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A VTP domain is made up of one or more
interconnected devices that share the same VTP domain name. A switch
can be configured to be in one VTP domain only. Global VLAN
information is propagated across the network by way of connected
switch trunk ports.
When transmitting VTP messages to
other switches in the network, the VTP message is encapsulated in a
trunking protocol frame such as ISL or IEEE 802.1Q. Figure
shows the generic encapsulation for VTP within an ISL
frame. The VTP header varies, depending upon the type of VTP
message, but generally, four items are found in all VTP messages:
- VTP protocol version - Either
Version 1 or 2
- VTP message type - Indicates one
of four types
- Management domain name length -
Indicates size of the name that follows
- Management domain name - The name
configured for the management domain
It is important to note that switches
can be configured not to accept VTP information. These switches will
forward VTP information on trunk ports in order to ensure that other
switches receive the update, but the switches will not modify their
database, nor will the switches send out an update indicating a
change in VLAN status. This is referred to as transparent mode.
By default, management domains are
set to a nonsecure mode, meaning that the switches interact without
using a password. Adding a password automatically sets the
management domain to secure mode. A password must be configured on
every switch in the management domain to use secure mode.
Detecting the addition of VLANs within the advertisements serves as a notification to the switches (servers and clients) that they should be prepared to receive traffic on their trunk ports with the newly defined VLAN IDs, emulated LAN names, or 802.10 SAIDs (Security Association IDentifier).
In Figure ,
C5000-3
transmits a VTP database entry with additions or deletions to
C5000-1 and C5000-2. The configuration database has a revision
number that is notification +1. A higher configuration revision
number indicates that the VLAN information that is being sent is
more current then the stored copy. Any time a switch receives an
update that has a higher configuration revision number, the switch
will overwrite the stored information with the new information being
sent in the VTP update.
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