Secure LAN Switching

This chapter focuses on the Cisco Catalyst 5000/5500 series switches. We will discuss private VLANs in the context of the 6000 series switches.

This excerpt is from Chapter 5, Secure LAN Switching, of Network Security Principles & Practices, written by Saadat

Malik and published by Cisco Press.


In order to provide comprehensive security on a network, it is important to take the concept of security to the last step and ensure that the Layer 2 devices such as the switches that manage the LANs are also operating in a secure manner.

This chapter focuses on the Cisco Catalyst 5000/5500 series switches. We will discuss private VLANs in the context of the 6000 series switches. Generally, similar concepts can be implemented in other types of switches (such as the 1900, 2900, 3000 and 4000 series switches) as well.

Security on the LAN is important because some security threats can be initiated on Layer 2 rather than at Layer 3 and above. An example of one such attack is one in which a compromised server on a DMZ LAN is used to connect to another server on the same segment despite access control lists on the firewall connected on the DMZ. Because the connection occurs at Layer 2, without suitable measures to restrict traffic on this layer, this type of access attempt cannot be blocked.

General switch and layer 2 security

Some of the basic rules to keep in mind when setting up a secure Layer 2 switching environment are as follows:

  • VLANs should be set up in ways that clearly separate the network's various logical components from each other. VLANs lend themselves to providing segregation between logical workgroups. This is a first step toward segregating portions of the network needing more security from portions needing lesser security. It is important to have a good understanding of what VLANs are. VLANs are a logical grouping of devices that might or might not be physically located close to each other.
  • If some ports are not being used, it is prudent to turn them off as well as place them in a special VLAN used to collect unused ports. This VLAN should have no Layer 3 access.
  • Although devices on a particular VLAN cannot access devices on another VLAN unless specific mechanisms for doing so (such as trunking or a device routing between the VLANs) are set up, VLANs should not be used as the sole mechanism for providing security to a particular group of devices on a VLAN. VLAN protocols are not constructed with security as the primary motivator behind them. The protocols that are used to establish VLANs can be compromised rather easily from a security perspective and allow loopholes into the network. As such, other mechanisms such as those discussed next should be used to secure them.
  • Because VLANs are not a security feature, devices at different security levels should be isolated on separate Layer 2 devices. For example, having the same switch chassis on both the inside and outside of a firewall is not recommended. Two separate switches should be used for the secure and insecure sides of the firewall.
  • Unless it is critical, Layer 3 connectivity such as Telnets and HTTP connections to a Layer 2 switch should be restricted and very limited.
  • It is important to make sure that trunking does not become a security risk in the switching environment. Trunks should not use port numbers that belong to a VLAN that is in use anywhere on the switched network. This can erroneously allow packets from the trunk port to reach other ports located in the same VLAN. Ports that do not require trunking should have trunking disabled. An attacker can use trunking to hop from one VLAN to another. The attacker can do this by pretending to be another switch with ISL or 802.1q signaling along with Dynamic Trunking Protocol (DTP). This allows the attacker's machine to become a part of all the VLANs on the switch being attacked. It is generally a good idea to set DTP on all ports not being used for trunking. It's also a good idea to use dedicated VLAN IDs for all trunks rather than using VLAN IDs that are also being used for nontrunking ports. This can allow an attacker to make itself part of a trunking VLAN rather easily and then use trunking to hop onto other VLANs as well.

Generally, it is difficult to protect against attacks launched from hosts sitting on a LAN. These hosts are often considered trusted entities. As such, if one of these hosts is used to launch an attack, it becomes difficult to stop it. Therefore, it is important to make sure that access to the LAN is secured and is provided only to trusted people.

Some of the features we will discuss in the upcoming sections show you ways to further secure the switching environment.

The discussion in this chapter revolves around the use of Catalyst 5xxx and 6xxx switches. The same principles can be applied to setting up security on other types of switches.

>> Read the rest of Chapter 5, Secure LAN Switching.


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This was first published in June 2003

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