Cisco 3 - LAN

Perrine. J Page 104/19/23

Chapter 9

VLAN Trunking Protocol

The concept of trunking comes from telephony where the ‘media’ connecting two central offices.

VLAN trunking protocols allow the traveling of frames from different VLANs through a single physical channel.

Two type of trunking mechanisms that exists are:

1. Frame filtering

2. Frame tagging

Cisco 3 - LAN

Perrine. J Page 204/19/23

Chapter 9

Macintosh SE¾

Macintosh SE¾

Macintosh SE¾

Macintosh SE¾

Macintosh SE¾

Macintosh SE¾

Switch

Switch

VLAN1 VLAN2VLAN3

2nd floor

1st floor

Backbone

VLAN2

Packet with VLANID, added by the switch

VLAN2

VLANID is removed by the switch before sending to the target host.

Cisco 3 - LAN

Perrine. J Page 304/19/23

Chapter 9

VLAN Trunking Protocol

The most common schemes for Ethernet are:

1. Inter-Switch Link (ISL)

2. 802.1q

A trunk link does not belong to a specific VLAN. The responsibility of a trunk link is to act as a conduit for VLANs between switches and routers.

The role of VTP is to maintain VLAN configuration consistency across a common network administration domain.

Cisco 3 - LAN

Perrine. J Page 404/19/23

Chapter 9

VLAN Trunking Protocol

VTP messages are encapsulated either

• Cisco proprietary ISL

• IEEE 8802.1Q

protocol frames.

Using VTP, each Catalyst Family Switch advertises the following on its trunk ports:

• Management domain

• Configuration revision number

• Known VLANs & their specific parameters

Cisco 3 - LAN

Perrine. J Page 504/19/23

Chapter 9

VLAN Trunking Protocol

VTP provides the following benefits:

• VLAN configuration consistency across the network

• Mapping scheme that allows a VLAN to be trunked over mixed media

• Accurate tracking and monitoring of VLANs

• Dynamic reporting of added VLANs across the network

• Plug-and-play configuration when adding new VLANs

Cisco 3 - LAN

Perrine. J Page 604/19/23

Chapter 9

VLAN Trunking Protocol

A VTP domain is made up of one or more interconnected devices that share the same VTP domain name.

VTP switches operate in one of three modes:

• Server

• Client

• Transparent

Cisco 3 - LAN

Perrine. J Page 704/19/23

Chapter 9

VTP Server Mode

If a switch is configured for server mode, you can create, modify and delete VLANs.

VTP servers advertise their VLAN configuration to other switches in the same VTP domain.

This is the default mode on the switch.

Cisco 3 - LAN

Perrine. J Page 804/19/23

Chapter 9

VTP Client Mode

A switch that is configured as a VTP client cannot create, modify or delete VLAN information.

Clients cannot save VLAN information.

Is useful for switches that lack sufficient memory to store large tables of VLAN information.

VTP clients process VLAN changes as servers do, and they send VTP messages out all trunk ports.

Cisco 3 - LAN

Perrine. J Page 904/19/23

Chapter 9

VTP Transparent Mode

Switches configured for transparent mode do not participate in VTP.

They forward VTP advertisements received on their trunk ports but ignore information contained in the message

They neither modify its database when updates are received nor sends an update indicating a change in its VLAN status.

Transparent mode switches can add VLANs of local significance only.

VTP is disable on a transparent switch.

Cisco 3 - LAN

Perrine. J Page 1004/19/23

Chapter 9

VTP Operation

VTP database entry has a revision number. A higher configuration revision number indicates that the VLAN information that is being sent is more current than the stored copy.

When a switch receives an update that has a higher configuration revision number, the switch overwrites the stored information with the new information being sent in the VTP update.

By default management domains are set to a nonsecure mode which means that the switches interact without using a password.

If a password is used, then the same password must be configured on every switch in the management domain.

Cisco 3 - LAN

Perrine. J Page 1104/19/23

Chapter 9

VTP Implementation

With VTP, each switch advertises on its trunk ports

• Its management domain

• Configuration revision number

• The VLANs that it knows about

• Certain parameters for each known VLAN

Cisco 3 - LAN

Perrine. J Page 1204/19/23

Chapter 9

VTP Implementation

What triggers the advertisements:

• Creating or deleting a VLAN

• Suspending or activating a VLAN

• Changing the name of a VLAN

• Changing the maximum transmission unit, MTU, of a VLAN

Cisco 3 - LAN

Perrine. J Page 1304/19/23

Chapter 9

VTP Implementation

Two types of VTP advertisements exists:

• Requests from clients that want information at bootup

• Responses from servers

VTP messages are one of three types:

• Advertisement request – CLIENT request VLAN information

• Summary advertisement – SERVER responds with summary

• Subset advertisement – SERVER responds with subset

By default, server and client Catalyst switches issue summary advertisements every five minutes.

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Perrine. J Page 1404/19/23

Chapter 9

VTP Pruning

Remember, the default behavior of a switch is to propagate broadcast and unknown packets across the network.

VTP pruning enhances network bandwidth use by reducing unnecessary flooding of traffic.

If no device from VLAN3 is available on a remote switch, pruning can keep the switch from sending VLAN3 traffic out the trunk and wasting bandwidth.

By default, VTP pruning is disabled.

VLAN 1 is always pruning ineligible.