What Is Native VLANs?
A Native VLAN is a term used in networking, mainly in the context of VLAN tagging which is a part of the IEEE 802.1Q standard. In a VLAN setup, multiple virtual networks are configured on a single physical network infrastructure to segment traffic for efficiency and security purposes.
The Native VLAN plays a special role in interVLAN communication. It is the VLAN to which a port is assigned when no specific VLAN tag (or 802.1Q tag) is present in the Ethernet frame. In other words, untagged traffic that arrives at a switch port is assumed to belong to the Native VLAN. This feature is particularly useful for backward compatibility with older network equipment that doesn’t understand VLAN tags.
It’s important to understand that the Native VLAN is a default setting, and it can be changed according to the network design requirements. For security reasons, it is often advised to change the default Native VLAN to a different one, as the default settings are widely known and could be exploited for network intrusions.
In a network environment, particularly one that involves trunk links (which are used to carry traffic from multiple VLANs), the Native VLAN plays a critical role in ensuring that untagged traffic is appropriately classified and handled. This contributes to maintaining orderly network operations and helps in segregating different types of network traffic for management and security purposes.
Lets use the figure below as an example of a switched network. Let also assume that one of the workstations on the network (PC 5) cannot connect to the internal web server WEB/TFTP, the first place you start troubleshooting will be at the Switch 2 to check if VLANs are properly configured.
Looking at the diagram, switch port fa0/3 on Switch 3 is configured as a trunk port.
Native VLAN Mismatch
When you connect to switch S2, if there is an error on the switch port, it will appear on your console window, in this case, there is and it looks like this:
S3#
#CDP-4-NATIVE VLAN_MISMATCH: Native VLAN mismatch discovered on
FastEthernet0/3 (100), with S1 FastEthernet0/3 (99).
Using the show interfaces fa0/3 switch port command will display connectivity details on the port.
e.g
S3# show interfaces fa0/3 switchport
Name: fa0/3
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Nagotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 100 (Inactive)
…
Trunking VLANs Enabled: 10, 99
Looking at the above detail, you will notice that the native VLAN has been set to VLAN 100 and it is inactive.
As you look further down the output, you see that the allowed VLANs are 10 and 99.
To sum it up, this is a case of a mismatched native VLAN
The solution:
You need to reconfigure the native VLAN on the Fast Ethernet F0/3 trunk port to be VLAN 99. e.g
S3#config t
S3#interface fa0/3
S3#switchport trunk native vlan 99
S3#end
After you have done that, use the show interfaces fa0/3 switchport to confirm you configuration.
And use the Ping to confirm connectivity with the server. e.g.
PC5> ping 192.168.10.30
Pinging 192.168.10.30 with 32 bytes of data:
Reply from 192.168.10.30: bytes-32 times-147ms TT-128
Reply from 192.168.10.30: bytes-32 times-147ms TT-128
Reply from 192.168.10.30: bytes-32 times-147ms TT-128
…
The screen output for the computer PC5 shows that connectivity has been restored to the WEB/TFTP server found at IP address 192.168.10.30.
in conclusion, Native VLAN inconsistencies can lead to security vulnerabilities and communication issues within a network. It is important to ensure that all devices are configured to use the same native VLAN to avoid any potential conflicts or miscommunications.
Regularly monitoring and auditing VLAN configurations can help identify and address any mismatches before they cause major disruptions. Additionally, implementing proper VLAN segmentation and access control measures can further enhance network security and stability.
