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With VTP, each switch advertises on its
trunk ports its management domain, configuration revision number,
the VLANs that it knows about, and certain parameters for each known
VLAN. These advertisement frames are sent to a multicast address so
that all neighboring devices can receive the frames; however, the
frames are not forwarded by normal bridging procedures. All devices
in the same management domain learn about any new VLANs now
configured in the transmitting device. A new VLAN must be created
and configured on one device only in the management domain. The
information is automatically learned by all the other devices in the
same management domain.
Advertisements on factory-default
VLANs are based on media types. User ports should not be configured
as VTP trunks.
Each advertisement starts as
configuration revision number 0. When changes are made, the
configuration revision number increments (n + 1). The revision
number in the management domain continues to increment until it
reaches 2,147,483,648, at which point the counter will reset back to
zero.
There are two types of
advertisements:
- Requests from clients that want
information at bootup
- Response from servers
There are three types of messages:
- Advertisement requests - Clients
request VLAN information, and the server responds with summary
and subset advertisements. See Figure
for examples of these.
- Summary advertisements - By
default, server and client Catalyst switches issue summary
advertisements every five minutes. They inform neighbor switches
what they believe to be the current VTP revision number.
Assuming the domain names match, the receiving server or client
compares the configuration revision number. If the revision
number in the advertisement is higher than the current revision
number in the receiving switch, the receiving switch then issues
an advertisement request for new VLAN information. See Figure
for an example.
- Subset advertisements - These
contain detailed information about VLANs such as VTP version
type, domain name and related fields, and the configuration
revision number. Creating or deleting a VLAN, suspending or
activating a VLAN, changing the name of a VLAN, and changing the
maximum transmission unit (MTU) of a VLAN can trigger these
advertisements. See Figure
for an example.
Advertisements may contain some or
all of the following information:
- Management domain name -
Advertisements with different names are ignored.
- Configuration revision number -
The
higher number indicates a more recent configuration.
- Message Digest 5 (MD5) - MD5 is
the key that is sent with the VTP when a password has been
assigned. If the key does not match, the update is ignored.
- Updater identity - The updater
identity is the identity of the switch that is sending the VTP
summary advertisement.
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