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VLAN Trunking protocol

CCNA Exploration Semester 3

Chapter 4

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Topics

The role of VLAN Trunking Protocol (VTP) Operation of VTP Configure VTP on switches

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Semester 3

LAN Design

Basic Switch Concepts

VLANs

VTP

STP

Inter-VLAN routing

Wireless

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Purpose of VTP

You can create VLANs on a switch. What if you have the same VLANs on 10

linked switches? Or 100 linked switches? Do you have to create the VLANs on every

switch and allow them on each trunk? VTP helps. But you still have to assign access ports to

VLANs on each switch.

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VTP domain

Group of layer 2 switches sharing VLAN data. Ends at router or layer 3 switch. Switch can be linked but not part of domain. Each switch can belong to only one domain. Domain is defined by its name. Proprietary to Cisco, so all switches in

domain must be Cisco switches.

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VTP server

One switch acts as server. Create VLANs on this switch. Information saved in vlan.dat. Server sends VLAN information to client

switches over active trunk links. Add, delete, rename VLANs on server. Default mode of switch is server.

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VTP client

Client receives VLAN information from server.

Client switches then have the same VLANs as the server.

Client does not save VLAN information. It is held only in RAM and lost if switch is powered off.

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VTP modes

Server – default mode. Sends VLAN information to other switches.

Client – receives VLAN information and forwards it to other switches.

Transparent – forward VTP traffic but do not originate or use it. They can have their own VLANs, not shared with other switches.

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VTP defaults

Version 1. (Versions 2 and 3 also exist.) VTP domain name is not set. VTP mode server One active VLAN, VLAN 1 Configuration revision number 0 Any switches added to a domain should be in

the default condition or they may send unwanted information to other switches.

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Show vtp status

VTP Version Maximum VLANs Supported Number of Existing VLANs VTP Operating Mode- server, client, or transparent. VTP Domain Name VTP Pruning Mode VTP V2 Mode (disabled by default) VTP Traps Generation MD5 Digest (checksum of VTP configuration) Configuration Last Modified

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VTP advertisements

Consist of VTP header and VTP message Encapsulated inside Ethernet frame with tag

to pass over trunk link. Destination MAC address is multicast

address 01-00-0C-CC-CC-CC

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VTP message header

Domain name Domain name length Version - VTP 1 or VTP 2, on Cisco 2960

switch. Configuration revision number Other fields, depending on type of message.

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Configuration revision number

32-bit number. Default value is 0. It is incremented each time a VLAN is added

or removed. Reset to 0 is domain name changes. Switch uses it to see if information is more

recent that what it already holds.

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Summary advertisement

Sent immediately after a change is made, with updated revision number.

Sent every 5 minutes by servers and clients to check on current VTP configuration revision number.

Contains the VTP domain name, the current revision number, and other VTP configuration details.

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VTP Summary advertisement

A switch receives a summary advertisement. Compares domain name to its own domain name. If name is different, the switch ignores the packet. If the name is the same, the switch compares the

revision number to its own revision number. Number higher or equal, ignored the packet. Number lower, sends an advertisement request.

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Request Advertisement

Sent by client to server if: The VTP domain name has been changed The switch receives a summary

advertisement with a higher configuration revision number than its own

A subset advertisement message is missed for some reason

The switch has been reset

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Subset advertisement

Contains VLAN information. Several may be needed if there is a lot of information.

Sent by server in response to a request or after: Creating or deleting a VLAN Suspending or activating a VLAN Changing the name of a VLAN Changing the MTU of a VLAN

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Subset advertisement

Version Code Seq-number Domain name length

Management domain name (padded to 32 bytes)

Configuration revision number

VLAN info field 1

VLAN info field 2

Etc.

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Subset advertisement VLAN info

VLAN-Info

Info length Status VLAN-Type VLAN-name Len

ISL VLAN ID MTU size

802.10 index

VLAN name (padded to multiple of 4 bytes)

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Pros and cons

Small network – don’t bother with VTP. Big network – good for consistency and

easier to make changes. Server switches need lots of flash memory,

clients do not. Redundancy – don’t have everything on one

server switch. Problems from large domains. Extreme care when adding a switch.

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Why transparent?

Put a switch in transparent mode if it has local VLANs that are not on other switches.

The other switches do not need to know about them.

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VTP pruning

Disabled by default Enable it on one server in domain. Stops VLAN traffic from being sent on links

that do not lead to devices on that VLAN. Cuts down on traffic on trunk links.

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Domain names and passwords

Configure a domain name on the first server switch. The other switches will learn it.

If you configure it on other switches, check that it is exactly the same. It is case sensitive.

If you use a password then it must be exactly the same on all switches.

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Versions

VTP version 1 is used by default on Catalyst switches, but they can use version 2.

If you configure version 2 on one switch then the other switches should learn the new version and change to it.

If a switch is not capable of running version 2 then it will not exchange advertisements.

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Configure VTP

Configure VTP with a domain name before creating VLANs on the server.

Existing VLANs are removed when you start to configure VTP.

Check that links are trunk links. Configure client switches to client mode.

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Commands on server

SW1(config)#vtp domain cisco1 SW1(config)#vtp password cisco (Password is optional) Server mode is default, but if it was changed: SW1(config)#vtp mode server Version 1 is default, but command is: SW1(config)#vtp version 1

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Commands on server

Create VLANs Check that link is a trunk. Check VTP operation SW1# show vtp status Assign switch ports to VLANs.

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Client configuration

SW2(config)#vtp mode client

Check that link is a trunk. Check VTP operation SW2# show vtp status Assign switch ports to VLANs.

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Things to check

VTP Version. It needs to be the same on all switches in the domain.

Domain name. Is it exactly the same on all switches?

VTP Password if any. Is it exactly the same on all switches?

Check that there is at least one server. Better to have at least two.

If you recently added a new switch, had its revision number been set to 0?

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The End