Cisco Configuring VTP: User & Service Manual PDF (EN)

Prerequisites for VTP

Configuring VTP Prerequisites for VTP, on page 1 Restrictions for VTP, on page 2 Information About VTP, on page 2 How to Configure VTP, on page 9 Monitoring VTP, on page 19 Configuration Examples for VTP, on page 19 Where to Go Next, on page 19 Additional References, on page 20 Feature History for VTP, on page 20 Prerequisites for VTP Before you create VLANs, you must decide whether to use the VLAN Trunking Protocol (VTP) in your network. Using VTP, you can make configuration changes centrally on one or more devices and have those changes automatically communicated to all the other devices in the network. Without VTP, you cannot send information about VLANs to other devices.

VTP is designed to work in an environment where updates are made on a single device and are sent through VTP to other devices in the domain. It does not work well in a situation where multiple updates to the VLAN database occur simultaneously on devices in the same domain, which would result in an inconsistency in the VLAN database. You can enable or disable VTP per port by entering the [ no ] vtp interface configuration command. When you disable VTP on trunking ports, all VTP instances for that port are disabled. You cannot set VTP to off for the MST database and on for the VLAN database on the same port.

When you globally set VTP mode to off, it applies to all the trunking ports in the system. However, you can specify on or off on a per-VTP instance basis. For example, you can configure the device as a VTP server for the VLAN database but with VTP off for the MST database. Because trunk ports send and receive VTP advertisements, you must ensure that at least one trunk port is configured on the device or device stack and that this trunk port is connected to the trunk port of another device. Otherwise, the device cannot receive any VTP advertisements. Configuring VTP 1

Стр. 1

VTP Domain

Restrictions for VTP The following are restrictions for a VTP: Caution Before adding a VTP client device to a VTP domain, always verify that its VTP configuration revision number is lower than the configuration revision number of the other devices in the VTP domain. Devices in a VTP domain always use the VLAN configuration of the device with the highest VTP configuration revision number. If you add a device that has a revision number higher than the revision number in the VTP domain, it can erase all VLAN information from the VTP server and VTP domain. Information About VTP The following sections provide information about VTP and VTP configuration:

VTP VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs on a network-wide basis. VTP minimizes misconfigurations and configuration inconsistencies that can cause several problems, such as duplicate VLAN names, incorrect VLAN-type specifications, and security violations. VTP functionality is supported across the stack, and all devices in the stack maintain the same VLAN and VTP configuration inherited from the active device. When a device learns of a new VLAN through VTP messages or when a new VLAN is configured by the user, the new VLAN information is communicated to all devices in the stack.

When a device joins the stack or when stacks merge, the new devices get VTP information from the active device. VTP Domain A VTP domain (also called a VLAN management domain) consists of one device or several interconnected devices under the same administrative responsibility sharing the same VTP domain name. A device can be in only one VTP domain. You make global VLAN configuration changes for the domain. By default, the device is in the VTP no-management-domain state until it receives an advertisement for a domain over a trunk link (a link that carries the traffic of multiple VLANs) or until you configure a domain name.

You can create or modify VLANs on a VTP server without specifying the domain name. However, when the management domain name is not specified VLAN information is not propagated over the network. If the device receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The device then ignores advertisements with a different domain name or an earlier configuration revision number. When you make a change to the VLAN configuration on a VTP server, the change is propagated to all devices in the VTP domain. VTP advertisements are sent over all IEEE trunk connections, including IEEE 802.1Q.

Configuring VTP Restrictions for VTP Configuring VTP 2

Стр. 2

VTP Modes

VTP dynamically maps VLANs with unique names and internal index associates across multiple LAN types. Mapping eliminates excessive device administration required from network administrators. If you configure a device for VTP transparent mode, you can create and modify VLANs, but the changes are not sent to other devices in the domain, and they affect only the individual device. However, configuration changes made when the device is in this mode are saved in the device running configuration and can be saved to the device startup configuration file. VTP Modes Table 1:

VTP Modes Description VTP Mode VTP server In VTP server mode, you can create, modify, and delete VLANs, and specify other configuration (such as the VTP version) for the entire VTP domain. VTP servers advertise their VLAN configura devices in the same VTP domain and synchronize their VLAN configurations with other devices advertisements received over trunk links. VTP server is the default mode. In VTP server mode, VLAN configurations are saved in NVRAM. If the device detects a failure w a configuration to NVRAM, VTP mode automatically changes from server mode to client mode. If the device cannot be returned to VTP server mode until the NVRAM is functioning.

VTP client A VTP client functions like a VTP server and transmits and receives VTP updates on its trunks, b create, change, or delete VLANs on a VTP client. VLANs are configured on another device in th is in server mode. In VTP versions 1 and 2 in VTP client mode, VLAN configurations are not saved in NVRAM. In 3, VLAN configurations are saved in NVRAM in client mode. VTP transparent VTP transparent devices do not participate in VTP.

A VTP transparent device does not advertise configuration and does not synchronize its VLAN configuration based on received advertisements VTP version 2 or version 3, transparent devices do forward VTP advertisements that they receive devices through their trunk interfaces. You can create, modify, and delete VLANs on a device in VT mode. In VTP versions 1 and 2, the device must be in VTP transparent mode when you create private VLA they are configured, you should not change the VTP mode from transparent to client or server mode 3 also supports private VLANs in client and server modes. When private VLANs are configured, the VTP mode from transparent to client or server mode.

When the device is in VTP transparent mode, the VTP and VLAN configurations are saved in NV they are not advertised to other devices. In this mode, VTP mode and domain name are saved in t running configuration, and you can save this information in the device startup configuration file b copy running-config startup-config privileged EXEC command. In a device stack, the running configuration and the saved configuration are the same for all devic A device in VTP off mode functions in the same manner as a VTP transparent device, except that forward VTP advertisements on trunks. VTP off Configuring VTP VTP Modes Configuring VTP 3

Стр. 3

VTP Version 2

VTP Advertisements Each device in the VTP domain sends periodic global configuration advertisements from each trunk port to a reserved multicast address. Neighboring devices receive these advertisements and update their VTP and VLAN configurations as necessary. Because trunk ports send and receive VTP advertisements, you must ensure that at least one trunk port is configured on the switch stack and that this trunk port is connected to the trunk port of another switch. Otherwise, the switch cannot receive any VTP advertisements. VTP advertisements distribute this global domain information:

• VTP domain name • VTP configuration revision number • Update identity and update timestamp • MD5 digest VLAN configuration, including maximum transmission unit (MTU) size for each VLAN • Frame format VTP advertisements distribute this VLAN information for each configured VLAN: • VLAN IDs (including IEEE 802.1Q) • VLAN name • VLAN type • VLAN state • Additional VLAN configuration information specific to the VLAN type In VTP version 3, VTP advertisements also include the primary server ID, an instance number, and a start index. VTP Version 2 If you use VTP in your network, you must decide which version of VTP to use. By default, VTP operates in version 1.

VTP version 2 supports these features that are not supported in version 1: • Token Ring support—VTP version 2 supports Token Ring Bridge Relay Function (TrBRF) and Token Ring Concentrator Relay Function (TrCRF) VLANs. • Unrecognized Type-Length-Value (TLV) support—A VTP server or client propagates configuration changes to its other trunks, even for TLVs it is not able to parse. The unrecognized TLV is saved in NVRAM when the device is operating in VTP server mode. • Version-Dependent Transparent Mode—In VTP version 1, a VTP transparent device inspects VTP messages for the domain name and version and forwards a message only if the version and domain name match.

Although VTP version 2 supports only one domain, a VTP version 2 transparent device forwards a message only when the domain name matches. • Consistency Checks—In VTP version 2, VLAN consistency checks (such as VLAN names and values) are performed only when you enter new information through the CLI or SNMP. Consistency checks are Configuring VTP VTP Advertisements Configuring VTP 4

Стр. 4