VTP Component


There are number of key components that you need to be familiar with when learning about VTP. Here is a brief description of the components, which will be further explained as you go through the chapter.
VTP Domain-Consists of one or more interconnected switches. All switches in a domain share VLAN configuration details using VTP advertisements. A router or Layer 3 switch defines the boundary of each domain.
VTP Advertisements-VTP uses a hierarchy of advertisements to distribute and synchronize VLAN configurations across the network.
VTP Modes- A switch can be configured in one of three modes: server, client, or transparent.
VTP Server-VTP servers advertise the VTP domain VLAN information to other VTP-enabled switches in the same VTP domain. VTP servers store the VLAN information for the entire domain in NVRAM. The server is where VLAN can created, deleted, or renamed for the domain.
VTP Client-VTP clients function the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client. A VTP client only stores the VLAN information for the entire domain while the switch is on. A switch reset deletes the VLAN information. You must configure VTP client mode on a switch.
VTP Transparent-Transparent switches forward VTP advertisements to VTP clients and VTP servers. Transparent switches do not participate in VTP. VLANs that are created, renamed, or deleted on transparent switches are local to that switch only.
VTP Pruning-VTP pruning increases network available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to reach the destination devices. Without VTP pruning, a switch floods broadcast, multicast, and unknown unicast traffic across all trunk links within a VTP domain even though receiving switches might discard them.
VTP Frame Structure
VTP advertisements (or messages) distribute VTP domain name and VLAN configuration changes to VTP-enabled switches. In this topic, you will learn about the VTP frame structure and how the three types of advertisements enable VTP to distribute and synchronize VLAN configurations throughout the network.
VTP Frame Encapsulation
A VTP frame consists of a header field and a message field. The VTP information is inserted into the data field of an Ethernet frame. The Ethernet frame is then encapsulated as a 802.1Q trunk frame (or ISL frame). Each switch in the domain sends periodic advertisements out each trunk port to a reserved multicast address. These advertisements are received by neighboring switches, which update their VTP and VLAN configurations as necessary.
VTP Frame Details
In the figure, you can see the VTP frame structure in more detail. Keep in mind that a VTP frame encapsulated as an 802.1Q frame is not static. The contents of the VTP message determines which fields are present. The receiving VTP-enabled switch looks for specific fields and values in the 802.1Q frame to know what to process. The following key fields are present when a VTP frame is encapsulated as an 802.1Q frame:
Destination MAC address-This address is set to 01-00-0C-CC-CC-CC, which is the reserved multicast address for all VTP messages.
LLC field-Logical link control (LLC) field contains a destination service access point (DSAP) and a source service access point (SSAP) set to the value of AA.
SNAP field-Subnetwork Access Protocol (SNAP) field has an OUI set to AAAA and type set to 2003.
VTP header field- The contents vary depending on the VTP message type-summary, subset, or request-but it always contains these VTP fields:
Domain name- Identifies the administrative domain for the switch.
Domain name length- Length of the domain name.
Version- Set to either VTP 1, VTP 2, or VTP 3. The Cisco 2960 switch only supports VTP 1 and VTP 2.
Configuration revision number- The current configuration revision number on this switch.
VTP message field-Varies depending on the message type.
VTP Message Contents
VTP frames contain the following fixed-length global domain information:
VTP domain name
Identity of the switch sending the message, and the time it was sent
MD5 digest VLAN configuration, including maximum transmission unit (MTU) size for each VLAN
Frame format: ISL or 802.1Q
VTP frames contain the following information for each configured VLAN:
VLAN IDs (IEEE 802.1Q)
VLAN name
VLAN type
VLAN state
Additional VLAN configuration information specific to the VLAN type

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