radius authentication. The second replacement command tries radius authentication first, and then uses the local database as a backup.

Device Configurations (Instructor version)

Switch DLS1

Switch DLS2

Switch ALS1

Switch ALS2

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