Switches within a VTP management domain synchronize their VLAN databases by
sending and receiving VTP advertisements over trunk links. VTP advertisements
are flooded throughout a management domain by switches running in specific
modes of operation. Advertisements are sent every five minutes or whenever
there is a change in VLAN configuration. VTP advertisements are transmitted
over VLAN1 using a Layer 2 multicast frame. VLAN advertisements are not
propagated from a switch until a management domain name is specified or
learned.
The process of VLAN synchronization over VTP follows this
general order:
Step 1 Configure the VTP domain, VTP mode, and VTP password
(optional) on each switch. This proactively determines which switches will send
updates.
Step 2 Switches running VTP server mode
then send VTP updates across trunk links.
Step 3
A device that receives a VTP advertisement will check that the VTP management
domain name and password in the advertisement match those configured in the
local switch.
Step 4 If a match is found, a
switch further inspects the VTP update to see the configuration revision
number.
Step 5 If the configuration revision
number of the message is greater than the number currently in use and the
switch is running in VTP server or client mode, the switch overwrites its
current VLAN information with that in the received update.
Step 6 The switch may also request more
information.
Configuration Revision Number
One of
the most critical components of VTP is the configuration revision number. When
initially configured, the VTP configuration revision number is set to 0. Each
time a VTP server modifies its VLAN information, it increments the VTP
configuration revision number by one. It then sends out a VTP advertisement
referencing the new configuration revision number. If the configuration
revision number being advertised is higher than the number stored on other
switches in the VTP domain, they will overwrite their VLAN configurations with
the new information.
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CAUTION:
Given this overwrite process, if the VTP server sending the
advertisement had deleted all VLANs and sent an advertisement with a higher
revision number, the receiving devices in the VTP domain will delete those
VLANs as well.
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VTP Advertisement Types
There are three types of VTP
advertisements exchanged between switches:
-
Summary Advertisements – an update sent by VTP servers every 300
seconds or when a VLAN database change occurs. Among other things, this
advertisement lists the management domain, VTP version, domain name,
configuration revision number, timestamp, and number of subset advertisements.
When the advertisement results from a VLAN database change, one or more subset
advertisements will follow.
-
Subset Advertisements – an update that follows a summary
advertisement that results from a change in the VLAN database. A subset
advertisement cites the specific change that was made to a specific VLAN entry
in the VLAN database. One subset advertisement will be sent for each VLAN ID
that encountered a change.
-
Advertisement Requests from Clients – an update sent by a switch
requesting information in order to update its VLAN database. If a client hears
a VTP summary advertisement with a configuration revision number higher than
its own, the switch may send an Advertisement Request. A switch operating in
VTP server mode, then responds with summary and subset advertisements.
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NOTE:
VTP advertisements are associated with VLAN database information only,
not VLAN information configured on specific switch ports. Likewise, on a
receiving switch, the receipt of new VLAN information does not change the VLAN
associations of trunk or access ports on that switch.
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VTP Versions
Currently Catalyst switches run VTP versions 1, 2,
or 3. Version 2 is the most prevalent, and provides these features:
- Forwarding of VTP updates from transparent mode switches without checking
the version number
- Consistency checks on new VTP and VLAN configuration parameters
- Support for Token Ring Switches
- Propagation of VTP updates that have an unrecognized type, length or
value
VTP version 3 is available on some switches now using the Cat OS
operating system version.