VTP Modes, CCNA Course Training in Delhi Gurgaon India

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VTP operates in one of three modes: server, transparent, or client. You can complete
different tasks depending on the VTP operation mode. The characteristics of the three VTP
modes are as follows:
■ Server: The default VTP mode is server mode, but VLANs are not propagated over the
network until a management domain name is specified or learned. When you change
(create, modify, or delete) the VLAN configuration on a VTP server, the change is
propagated to all switches in the VTP domain. VTP messages are transmitted out of all
the trunk connections. A VTP server synchronizes its VLAN database file with other
VTP servers and clients.
802.1Q
Trunk
VTP Client VTP Client VTP Client VTP Client
802.1Q
Trunk
802.1Q
Trunk
VTP Server
VTP Domain ICND
1. VLAN Added/Deleted
2. Change Propagated
3. Synch to Latest Change
802.1Q
Trunk
28 Chapter 2: Medium-Sized Switched Network Construction
■ Transparent: When you change the VLAN configuration in VTP transparent mode,
the change affects only the local switch and does not propagate to other switches in the
VTP domain. VTP transparent mode does forward VTP advertisements that it receives
within the domain. A VTP transparent device does not synchronize its database with
any other device.
■ Client:You cannot change the VLAN configuration when in VTP client mode;
however, a VTP client can send any VLANs currently listed in its database to other
VTP switches. VTP advertisements are forwarded in VTP client mode. A VTP client
synchronizes its database with other VTP servers and clients.
VTP clients that run Cisco Catalyst operating systems do not save the VLANs to NVRAM.
When the switch is reloaded, the VLANs are not retained, and the revision number is zero.
However, Cisco IOS VTP clients save VLANs to the vlan.dat file in flash memory, retaining
the VLAN table and revision number.
VTP Operation
VTP advertisements are flooded throughout the management domain. VTP advertisements
are sent every 5 minutes or whenever VLAN configurations change. Advertisements are
transmitted over the default VLAN (VLAN 1) using a multicast frame. A configuration
revision number is included in each VTP advertisement. A higher configuration revision
number indicates that the VLAN information being advertised is more current than the
stored information. Figure 2-12 illustrates this operation.
Figure 2-12 VTP Operation
CAUTION The erase startup-config command does not affect the vlan.dat file on
Cisco IOS switches. VTP clients with a higher configuration revision number can
overwrite VLANs on a VTP server in the same VTP domain. Delete the vlan.dat file and
reload the switch to clear the VTP and VLAN information. See documentation for your
specific switch model to determine how to delete the vlan.dat file.
1. Add New VLAN
2. Rev 3 Rev 4
Client Client
4 Server 4
3. Rev 3 Rev 4
4. Sync New VLAN
Information
3. Rev 3 Rev 4
4. Sync New VLAN
Information
Implementing VLANs and Trunks 29
One of the most critical components of VTP is the configuration revision number. Each
time a VTP server modifies its VLAN information, the VTP server increments the
configuration revision number by one. The server then sends a VTP advertisement with the
new configuration revision number. If the configuration revision number being advertised
is higher than the number stored on the other switches in the VTP domain, the switches
overwrite their VLAN configurations with the new information being advertised.
The configuration revision number in VTP transparent mode is always zero.
A device that receives VTP advertisements must check various parameters before
incorporating the received VLAN information. First, the management domain name and
password in the advertisement must match those configured in the local switch. Next, if the
configuration revision number indicates that the message was created after the
configuration currently in use, the switch incorporates the advertised VLAN information.
To reset the configuration revision number on some Cisco Catalyst switches, you can
change the VTP domain to another name and then change it back. You can also change the
VTP mode to transparent and then change it back to client or server.
VTP Pruning
VTP pruning uses VLAN advertisements to determine when a trunk connection is flooding
traffic needlessly.
By default, a trunk connection carries traffic for all VLANs in the VTP management
domain. In many enterprise networks, not every switch will have ports assigned to every
VLAN.
Figure 2-13 shows a switched network with VTP pruning enabled. Only switches 2, 4, and
5 support ports configured in VLAN 3. Switch 5 does not forward the broadcast traffic from
host X to switches 1 and 3 because traffic for VLAN 3 has been pruned on the links between
switch 5 and switch 1 and switch 3, as indicated in the figure.
VTP pruning increases available bandwidth by restricting flooded traffic to those trunk
links that the traffic must use to access the appropriate network devices.
You can enable pruning only on Cisco Catalyst switches that are configured for VTP
servers, and not on clients.