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 Wireless
Concepts
VLANs STP
VTP Inter-VLAN
routing
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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
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