Cisco VTP: VLAN Trunking Protocol

VLAN Trunking Protocol (VTP) is a Cisco Layer 2 messaging protocol that manages the addition, deletion, and renaming of VLANs on a network-wide basis. Virtual Local Area Network (VLAN) Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst Family products.

VTP ensures that all switches in the VTP domain are aware of all VLANs. There are occasions, however, when VTP can create unnecessary traffic. All unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations where few users are connected in that VLAN. VTP pruning is a feature used to eliminate (or prune) this unnecessary traffic.

By default, all Cisco Catalyst switches are configured to be VTP servers. This is

suitable for small-scale networks where the size of the VLAN information is small and easily stored in all switches (in NVRAM). In a large network, a judgment call must be made at some point when the NVRAM storage needed is wasted, because it is duplicated on every switch. At this point, the network administrator should choose a few well-equipped switches and keep them as VTP servers. Everything else participating in VTP can be turned into a client. The number of VTP servers should be chosen so as to provide the degree of redundancy desired in the network.

There are three version of VTP so far. VTP Version 2 (V2) is not much different than VTP Version 1 (V1). The major difference is that VTP V2 introduces the support for Token Ring VLANs. If you are using Token Ring VLANs, you need to enable VTP V2. Otherwise, there is no reason to use VTP V2. VTP version 3 differs from earlier VTP versions in that it does not directly handle VLANs. VTP version 3 is a protocol that is only responsible for distributing a list of opaque databases over an administrative domain. When enabled, VTP version 3 provides the following enhancements to previous VTP versions:

• Support for extended VLANs.
• Support for the creation and advertising of private VLANs.
• Improved server authentication.
• Protection from the "wrong" database accidentally being inserted into a VTP domain.
• Interaction with VTP version 1 and VTP version 2.
• Provides the ability to be configured on a per-port basis.
• Provides the ability to propagate the VLAN database andother databases.

Protocol Structure - VTP: VLAN Trunking Protocol

The format of the VTP header can vary depending on the type of VTP message. However, they all contain the following fields in the header:

• VTP protocol version: 1 or 2 or 3
• VTP message types:
• Summary advertisements
• Subset advertisement
• Advertisement requests
• VTP join messages
• Management domain length
• Management domain name

Summary Advertisements

When the switch receives a summary advertisement packet, it compares the VTP domain name to its own VTP domain name. If the name is different, the switch simply ignores the packet. If the name is the same, the switch then compares the configuration revision to its own revision. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.

• Followers indicate that this packet is followed by a Subset Advertisement packet.
• The updater identity is the IP address of the switch that is the last to have incremented the configuration revision.
• Update timestamps are the date and time of the last increment of the configuration revision.
• Message Digest 5 (MD5) carries the VTP password if it is configured and used to authenticate the validation of a VTP update.

Subset Advertisements

When you add, delete, or change a VLAN in a switch, the server switch where the changes were made increments the configuration revision and issues a summary advertisement, followed by one or several subset advertisements. A subset advertisement contains a list of VLAN information. If there are several VLANS, more than one subset advertisement may be required in order to advertise them all.

The following formatted example shows that each VLAN information field contains information for a different VLAN (ordered with lowered-valued ISL VLAN IDs occurring first):

Most of the fields in this packet are easy to understand. Below are two clarifications:

• Code- The format for this is 0x02 for subset advertisement.
• Sequence number- This is the sequence of the packet in the stream of packets following a summary advertisement. The sequence starts with 1.

Advertisement Requests

A switch needs a VTP advertisement request in the following situations:

• The switch has been reset.
• The VTP domain name has been changed.
• The switch has received a VTP summary advertisement with a higher configuration revision than its own.

Upon receipt of an advertisement request, a VTP device sends a summary advertisement, followed by one or more subset advertisements. Below is an example.

• Code- The format for this is 0x03 for an advertisement request
• Start Value - This is used in cases where there are several subset advertisements. If the first (N) subset advertisement has been received and the subsequent one (N+1) has not, the Catalyst only requests advertisements from the (N+1)th one.

Related Protocols

IEEE 802.1Q

Sponsor Source

VLAN Trunking Protocol (VTP) is a Cisco protocol.

Reference

http://www.cisco.com : Understanding and Configuring VLAN Trunking Protocol