A growing challenge facing network administrators is determining how to control who can access the organization’s internal network — and who can’t. For example, can anyone walk into your office, plug in a laptop, and access your network? You might argue that the wall jack has no connection to a switch, but couldn’t someone just pull the Ethernet cable from a working PC and connect to the network that way?
You might think this an unlikely scenario, but it does happen. At my organization, we had salesmen coming in to demo products, and they would just pull the Ethernet jack off a PC and connect it to their laptop, hoping to get Internet access.
The idea that anyone could just come in and access our network scared me — and the possibility should scare you too. What frightened me the most were the various viruses or worms that their PCs might contain. Remember, not everyone recognizes the importance of effective security measures, and you don’t want to trust your network’s security to their apathy.
I turned to switch port security to help solve the problem. Let’s look at how you can use Cisco’s Port Security feature to protect your organization.
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-security Interface Mode command. Here’s an example:
Switch)# config t
Switch(config)# int fa0/18
Switch(config-if)# switchport port-security ?
aging Port-security aging commands
mac-address Secure mac address
maximum Max secure addresses
violation Security violation mode
Switch(config-if)# switchport port-security
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 -
-------------------------------------------------------------------
Total Addresses in System (excluding one mac per port) : 0
Max Addresses limit in System (excluding one mac per port) : 1024
Switch# show port-security interface fa0/18
Port Security : Enabled
Port Status : Secure-up
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses : 1
Total MAC Addresses : 1
Configured MAC Addresses : 0
Sticky MAC Addresses : 0
Last Source Address : 0004.00d5.285d
Security Violation Count : 0
Switch#
For more information on switch port commands and configuring the Port Security feature, check out Cisco’s Enabling Port Security documentation for the Catalyst 2950. What steps have you taken to lock down switch port security? Share your tips in this article’s discussion.
David Davis has worked in the IT industry for more than 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.
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