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Protecting Cisco Switches: Port Security

with 5 comments

I figure I would tackle some Cisco switch security, which oddly enough is typically overlooked, it seems most people primarily focus on their firewalls protecting layer 3 and above meanwhile you can plug right into a layer 2 switch and your in. So it just seems to me that layer 2 security is occasionally overlooked, but we shall cover a few good commands within Cisco switches to protect layer 2. Please keep in mind that security is a broad enough subject as it is, so this post itself will cover parts of port security here, and the discussion will be continued in another post.

I’ll start off by pointing out the most common kind of  attack port security protects against, and that would be MAC Flooding in this type of attack an attacker sends multitudes of spoofed MAC addresses to the switch so the switch stores all the address in the CAM table, and at some point the CAM table overflows and the MAC addresses of legitimate clients are flushed out, at which point the switch forgets how to get data to the legitimately attached clients, so when any legitimate data is sent it gets flooded out all ports. This could have multiple effects one, the attacker can sniff the data off the network and, two any other attached network devices/workstations/servers will get flooded with the data as well so it will simulate a DoS attacks well. This can also protect against a MAC spoofing attacks, where an attacker will change their MAC address so the switch thinks the attacker is a different device like a router, at which point all data leaving the subnet is sent to the attacker so the attacker in essence performs a Man-In-The-Middle attack, where they can do whatever they want with the data they receive.

I would also like to include this will protect against DHCP attacks, after all DHCP leases are given to the MAC address so MAC flooding attacks could also be used to fill up your DHCP scopes (Another type of DoS attack), and when you sit back and think how important DHCP really is and how dependent most networks are of it, imagine losing it for a little while. As stated above however I will cover DHCP Snooping and Dynamic ARP inspection in the future, I just wanted to include this little tidbit.

You can protect against these types of attacks by implementing port security. Keep in mind when you implement port security you can either configure it to be really restrictive or just to protect against the mentioned attacks. The syntax to configure port security is below:

See quite simple really.

The first command simply activates port security on the switch port, but you still have a few more parameters to set. The next command sets the maximum number of MAC addresses the switch port can learn before a violation occurs, which is what is configured in the third line. The default is shutdown, which as you can guess shuts the switch port down, requiring IT to intervene to see what is happening. Their are two other violation modes you can configure instead of shutdown one of which is protect which will allow the computer with the configured MAC address(es) to communicate with the network while dropping the frames of any other computer with a different MAC address after the violation occurs, however the switch will not increase the violation count so their will be no indication at all that anything is happening until a legitimate user complains, so this will happen without your knowledge. The other violation parameter is restrict which acts very similar to protect but restrict will increase the violation count and sends an SNMP trap so at least their is an indication something is happening. I’ve honestly only used the shutdown parameter, since I really do not see the point behind the restrict/protect since an attack could be attempted without your knowledge, where as shutdown will stop any attempt in it’s tracks, and if your smart and this happens enough you will investigate. Now the mac-address sticky command, simple takes the first mac address the port learns and adds it to the running config, so if the device on other side of the switch port needs to be changed it requires administrator intervention.

Now when I said this could be configured to be really restrictive or just to prevent L2 attacks. A switch can hold a very large amount of MAC address, so you can simply set the maximum number of MAC address to 100 per port and forget about it for a while, or you can be really restrictive and set the number of MAC address to 1 or 2. Just keep in mind how often your users get new PC’s or if they move between offices with laptops. You’ll need to decide how restrictive you want to depending on your environment.

Another thing you want to plan for is how are you going to manage port security, just think when a user at a remote site calls up and says “I can’t connect” and you know it’s because of the port security and you have 3+ switches at that site, it doesn’t make much sense to go in their and clear every port security violation now does it or to connect to all 3 switches blindly issuing show port-security hunting for violations? I’ve got my switches sending port security and err-disable traps to a syslog server of mine where I can filter by those specific traps where I see the switch, the port, and the time it went down, from there is just takes a few questions to the user and it’s easier to figure out. (Thank you SolarWinds NPM!) Then simply issuing a shut no shut on the interface will clear it, now if you have issued the mac-address sticky command you will want to clear the original mac address or else the port will simply go back into and err-disable state, to clear the sticky mac address issue the clear port-security sticky int <specify single interface> from global config mode, then proceed to shut no shut the interface.


Written by Stephen J. Occhiogrosso

October 21, 2010 at 12:36 PM

5 Responses

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  1. Loving your blog! Even though this post is a few months it got me thinking a bit.

    What breaks ‘port security’? When user x from Site A is having trouble connecting did they plug a different device in without telling you?

    Or there something else going on?


    February 19, 2011 at 2:25 AM

  2. Thanks for the feedback!

    In my experience, and most of my ports that experience violations (and move to an err-disabled state) are from end-users deciding to randomly change offices and they either take PC with them or decide to dock their laptop in someone else’s office as they walk around the office.

    I have had only a few instances were users have plugged in a rouge device. In those instances port security will disable the port just as quickly as you clear the violation. The first time I ever encountered this I honestly reset the port security 3-4 times before had a local technician check the location. Normally you only hear about users plugging in rouge devices in security material but it does happen and it does pose a security threat to the network.

    Other then the security risk it poses, if multiple users start plugging in multiple rouge devices they generate too much traffic. Which may cause a bottleneck. Since the network may have only been designed to handle so much throughput.


    February 19, 2011 at 2:45 AM

  3. This is very well written, I am glad I ran across your blog, I am surprised you are not an author yet! Keep up the good work.

    Andreja Trbojevic

    March 5, 2011 at 6:59 AM

  4. […] […]

  5. […] […]

    Port Security

    November 17, 2012 at 10:21 AM

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