Native VLAN is a fundamental concept in Cisco networking that refers to the VLAN assigned to untagged traffic on an 802.1Q trunk port. When frames traverse a trunk link, they typically carry a VLAN tag that identifies which VLAN the traffic belongs to. However, the Native VLAN operates differently …Native VLAN is a fundamental concept in Cisco networking that refers to the VLAN assigned to untagged traffic on an 802.1Q trunk port. When frames traverse a trunk link, they typically carry a VLAN tag that identifies which VLAN the traffic belongs to. However, the Native VLAN operates differently because frames belonging to this VLAN are transmitted across the trunk link in an untagged format. By default, Cisco switches configure VLAN 1 as the Native VLAN on all trunk ports. This means any traffic that arrives on a trunk port and lacks a VLAN tag will be associated with VLAN 1. Similarly, when traffic from the Native VLAN needs to be sent out a trunk port, the switch forwards it as untagged frames. This behavior exists primarily for backward compatibility with older devices that do not support 802.1Q tagging. Understanding Native VLAN configuration is essential for network security and proper network operation. A common security best practice involves changing the Native VLAN from the default VLAN 1 to an unused VLAN. This helps mitigate VLAN hopping attacks, where malicious actors could potentially exploit Native VLAN misconfigurations to access unauthorized network segments. It is crucial that the Native VLAN matches on both ends of a trunk link. When there is a Native VLAN mismatch between two switches, several problems can occur including connectivity issues, traffic being placed in incorrect VLANs, and Spanning Tree Protocol problems. Cisco switches will generate CDP or console messages alerting administrators to Native VLAN mismatches. To configure the Native VLAN on a Cisco switch, administrators use the command switchport trunk native vlan followed by the desired VLAN number while in interface configuration mode for the trunk port. Proper Native VLAN configuration ensures efficient traffic flow and maintains network security across your switching infrastructure.
Native VLAN - Complete Guide for CCNA
What is Native VLAN?
The Native VLAN is a special VLAN designated on 802.1Q trunk ports where frames are sent and received untagged. By default, VLAN 1 is the native VLAN on Cisco switches. When a switch receives an untagged frame on a trunk port, it assumes the frame belongs to the native VLAN.
Why is Native VLAN Important?
1. Backward Compatibility: Native VLAN allows trunk ports to communicate with devices that do not understand VLAN tagging.
2. Control Traffic: Certain control protocols like CDP, VTP, and DTP send their frames untagged on the native VLAN.
3. Security Implications: Misconfigured native VLANs can lead to VLAN hopping attacks, making proper configuration critical for network security.
How Native VLAN Works
On an 802.1Q trunk link: - Frames belonging to the native VLAN are transmitted untagged - All other VLAN frames are transmitted with an 802.1Q tag - When an untagged frame arrives, the switch places it into the native VLAN - Both ends of a trunk link must have matching native VLAN configurations
Configuration Commands:
To change the native VLAN on a trunk port: Switch(config-if)# switchport trunk native vlan
To verify native VLAN settings: Switch# show interfaces trunk Switch# show interfaces switchport
Native VLAN Mismatch
When the native VLAN differs between two connected trunk ports: - CDP will detect and report the mismatch - Traffic may be placed into the wrong VLAN - Network connectivity issues will occur - Security vulnerabilities may be introduced
Best Practices
1. Change the native VLAN from the default VLAN 1 to an unused VLAN 2. Ensure native VLAN matches on both ends of trunk links 3. Consider using switchport trunk native vlan tag to tag native VLAN traffic (on supported platforms) 4. Do not use the native VLAN for regular user traffic
Exam Tips: Answering Questions on Native VLAN
Key Facts to Remember: - Default native VLAN is VLAN 1 - Native VLAN frames are untagged on 802.1Q trunks - Native VLAN must match on both sides of a trunk - CDP detects and reports native VLAN mismatches
Common Exam Scenarios:
1. Troubleshooting Questions: If hosts on specific VLANs cannot communicate across a trunk, check for native VLAN mismatch first.
2. Security Questions: When asked about VLAN hopping prevention, changing the native VLAN from default is a correct answer.
3. Configuration Questions: Know the exact syntax: switchport trunk native vlan followed by the VLAN number.
4. Verification Questions: The command show interfaces trunk displays native VLAN information.
Watch Out For: - Questions that confuse native VLAN with management VLAN - they are different concepts - Scenarios where only one side of the trunk has a modified native VLAN - Questions asking about which traffic type remains untagged (answer: native VLAN traffic)
Quick Reference for Exams: - 802.1Q = Native VLAN concept (ISL does not use native VLAN the same way) - Untagged frame on trunk = Goes to native VLAN - VLAN 1 = Default but not recommended for production use