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| 2.7 |  | VLAN Trunking Protocol (VTP) |
| 2.7.5 |  | VTP advertisements |

Periodic VTP advertisements are sent out each trunk port with the multicast destination MAC address 01-00-0C-CC-CC-CC. VTP advertisements contain the following configuration information:

- VLAN IDs (ISL and 802.1Q)
- Emulated LAN names (ATM LANE)
- 802.10 SAID values (FDDI)
- VTP domain name
- VTP configuration revision number
- VLAN configuration, including the maximum transmission unit (MTU) size for each VLAN
- Frame format

VTP messages are encapsulated in either ISL frames or IEEE 802.1Q frames.

VTP messages are sent with the following Ethernet frame field values:

- Multicast destination MAC address 01-00-0C-CC-CC-CC
- Destination Service Access Point (DSAP) 0xAA in the Local Link Control (LLC) header
- Source Service Access Point (SSAP) 0xAA in the LLC header
- Organizational Unique Identifier (OUI) of 00-00-0C (for Cisco) in the Subnetwork Access Protocol (SNAP) header
- EthertType of 2003 in the SNAP header

VTP packets can be encapsulated into an ISL frame. The VTP header’s format varies, depending on the type of VTP message. However, all packets contain the following fields in the header:

- VTP protocol version: 1 or 2
- VTP message type
- Management domain length
- Management domain name

The following message types are used by VTP:

- Summary advertisements
- Subset advertisements
- Advertisement requests
- VTP join messages

VTP join messages, which rely on the VTP Pruning Protocol will be examined in the VTP Pruning section.

Advertisements on factory-default VLANs are based on media types. User ports should not be configured as VTP trunks.

There are two types of advertisements:

- Requests from clients that want information at bootup
- Response from servers

Advertisement Requests
A switch issues 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 number than its own.

The server responds with summary and subset advertisements.

- Code is 0x03 for advertisement request (type 3).
- Rsvd is reserved and always set to 0.
- Start-Value is used in cases in which there are several subset advertisements. If subset advertisements n is the first advertisement that has not been received in a sequence of advertisements, an advertisement request is sent for advertisements starting with n. If the start value is 0, all subset advertisements are sent for the particular management domain.

Summary advertisements
By default, Catalyst switches issue summary advertisements every five minutes. They inform neighbor switches of the current domain name and the configuration revision number.

When the switch receives a summary advertisement packet, it compares the 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. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.

- The VTP version is either 1 or 2.
- Code indicates which of the four VTP message types is included—here, 0x01 or type 1.
- Followers indicates how many VTP subset advertisement messages (type 2) follow the summary advertisement frame. The value can range from 0 to 255; 0 indicates that no subset advertisements follow. A Catalyst transmits the subset advertisement only if there is a change in the system or as a response to an advertisement request.
- MgmtD Len specifies the length of the VTP domain name.
- Management Domain Name specifies the VTP domain name.
- The Configuration Revision Number field is 32 bytes.
- The Updater Identity is the IP address of the last switch that incremented the configuration revision.
- Update Timestamps are the date and time of the last increment of the configuration revision.
- MD5 Digest consists of a message-digest hash—a function of the VTP password and the VTP header contents (excluding the MD5 Digest field). If the receiving Catalyst hash computation does not match, the packet is discarded.

Subset advertisements
When a VLAN is added, changed, or deleted on a VTP server, the configuration revision number is incremented, and a summary advertisement is issued, followed by one or several subset advertisements. Subset advertisements are also triggered by suspending or activating a VLAN, changing its name, or changing the maximum transmission unit (MTU). A subset advertisement contains a list of VLANs and corresponding VLAN information. If there are several VLANs, more than one subset advertisement might be required in order to advertise all the information.

- Code is 0x02 for subset advertisement (type 2).
- Seq-Number represents the sequence number of the packet in the stream of subset advertisements following a summary advertisement. The sequence starts with 1. The receiving Catalyst uses this value to ensure that it receives all subset advertisements. If it does not receive all of the subsets, it requests a resend, starting with a specific subset advertisement.

VLAN-info fields each contain the following information:

- The VLANs status (active or suspended)
- VLAN-Type (Ethernet, Token Ring, FDDI, or otherwise)
- VLAN-Name Len – Length of the VLAN name
- ISL VLAN-ID – VLAN number of this named VLAN
- MTU size – Maximum frame size supported for this VLAN
- 802.10 Index – SAID value used if the frame passed over an FDDI trunk
- VLAN-name

The VTP subset advertisement lists this information for each individual VLAN, including default VLANs.