VLANS Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.



  • Best practice – 1 VLAN per IP
  • Broadcast – Routers can accept and generate broadcasts, but they cannot forward them
  • VLAN 1. 1002 – 1005 are shipped by default
  • Native VLAN – Default on Cisco is VLAN 1. All unassigned hosts are on Native Vlan
  • Static VLAN – Dependent on Host Port
  • Dynamic VLAN – Dependent on Host MAC Address, uses VLAN Membership Policy Server (VMPS – uses UDP). Host can most from port to port or switch to switch and vlan assignment is based on their MAC address. Uses TFTP server to map addresses. Port fast is enabled by default for dynamic vlans. Don’t use port security. Dynamic port cannot be a trunk port
  • VLAN.dat – VLANs kept in separate file. Most delete separately not done with erase
  • Vlan database – using CTRL+Z will not save the config. Must type apply
  • Dynamic Desirable Trunking – Port is actively trying to form a trunk
  • Troubleshooting – Check port speed/duplex and check MAC table
  • ISL – Cisco own trunk protocol. Places both header & trailer in frame, then encapsulates it = overhead. No native vlans = every frame is encapsulated = overhead
    • 26 byte header + 4 byte trailer CRC = 30BYTES. Too large for switch, considered as giant frames
    • 802.1Q – No encapsulation. Adds 4 byte header to frame
    • Trunk – Port must agree on duplex, speed and encapsulation
    • Giants are frame larger than 1518 or 1522 (802.3ac). Runts are frames smaller tan 64bytes. Baby Giants are 1500 < 2000 bytes
    • Dynamic Trunking Protocol – Attempts to negotiate a trunk with remote switch. Sends DTP frames every 30 sec (overhead)
    • VLAN Design – Keep broadcasts and multicast away from core
      • End to end (80/20) – 80% of local traffic stays local and 20% go to core. These vlans must be on every access-layer switch
      • Local (20/80) – 20% local, 80% core.
      • Port status
        • Trunk – Trunk port and no DTP negotiation.
        • Dynamic Desirable – Default. Responds to DTP and becomes a trunk, otherwise access.
        • Dynamic Auto – NOT actively negotiate a trunk, but will respond to DTP and become one if remote is trunk or dynamic desirable. Trunk will not form if both port are dynamic auto


# Show vlan brief Shows VLAN, name, status and ports. Does not show trunk ports

# Show int trunk Shows trunk ports, mode, encapsulation, status and native vlan, allowed VLANS

# Show vlan id 5 Shows VLAN 5 ports etc


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