native VLAN mismatch, traffic is not transmitted correctly on the trunk. Use show commands to display port information, switch port information, or trunking information. The output in Figure shows that DTP has negotiated with the other switch to enable 802.1Q trunking. Also note that the native VLAN has been configured to be VLAN99. It is best practice that the native VLAN is not left as the default of VLAN1 and should be an “unused” VLAN. This will be discussed in more detail later. In Figure , interface Fast Ethernet 2/1 has been configured as a trunk link for ISL that is permanently on. DTP negotiation is not allowed. The trunk link will carry VLAN traffic for VLANs 1 through 5 and 1002 through 1005. VLANs 2 through 5 are configured on various access ports on the switch, and the trunk links need to carry the frames for these VLANs in addition to the frames for the system VLANs 1 and 1002-1005. Note: It is best practice to shut down an interface while configuring trunking attributes so that premature autonegotiation cannot occur. When configuring the Layer 2 trunk to not use DTP, the following syntax is used so that the trunk mode is set to “on” and no DTP messages are sent on the interface:

Enter the shutdown command in the interface mode.
Enter the switchport trunk encapsulation command.
Enter the switchport mode trunk command.
Enter the switchport nonegotiate command.
Enter the no shutdown command.

Use show commands to display port information, switch port information, or trunking information.

Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.1 Explaining VTP Domains In an enterprise network with many interconnected switches, maintaining a consistent list of VLANs across those switches can be administratively cumbersome and potentially error prone. The VLAN Trunking Protocol (VTP) is designed to automate this administrative task. Switches that share common VLAN information are organized into logical groups called VTP management domains. The VLAN information within a VTP domain is propagated through trunk links and is updated via the VTP, allowing all switches within a particular domain to maintain identical VLAN databases. Only “global” VLAN information regarding VLAN number, name, and description is exchanged. Information on how ports are assigned to VLANs on a given switch is kept local to the switch and is not part of a VTP advertisement. These are the attributes of a VTP Domain:

A switch may be in only one VTP domain.
A VTP domain may be as small as only one switch.
VTP updates are exchanged only with other switches in the same domain.
The way VLAN information is exchanged between switches in the same domain depends upon the VTP mode of the switch.
By default, a Catalyst switch is in the no-management-domain state until it receives an advertisement for a domain over a trunk link, or until a management domain is configured.

Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.2 Describing VTP VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the additions, deletions, and name changes of VLANs on all switches in a VTP domain. Switches sharing a single VTP domain exchange VTP updates to distribute and synchronize VLAN information. VTP runs over trunk links, allowing interconnected switches to distribute and synchronize a single list of configured VLANs. This process reduces the manual configuration required at each switch; VLANs can be created on one switch and then propagated to others. VTP has the following attributes:

It is a Cisco proprietary protocol.
Advertises VLANs 1 through 1005 only.
Updates are exchanged only across trunk links.
Each switch operates in a given VTP mode that determines how updates are sent from and received by that switch.

Currently, Catalyst switches run VTP versions 1, 2, and 3. Version 2 is the most common, although within version 2, the default operating mode of the switch is version 1. Version 2 provides these features:

Support for Token Ring switches
Consistency checks on new VTP and VLAN configuration parameters
Propagation of VTP updates that have an unrecognized type, length, or value
Forwarding of VTP updates from transparent mode switches without checking the version number

VTP version 3 is now available on some switches that use the Cisco Catalyst operating system. When enabled, VTP version 3 provides these enhancements to previous VTP versions:

Support for extended VLANs
Support for the creation and advertising of private VLANs
Support for VLAN instances and MST mapping propagation instances
Improved server authentication
Protection from the wrong database accidentally being inserted into a VTP domain
Interaction with VTP versions 1 and 2
Ability to be configured on a per-port basis

CAUTION: VTP versions 1 and 2 are not interoperable on switches in the same VTP domain. Every switch in the VTP domain must use the same VTP version. Do not enable VTP version 2 unless every switch in the VTP domain supports version 2. There are some guidelines to using VTP within the Campus Infrastructure module:

The VTP domain is restricted to building switch blocks.
VTP keeps VLAN information consistent between the Building Distribution layer and Building Access layer switches.
VLAN configuration errors or failures are confined to the distribution and access layer switch blocks.
Knowledge of all VLANs does not need to exist on all switches within the Campus Infrastructure module. Use of VTP is optional, and in high-availability environments it is best practice to set all switches to ignore VTP updates.

CAUTION: VLANs deleted on one switch may be deleted on all switches in the VTP domain, and thus all ports are removed from that VLAN. Delete VLANs with caution on a switch participating in a VTP domain with other switches.

Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.3 VTP Modes VTP can be configured on each switch to operate in one of three modes: server, client, or transparent. The default mode is server. The mode determines if VLANs can be created on the switch and how the switch participates in sending and receiving VTP advertisements. The number of VLANs that can be configured on a switch varies by mode. Figure describes the features of the VTP client, server, and transparent modes. CAUTION: Before adding a VTP client or server to a VTP domain, always verify that its VTP configuration revision number is lower than the configuration revision number of the other switches in the VTP domain. Switches in a VTP domain always use the VLAN configuration of the switch with the highest VTP configuration revision number. If you add a switch in server or client mode that has a revision number that is higher than the revision number in the VTP domain, it can erase all VLAN information from the VTP server and VTP domain. To reset the VTP revision number on the switch that is being added, either modify the VTP domain name or set the VTP mode to transparent.

Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.4 Describing 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. Commonly, some switches in an enterprise network do not have local ports configured in each VLAN. In Figure , only switches 1 and 4 support ports statically configured in the red VLAN. VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate