Secure port on cisco switch

Understand the basics

In its most basic form, the Port Security feature remembers the Ethernet MAC address connected to the switch port and allows only that MAC address to communicate on that port. If any other MAC address tries to communicate through the port, port security will disable the port. Most of the time, network administrators configure the switch to send a SNMP trap to their network monitoring solution that the port's disabled for security reasons.

Of course, implementing any security solution always involves a trade-off—most often, you trade increased security for less convenience. When using port security, you can prevent devices from accessing the network, which increases security.

However, as you know, there's usually a downside. In this case, it's that the network administrator is the only one who can "unlock" the port, which can cause problems when there are legitimate reasons to change out devices.

Configure port security

Configuring the Port Security feature is relatively easy. In its simplest form, port security requires going to an already enabled switch port and entering the port-securityInterface Mode command. Here's an example:

"First go to the interface of port that you want to configure"
Switch)# config t
Switch(config)# int fa0/18


"Then change the mode of port to Access, Port-security can't be enabled on a port which is on trunk mode"
Switch(config-if)# switchport mode access


"Enable port-security on port"
Switch(config-if)# switchport port-security


"Assign specific MAC address to the port"
Switch(config-if)# switchport port-security mac address XXXX.XXXX.XXXX


"Limit the number of MAC addresses allowed through the port"
Switch(config-if)# switchport port-security maximum 1


"Set the action to take in case of violating the rule"
Switch(config-if)# switchport port-security violation shutdown
Switch(config-if)#^Z

By entering the most basic command to configure port security, we accepted the default settings of only allowing one MAC address, determining that MAC address from the first device that communicates on this switch port, and shutting down that switch port if another MAC address attempts to communicate via the port. But you don't have to accept the defaults.

Know your options

As you can see in the example, there are a number of other port security commands that you can configure. Here are some of your options:

• switchport port-security maximum {max # of MAC addresses allowed}: You can use this option to allow more than the default number of MAC addresses, which is one. For example, if you had a 12-port hub connected to this switch port, you would want to allow 12 MAC addresses—one for each device. The maximum number of secure MAC addresses per port is 132.
• switchport port-security violation {shutdown | restrict | protect}: This command tells the switch what to do when the number of MAC addresses on the port has exceeded the maximum. The default is to shut down the port. However, you can also choose to alert the network administrator (i.e., restrict) or only allow traffic from the secure port and drop packets from other MAC addresses (i.e., protect).
• switchport port-security mac-address {MAC address}: You can use this option to manually define the MAC address allowed for this port rather than letting the port dynamically determine the MAC address.

Of course, you can also configure port security on a range of ports. Here's an example:

Switch)# config t
Switch(config)# int range fastEthernet 0/1 - 24  
Switch(config-if)# switchport port-security 

However, you need to be very careful with this option if you enter this command on an uplink port that goes to more than one device. As soon as the second device sends a packet, the entire port will shut down.

View the status of port security

Once you've configured port security and the Ethernet device on that port has sent traffic, the switch will record the MAC address and secure the port using that address. To find out the status of port security on the switch, you can use the show port-security address and show port-security interface commands. Below are examples for each command's output:

Switch# show port-security address         
          Secure Mac Address Table
-------------------------------------------------------------------
Vlan    Mac Address       Type                Ports   Remaining Age


----    -----------       ----                -----   -------------
                                                         (mins)    
   1    0004.00d5.285d    SecureDynamic       Fa0/18       -


0
Max Addresses limit in System (excluding one mac per port) : 1024
-------------------------------------------------------------------
Total Addresses in System (excluding one mac per port)     :
 



tch# show port-security interface fa0/18
port Security                        : Enabled
PPort Status                        : Secure-up
Violation Mode                     : Shutdown
Aging Time                          : 0 mins
SecureStatic Address Aging  : Disabled
Maximum MAC Addresses    : 1

MAC Addresses                         : 0
Total MAC Addresses                 : 1
Configured MAC Addresses         : 0
Source MAC Address                 : 0004.00d5.285d
Security Violation Count              : 0

Switch#