VTP Advertisements

Each switch participating in VTP advertises VLANs, revision numbers, and VLAN parameters on its trunk ports to notify other switches in the management domain. VTP advertisements are sent as multicast frames. The switch intercepts frames sent to the VTP multicast address and processes them with its supervisory processor. VTP frames are forwarded out trunk links as a special case.

Because all switches in a management domain learn of new VLAN configuration changes, a VLAN need only be created and configured on just one VTP server switch in the domain.

By default, management domains are set to use non-secure advertisements without a password. A password can be added to set the domain to secure mode. The same password has to be configured on every switch in the domain so that all switches exchanging VTP information will use identical encryption methods.

The VTP advertisement process starts with configuration revision number 0 (zero). When subsequent changes are made, the revision number is incremented before advertisements are sent out. When listening switches receive an advertisement with a greater revision number than is locally stored, the advertisement will overwrite any stored VLAN information. Because of this, forcing any newly added network switches to have revision number zero is important. The VTP revision number is stored in NVRAM and is not altered by a power cycle of the switch. Therefore, the revision number can only be initialized to zero using one of the following methods:

• Change the VTP mode of the switch to transparent and then change the mode back to server.

• Change the VTP domain of the switch to a bogus name (a non-existent VTP domain) and then change the VTP domain back to the original name.

• Issue a clear config all command, which will clear the switch configuration and the VTP information stored in NVRAM. Power cycle the switch so that it boots up with a nonexistent VTP domain name and a VTP revision number of zero. (Use caution. This is the most drastic method because it will erase all configuration data.)

If the VTP revision number is not reset to zero, a new server switch might advertise VLANs as non-existent or deleted. If the advertised revision number happens to be greater than previous legitimate advertisements, listening switches would overwrite good VLAN database entries with null or deleted VLAN status information. This is referred to as a VTP synchronization problem.

Advertisements can originate as requests from client-mode switches that want to learn about the VTP database at boot-up time. As well, advertisements can originate from server-mode switches as VLAN configuration changes occur.

VTP advertisements can occur in three forms:

• Summary advertisements—VTP domain servers will send summary advertisements every 300 seconds and every time a VLAN topology change occurs. The summary advertisement lists information about the management domain, including VTP version, domain name, configuration revision number, timestamp, MD5 encryption hash code, and the number of subset advertisements to follow. For VLAN configuration changes, summary advertisements are followed by one or more subset advertisements, with more specific VLAN configuration data. Figure 4-6 shows the summary advertisement format.

Figure 4-6 VTP Summary Advertisement Format

Version (1 byte)

Type

Number of subset advertisements to follow (1 byte)

Domain name length (1 byte)

Management Domain Name (zero-padded to 32 bytes)

Configuration Revision Number (4 bytes)

Updater Identity (orginating IP address: 4 bytes)

Update Timestamp (12 bytes)

MD5 Digest hash code (16 bytes)

• Subset advertisements—VTP domain servers will send subset advertisements after a VLAN configuration change occurs. These advertisements list the specific changes that have been performed, such as creation or deletion of a VLAN, suspending or activating a VLAN, changing the name of a VLAN, and changing the MTU of a VLAN. Subset advertisements can list the following VLAN parameters: status of the VLAN, VLAN type (like Ethernet or Token Ring), MTU, length of the VLAN name, VLAN number, SAID value, and the VLAN name. VLANs are listed individually in sequential subset advertisements. Figure 4-7 shows the VTP subset advertisement format.

• Advertisement requests from clients—A VTP client can request any lacking VLAN information. For example, a client switch might be reset and have its VLAN database cleared, its VTP domain membership might be changed, or it might hear a VTP summary advertisement with a higher revision number than it currently has. After a client advertisement request, the VTP domain servers respond with summary and subset advertisements. Figure 4-8 shows the advertisement request format.

Figure 4-7 VTP Subset Advertisement and VLAN Info Field Formats VTP Subset Advertisement

Version (1 byte)

Type

Subset sequence number (1 byte)

Domain name length (1 byte)

Management Domain Name (zero-padded to 32 bytes)

Configuration Revision Number (4 bytes)

VLAN Info Field 1 (see below)

VLAN Info Field N

VTP VLAN Info Field

0

1

2

3

Info Length

VLAN Status

VLAN Type

VLAN Name Length

ISL VLAN ID

MTU Size

802.10 SAID

VLAN Name (padded with zeros to multiple of 4 bytes)

Figure 4-8 VTP Advertisement Request Format

0

1

2

3

(Adv request) (1 byte)

Reserved (1 byte)

Domain name length (1 byte)

Management Domain Name (zero-padded to 32 bytes)

Starting advertisement to request

Catalyst switches in server mode use a separate nonvolatile random-access memory (NVRAM) for VTP, different from the configuration NVRAM. All VTP information, including the VTP configuration revision number, is retained even when the switch power is off. In this manner, a switch is able to recover the last known VLAN configuration from its VTP database once it reboots.

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