Why You Should Avoid Using VLAN 1 in Cisco Networks

VLAN 1 is one of those topics that every network engineer encounters early in their career. It’s the default VLAN on Cisco switches and plays a critical role in how switches communicate with each other.

While VLAN 1 cannot be deleted, it’s a best practice to avoid using it for any user or production traffic. In this post, we’ll break down why VLAN 1 exists, what happens if you prune it, and why isolating it improves both security and stability in your network.

What Is VLAN 1?

When a Cisco switch is first powered on, all switch ports are assigned to VLAN 1 by default. VLAN 1 also serves as the native VLAN on IEEE 802.1Q trunks, which means that untagged traffic is associated with VLAN 1.

By default, several important Layer 2 control-plane protocols use VLAN 1, including:

CDP (Cisco Discovery Protocol)
STP (Spanning Tree Protocol)
VTP (VLAN Trunking Protocol)
LACP and PAgP (Link Aggregation Control Protocols)
DTP (Dynamic Trunking Protocol)

Because of this, VLAN 1 is integral to switch operation and cannot be deleted or suspended.

Can VLAN 1 Be Disabled or Deleted in NX-OS?

No, VLAN 1 cannot be deleted or disabled in Cisco NX-OS.

When you try, NX-OS displays an error message such as:

N9K(config)# no vlan 1
Deletion of VLAN 1 is not allowed!!
N9K(config)# no int vlan 1
Warning: Vlan 1 is default vlan and cannot be deleted

This behavior is intentional. VLAN 1 is reserved by the system for essential control-plane operations. You cannot modify it beyond pruning it from trunks or changing the native VLAN to another number.

Should You Prune VLAN 1 from Trunks?

Yes. While you cannot delete VLAN 1, you should prune it from trunk ports wherever possible.

Pruning VLAN 1 means that no data-plane traffic tagged (or untagged) for VLAN 1 will traverse those trunk links. However, control-plane protocols such as CDP, LLDP, LACP, and STP will continue to function because NX-OS handles these packets internally through system-level forwarding entries.

Example output showing CDP packets sent using VLAN 1 tagging:

N9K# show cdp internal event-history debugs
...
Sent CDP packet On Netstack with vlan 1 tagged, interface 0x1a006a00

Even if VLAN 1 is pruned, CDP and other discovery protocols still work. NX-OS ensures this through internal TCAM redirection mechanisms.

How Control Protocols Behave with VLAN 1

The behavior of control protocols, especially STP, changes slightly depending on which VLAN is configured as the native VLAN on a trunk.

When the native VLAN is VLAN 1:

VLAN 1 STP BPDUs are sent untagged to the IEEE STP MAC address (0180.c200.0000).
VLAN 1 STP BPDUs are also sent untagged to the PVST+ MAC address.
Non-VLAN 1 STP BPDUs are sent tagged to the PVST+ MAC address (0100.0ccc.cccd).

When the native VLAN is not VLAN 1:

VLAN 1 STP BPDUs are sent tagged to the PVST+ MAC address.
VLAN 1 STP BPDUs are also sent untagged on the native VLAN.
Non-VLAN 1 BPDUs are sent tagged as normal.

In summary, standard STP BPDUs are derived from VLAN 1 and typically sent untagged, allowing backward compatibility with CST (Common Spanning Tree) and interoperability across platforms.

Reference: Cisco – Understanding PVID Inconsistencies

Why VLAN 1 Should Be Avoided

Even though VLAN 1 is built-in and functional, using it for regular data or management traffic is not recommended. There are several reasons for this.

1. VLAN Hopping Attacks

VLAN 1 is a common target in VLAN hopping attacks. In these scenarios, an attacker sends crafted 802.1Q frames pretending to originate from VLAN 1 to gain unauthorized access to other VLANs. Because VLAN 1 is predictable and widely used, it becomes a natural attack vector.

2. Broadcast and Control-Plane Contention

If VLAN 1 carries user or broadcast traffic, it can interfere with essential control-plane protocols. A broadcast storm or loop on VLAN 1 could impact STP or CDP packets and cause instability.

3. Predictability

Every attacker, tester, and network scanning tool knows VLAN 1 is the default. Leaving it active for user traffic is similar to using “admin” as your default login name—it’s an unnecessary risk.

Cisco Best Practices

Cisco’s best practice recommendations are consistent across IOS and NX-OS platforms:

Do not use VLAN 1 for any user or management traffic.
Change the native VLAN on all 802.1Q trunks to an unused VLAN.
Prune VLAN 1 from all trunk ports.
Assign all unused access ports to a “dead-end” VLAN (commonly VLAN 999 or 666).
Shut down unused interfaces to prevent rogue connections.

This isolates VLAN 1 strictly for internal switch functions and protects against unintended traffic leakage or attacks.

Example Configuration

! Create a dead-end VLAN for unused ports
vlan 999
  name BLACKHOLE

! Change the native VLAN and prune VLAN 1
interface range GigabitEthernet1/0/1 - 48
  switchport trunk native vlan 999
  switchport trunk allowed vlan remove 1
  switchport mode trunk

! Move unused access ports to VLAN 999 and disable them
interface range GigabitEthernet1/0/10 - 48
  switchport access vlan 999
  shutdown

Common Misconceptions About VLAN 1

“Pruning VLAN 1 breaks CDP or STP.”
False. These control-plane packets continue to work even when VLAN 1 is pruned from a trunk because they are handled internally by the switch.

“I can delete VLAN 1 to make the network more secure.”
False. VLAN 1 cannot be deleted, but it can and should be isolated from data traffic.

“Only VLAN 1 can be used as the native VLAN.”
False. Any VLAN can be configured as the native VLAN. Using a dedicated, unused VLAN is a best practice.

Summary

Topic	Recommendation
Deleting VLAN 1	Not possible
Using VLAN 1 for data	Strongly discouraged
Control-plane traffic	Still uses VLAN 1 internally
Native VLAN	Should be changed to an unused VLAN
Unused ports	Move to a dead-end VLAN and disable

Final Thoughts

VLAN 1 is an integral part of Cisco’s architecture and cannot be removed. However, it should never be used for user, management, or server traffic.

Treat VLAN 1 as a control-plane VLAN — a background function of the switch — and isolate it from all production operations. By doing so, you protect your network from unnecessary risks and maintain a cleaner, more predictable Layer 2 environment.

VLAN 1 and Why You Should Not Use It