network devices. VLAN1 is always ineligible for pruning; traffic from VLAN1 cannot be pruned. Figure shows a switched network without VTP pruning enabled on the left. Port 1 on switch 1 and port 2 on switch 4 are assigned to the red VLAN. A broadcast is sent from the host connected to switch 1. Switch 1 floods the broadcast, and every network device in the network receives it, even though switches 3, 5, and 6 have no ports in the red VLAN. With VTP pruning enabled, the broadcast traffic from station A is not forwarded to switches 3, 5, and 6 because traffic for the red VLAN has been pruned on the links indicated on switches 2 and 4. Note: You can implement VTP pruning only on VTP servers, not on clients. Consider VTP pruning support to minimize traffic on trunk links. Note: A switch runs an instance of spanning tree for each VLAN that it is aware of, even if no ports are active or if VTP pruning is enabled. VTP pruning prevents unnecessary flooded traffic but does not eliminate the switch knowledge of pruned VLANs.
Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.5 Describing VTP Operation Switches within a VTP management domain synchronize their VLAN databases by sending and receiving VTP advertisements over trunk links. VTP advertisements are flooded throughout a management domain by switches running in specific modes of operation. Advertisements are sent every 5 minutes or whenever there is a change in VLAN configuration. VTP advertisements are transmitted over VLAN1, using a Layer 2 multicast frame. VLAN advertisements are not propagated from a switch until a management domain name is specified or learned. Figure shows the general order of VLAN synchronization over VTP. One of the most critical components of VTP is the configuration revision number. When initially configured, the VTP configuration revision number is set to 0. Each time a VTP server modifies its VLAN information, it increments the VTP configuration revision number by one. It then sends out a VTP advertisement referencing the new configuration revision number. If the configuration revision number being advertised is higher than the number stored on other switches in the VTP domain, they overwrite their VLAN configurations with the new information. CAUTION: Because of the overwrite process, if all VLANs on a VTP server are deleted, the VTP server sends an advertisement with a higher revision number. The receiving devices in the VTP domain accept the advertisement and delete those VLANs as well. Three types of VTP advertisements are exchanged between switches: Note: VTP advertisements are associated with VLAN database information only, not with VLAN information configured on specific switch ports. Likewise, on a receiving switch, the receipt of new VLAN information does not change the VLAN associations of trunk or access ports on that switch.
Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.6 Describing VTP Configuration Commands The vtp configuration command is used to configure VTP characteristics for a switch. All switches in the same VTP domain share the same VTP domain name and password, if one is configured. It is a good idea to set the VTP mode to “client” if switches are being added to an existing switched network. The show vtp commands are used to verify the current VTP parameter values. Figure describes the commands that are used to configure VTP.
Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.7 Configuring a VTP Management Domain Default VTP configuration values depend on the switch model and the software version. The default values for the Catalyst 2900, 4000, and 6000 series switches are as follows: The VTP domain name can be specified or learned from VTP updates received from other switches. By default, the domain name is not set. A password can be set for the VTP management domain. The password must be the same for all switches in the domain in order for the VLAN database to be synchronized among switches. The steps for configuring VTP vary per design and switch mode, but the general steps for configuring a switch are as follows : Step 1 Establish a design specifying which switches are server, client, or transparent, and what the boundaries are for the VTP domain.
Step 2 Verify the current VLAN information on any switch that will be configured as server.
Step 3 Specify the VTP password (optional).
Step 4 Specify the version number, if other than the default.
Step 5 Specify the VTP domain name (case-sensitive).
Step 6 Configure the VTP mode.
Step 7 Verify the configuration.
Step 8 Verify that updates are being sent from or received by the switch as intended. Figure describes the commands used to configure a switch to become part of a VTP domain. Follow these steps from privileged EXEC mode. Use the show vtp status command to verify the VTP configuration. When initially configuring switches in a VTP domain, pay close attention to the configuration revision number. Check to see that it increments only when changes are made at intended VTP servers. In Figure , “Configuration last modified by 10.1.1.1” specifies the IP address of the switch that last updated the VLAN database of this switch. Note: In this example, VTP version 2 is available (as shown by the “VTP Version” line of the output), but not enabled (as shown by the “VTP V2 Mode”). Use the show vtp counters command to display statistics about VTP operation. Output from this command verifies if VTP updates are being sent and received by the switch, and it records the number of updates that have been seen.
Content 2.4 Propagating VLAN Configurations with VLAN Trunking Protocol 2.4.8 Adding New Switches to an Existing VTP Domain The configuration revision number is used when determining if a switch should keep its existing VLAN database or overwrite it with the VTP update sent by another switch in the same domain with the same password. Therefore, when a switch is added to a network, it is important that it does not inject spurious information into the domain. CAUTION: This overwrite occurs whether the switch is a VTP client or server. A VTP client can erase VLAN information on a VTP server. One indication that information has been erased is when many of the ports in the network go into inactive state because the ports are now assigned to a nonexistent VLAN. An example of a VTP client overwriting a VTP server will be shown later. Figure describes the procedure for adding a new switch to a network. For VLAN stability, it is