What are the security implications of using ISL and dot1q for VLAN trunking?

2 dot1q Overview

dot1q is a standard protocol that is supported by many vendors. It works by inserting a 4-byte tag into the original Ethernet frame, creating a new dot1q frame. The tag contains the VLAN ID, which identifies the VLAN to which the frame belongs. dot1q operates at layer 2 of the OSI model, and it modifies the original frame by changing the frame check sequence (FCS) field.

3 ISL Advantages and Disadvantages

One advantage of ISL is that it preserves the original frame, which means it can carry any type of traffic, such as IPX or AppleTalk. Another advantage is that it supports up to 4096 VLANs, which is more than dot1q. However, ISL also has some disadvantages. One disadvantage is that it adds 30 bytes of overhead to each frame, which reduces the available bandwidth and increases the latency. Another disadvantage is that it is not compatible with other vendors' devices, which limits the interoperability and flexibility of the network.

4 dot1q Advantages and Disadvantages

One advantage of dot1q is that it adds only 4 bytes of overhead to each frame, which means it has less impact on the bandwidth and latency. Another advantage is that it is widely supported by many vendors, which enables the interoperability and flexibility of the network. However, dot1q also has some disadvantages. One disadvantage is that it modifies the original frame, which means it cannot carry some types of traffic, such as IPX or AppleTalk. Another disadvantage is that it supports only 4094 VLANs, which is less than ISL.

5 Security Implications of ISL and dot1q

Both ISL and dot1q can enhance the security of the network by isolating different VLANs and preventing unauthorized access. However, they also have some security risks that need to be addressed. One risk is that an attacker can capture and modify the VLAN ID in the ISL or dot1q frame, and send it to another VLAN, creating a VLAN hopping attack. This can compromise the confidentiality and integrity of the data. Another risk is that an attacker can send frames with different VLAN IDs to a trunk port, creating a double-tagging attack. This can bypass the VLAN filtering and reach the native VLAN or other VLANs. To mitigate these risks, some best practices are to use strong encryption, disable unused ports, assign a non-default native VLAN, and enable port security features.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?