Frequently Asked Questions of VLAN
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FAQ Execution
A VLAN (Virtual LAN) logically segments a switch into multiple broadcast domains, reducing broadcast traffic, improving performance, enhancing security, and simplifying network management.
802.1Q inserts a 4-byte tag into Ethernet frames containing VLAN ID, priority (PCP), and DEI. Tagged frames allow multiple VLANs over a single trunk link.
Access port: Carries traffic for one VLAN (untagged)
Trunk port: Carries multiple VLANs using 802.1Q tagging.
Native VLAN carries untagged traffic on a trunk. Misuse can lead to VLAN hopping attacks if not secured.
Causes traffic leakage, STP issues, and potential security breaches.
They are assigned to the native VLAN.
An attack allowing access to unauthorized VLANs. Switch spoofing
Double tagging.
Disable DTP, Use unused native VLAN, Force access mode on ports, Disable unused ports.
Dynamic Trunking Protocol automatically negotiates trunking between Cisco switches.
Attackers can force trunk formation to access multiple VLANs.
Static: Port-based manual assignment
Dynamic: VLAN assigned via VMPS/RADIUS (Old one), Now DHCP (New one technology).
A special VLAN for IP phones to prioritize voice traffic using QoS.
STP prevents loops per VLAN or per instance depending on STP mode.
Cisco STP implementation running one STP instance per VLAN.
PVST+: Slow convergence
Rapid PVST+: Faster (RSTP)
MST: Groups VLANs into instances.
Restricts unnecessary VLANs from trunk links.
VLAN Trunking Protocol distributes VLAN info across switches and modes also Server, Client, Transparent, Off.
Automatically removes unused VLANs from trunks.
Wrong revision number can delete VLANs across the network.
Sol:
| Existing Production Network | A new switch is added to the network, but it was previously used |
|---|---|
| VTP Domain: HTIS VTP Mode: Server VLANs: 10,20,30,40 Current Revision Number: 10 |
Same VTP domain (HTIS) VTP mode = Server Revision number = 50 VLANs = only default VLAN 1 |
| 🔹What Happens When You Connect It? | |
📉 Result:Users disconnected, Servers unreachable, Entire network outage | |
| 🔹Live Command Output (After Damage) | |
| SW1# show vlan brief VLAN Name Status Ports 1 default active | |
1. Set VTP transparent mode
2. Delete vlan.dat
3. Verify revision number is 0
3. Then connect trunk
Traffic loss, broadcast issues, and connectivity failure.
4094 VLANs (12-bit VLAN ID field).
Routing traffic between VLANs using a router or L3 switch.
Router-on-a-stick: External router, trunk link
L3 switch: Internal routing via SVIs (faster)
Logical L3 interface representing a VLAN.
Uses SVIs and routing table for packet forwarding.
Inter-VLAN communication for that VLAN stops.
No, It causes IP conflicts and routing ambiguity.
interface gi0/1
no switchport
ip address x.x.x.x y.y.y.y
ARP stays within VLAN; router performs ARP for next hop.
Router replies on behalf of another device; not recommended.
Provide gateway redundancy per VLAN using virtual IP.
Supports load balancing, failover, and high availability.
L3 switch forwards DHCP requests to server using helper-address.
Technically yes, but not recommended.
VLANs integrate with VRF, VXLAN, and MPLS for scalable segmentation.
1. Functional segmentation, 2. Avoid VLAN 1, 3. Limit trunk VLANs, 4. Use consistent numbering.
