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Virtual LANs and Trunking

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Virtual LANs and Trunking

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Cisco CCNA ICND Study Guide

By Cbrzana

Table Of Contents

Virtual LANs and Trunking

Things to remember from INTRO:

  • A VLAN is essentially a broadcast domain
  • L2 switches forward frames between devices in the same VLAN
  • L3 switch or router needed to communicate between VLANS
  • ISL is proprietary, fully encapsulates original frame (26-byte header, 4-byte trailer, 12-bit VLAN ID)
  • 802.1q adds 4-byte header after destination MAC address, CRC needs to recalculate
  • Both support separate instance of STP per VLAN

ISL supports multiple spanning trees using Per-VLAN Spanning Tree (PVST+).

-> Cisco’s PVST+ allows multiple STP instances over 802.1q trunks

802.1S: IEEE’s new specification that adds to 802.1q specification, allowing multiple spanning trees.

Native VLAN: 802.1q defines one VLAN on each trunk (by default, VLAN1), does not encapsulate frames. When receiving switch receives frame not encapsulated, assumes it is native VLAN.

-> ISL does not use a concept like this, all frame have an ISL header

Function ISL 802.1Q
Standards body Cisco-proprietary IEEE
Encapsulate original frame Yes No
Allows multiple spanning trees Yes (PVST+) Yes (PVST+ or 802.1S)
Uses a native VLAN No Yes

VLAN Trunking Protocol (VTP): Cisco proprietary protocol (L2) that broadcasts switch VLAN configuration information. Configure settings on one switch, and all other switches learn the VLAN settings dynamically.

1. Creation of switch called the VTP Server.

2. Scale to large sizes by reducing need for manual configuration.

3. Advertises every 5 minutes, or whenever change occurs

Broadcasts Include:

  • Configuration Revision Number (each time switch modifies config, increment by 1). When switch receives broadcast with larger revision number, updates config.
  • VLAN names/numbers
  • Info about which switches have ports assigned to which VLAN

VTP Operates in 3 Modes

  • Server Mode (switch acts as server) -> create, modify, delete VLAN configuration, stored in NVRAM
  • Client Mode -> Configuration not stored at all, can’t modify/create/delete
  • Transparent Mode -> Avoid’s using VTP to exchange configuration info, to be transparent = ignore VTP broadcasts (they still forward to other switches, however).
    • Changes to VLAN can be made, but only saved for that switch.

Function Server Client Transparent
Originates VTP advertisements
Yes No No
Processes received advertisements and synchronizes VLAN
configuration information with other switches
Yes Yes No
Forwards VTP advertisements received in a trunk
Yes Yes Yes
Saves VLAN configuration in NVRAM
Yes No Yes
Can create, modify, or delete VLANs using configuration
Yes No Yes

VTP Pruning: Since switches usually don’t have interfaces for every VLAN in the network, bandwidth is usually wasted in broadcasting those updates. VTP pruning allows switches to prevent broadcasts and unknown unicasts from going to switches with no interfaces in those VLANs.

  • Broadcast is only flooded toward switches with ports in a given VLAN

VLAN Trunking and Configuration

  • VTP is enabled by default
  • Trunking negotiation is attempted on all ports by default
Command Command Description
vlan database Exec command to enter VLAN config mode
vtp {domain | password | pruning | v2 mode} Defines VTP parameters in VLAN config mode
vlan vlan id [name vlan name] Creates/names a VLAN
switchport mode {access | dynamic {auto |
desirable} | trunk}
Interface subcommand that configures the interface for trunking.
switchport trunk {{allowed vlan vlan-list} |
{native vlan vlan-id} | {pruning vlan vlan-list}}
Refines the list of allowed VLANs, defines the 802.1Q native VLAN, and limits the range of VLANs for which pruning can occur.
switchport access vlan vlan-id
Interface subcommand that statically
configures the interface into that one VLAN.
show interfaces [interface-id | vlan vlan-id]
[switchport | trunk]
Displays trunk status.
show vlan [brief | id vlan-id | name vlan-name
| summary]
EXEC command that lists information about
the VLAN.
show vlan [vlan]
Displays VLAN information.
show vtp status
Lists VTP configuration and status information.
show spanning-tree vlan vlan-id
EXEC command that lists information about the spanning tree for a particular VLAN.

*To configure multiple interfaces simultaneously for same VLAN, use these steps:

vlan database

vlan 1 name test1

exit (apply update/increments revision number – if abort used, no changes saved)

config t

interface range [fastEthernet 0/1 – 5]

switchport mode access (trunking negotiations disabled, ports are access ports)

switchport access vlan 1


show vlan brief (to verify updates, VLAN name, status, and ports)


show vlan id 1

** Note that vlan 1 is configured by default **

Option Description Action
access Disables port trunk mode and does not even
attempt to form a trunk on the interface.
Does not trunk.
trunk Configures the port in permanent trunk mode.
Always tries to trunk.
dynamic desirable The port negotiates to a trunk port if the connected device is in the trunk, dynamic desirable, or dynamic auto state.
Otherwise, the port becomes a nontrunk port.
Trunks to switches
set to the trunk,
dynamic desirable, or
dynamic auto state.
dynamic auto Lets a port become a trunk only if the connected device is in the dynamic desirable or trunk state.
Trunks to switches
set to the trunk
dynamic desirable

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