Dynamic Trunking
Protocol (DTP)
Recall that a trunk’s frame
tagging protocol can be auto negotiated, through the use of the Dynamic
Trunking Protocol (DTP). DTP can also negotiate whether a port becomes a trunk
at all.
Examples demonstrated how to
manually configure a port to trunk:
Switch(config)# interface gi2/24
Switch(config-if)# switchport
mode trunk
DTP has two modes to dynamically decide
whether a port becomes a trunk:
Desirable
– the
port will actively attempt to form a trunk with the remote switch. This is the
default setting.
Auto
–
the port will passively wait for the remote switch to initiate the trunk.
What is VTP (VLAN Trunking Protocol)?
VTP (VLAN Trunking Protocol) is a Cisco proprietary
protocol used by Cisco switches to exchange VLAN information. With VTP, you can
synchronize VLAN information (like VLAN ID or VLAN name) with switches inside
the same VTP domain.
VTP Versions
There are three versions of VTP. VTP version 1 supports
the standard 1 – 1005 VLAN range. VTP version 1 is also default on Catalyst
switches.
VTP version 2 introduces
some additional features:
Token Ring support
VLAN
consistency checks
Domain-independent
transparent pass through VTPv1 and v2 are not compatible. The VTP version is
dictated by the VTP server, discussed in detail shortly. If the VTP server is
configured for VTPv2, all other switches in the VTP domain will change to v2 as
well. Until recently, VTP Version 3 was supported on only limited Cisco switch platforms.
VTPv3 was built to be flexible, and can forward both VLAN and other database
information, such as Multiple Spanning Tree (MST) protocol.
Other enhancements
provided by VTPv3 include:
Support for the extended 1006-4094 VLAN range.
Support for private VLANs.
Improved VTP authentication.
Protection from accidental database overwrites, by
using VTP primary and secondary servers.
Ability to enable VTP on a per-port basis.
Each switch can use one of
three different VTP modes:
1. VTP client mode –
a switch using this mode can’t change its VLAN configuration. That means that a
VTP client switch can’t create or delete VLANs. Received VTP updates are
processed and forwarded.
2. VTP server mode –
a switch using this mode can create and delete VLANs. A VTP server switch will
propagate VLAN changes. This is the default mode for Cisco switches.
3. VTP transparent mode –
a switch using this mode doesn’t share its VLAN database, but it forwards
received VTP advertisements. You can create and delete VLANs on a VTP
transparent switch, but the changes are not sent to other switches.
VTP Server
|
VTP Client
|
VTP Transparent
|
|
Create/Modify/Delete VLANs
|
Yes
|
No
|
Only local
|
Synchronizes itself
|
Yes
|
Yes
|
No
|
Forwards advertisements
|
Yes
|
Yes
|
Yes
|
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