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121 Apr 2023 S Ward Abingdon and Witney College
VLAN Trunking protocol
CCNA Exploration Semester 3
Chapter 4
221 Apr 2023 S Ward Abingdon and Witney College
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 lower or equal, ignored the packet. Number higher, 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