ICND2 – Chapter 1 – VLANs and VTP

Chapter 1 was a very long chapter that covered a lot of material. This is the chapter that I started and did not finish on the first 3 occasions. I just finished the review and here are my notes from the review:


VLANs created separate broadcast domains.

ISL fully encapsulates the original Ethernet frame, adds its own header and trailer (CRC).

The source and destination MAC addresses of an ISL header is that of the switch and not those on the original packet.

802.1Q inserts an extra 4-BYTE VLAN header into the original frame and thus keeps the original source and destination MAC addresses.

Both ISL and 802.1Q use 12 bits of of the VLAN header for the VLAN number. This results in 2^12 – 2 possible VLANs or 4094.

VLANs 1-1005 are considered to be normal range VLANs and 1006 and above are considered to be extended range VLANs.

802.1Q uses 1 VLAN on each trunk as the native VLAN. By default, this is VLAN 1. 802.1Q does not tag frames in the native VLAN and simply passes them as a normal frame.

Commands for creating a new VLAN (with optional name):

Switch(config)#vlan 10
Switch(config-vlan)#name My_Wonderful_Vlan

Commands to assign an access port to a VLAN:

Switch(config)#interface fa0/1
Switch(config-if)#switchport access vlan 10
Switch(config-if)#switchport mode access

Omitting the switchport mode access interface subcommand would allow the port to negotiate as a trunk.

Commands to assign a trunk port to a VLAN:

Switch(config)#interface fa0/24
Switch(config-if)#switchport mode trunk

You can also use switchport mode desirable or switchport mode dynamic auto.
By default (dynamic desirable), Cisco switches negotiate the type of trunking to use with the far-end using DPT (Dynamic Trunking Protocol) With auto, the switch port will not initiate DTP but will respond if the other switch does.

To assign the type of trunking used and not use negotiation use:

switchport trunk encapsulation [dot1q | isl | negotiate] – Use negotiate to return to default

To see a list of configured VLANs on a switch use:

Switch#show vlan brief
Switch#show vlan

To see details on a port use:

Switch#show interfaces FA0/1 switchport

To see a list of trunks on the switch use:

Switch#show interface trunk

This will also show which VLANs are allowed on the trunk.

To configure what VLANs are allowed on a trunk use:

Switch(config)#int fa0/24
Switch(config-if)#switchport allowed vlan [add | all | except | remove] 20

Configuring a port for Cisco Phones with voice VLAN:

Switch(config)#int Fa0/1
Switch(config-if)#switchport voice vlan 99


VTP (VLAN Trunking Protocol) is a Layer 2 messaging protocol that switches use to exchange VLAN configuration information.

VTP servers and clients examine the VLAN database config revision number to determine whether or not to make changes based on a received VTP message.

New VLANs can only be added to switches running in Server mode or Transparent mode. Changes made on a switch using VTP server mode result in the config number increasing by one and a VTP update message being sent to neighboring switches. VTP Transparent configured switches ignore the messages but pass them along to other switches.

VTP servers and clients send periodic VTP messages every 5 minutes.

There are 3 types of VTP messages:

Summary advertisement: The periodic VTP messages that occur every 5 minutes that list revision number, domain name, etc. but no VLAN information
Subset advertisement: Message that contain the VLAN information that changed since the last revision
Advertisement request: Immediate request from a switch to another switch to send VTP information

In order for switches to exchange VTP messages, ALL of the following must be true:

1. The link between the two switches must be a trunk with ISL or 802.1Q
2. The VTP domain name must match
3. The VTP password must match (if configured)

VTP clients and servers store the VLAN database in a file named vlan.dat in flash memory. When wiping a switch you need to use “delete flash:vlan.dat” to get rid of the VLAN database.

VTP pruning allows VTP to determine which trunks do not have any ports in a certain VLAN and then it does not flood frames for those VLANs to those trunks

Configuring VTP on a Switch:

Switch(config)#vtp mode [server | client]
Switch(config)#vtp domain MyDomain
Switch(config)#vtp password MyPass (optional password)
Switch(config)#vtp pruning (optional)
Switch(config)#vtp version 2 (optional)

To see the status of VTP use:

Switch#show vtp status

Before connecting a new switch to a VTP domain, reset the new switch’s VTP revision number to 0 by doing one of the following:

1. Configure the switch to transparent mode and then back to client/server mode
2. Erase the vlan.dat file from flash


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