VTP – The three Advertisement types, what information they contain, and their behaviors explained!

Above is the current switched Topology and the VTP mode / Trunk connections.

Without further ado:

The three different types of Advertisements, and details regarding them

Summary Advertisements – Sent every 5 minutes by VTP Servers or triggered by a change in the VLAN Database (vlan.dat in Flash), these contain the VTP Domain name / Config Revision # / MD5 hash code / timestamp / a number indicating how many “Subset Advertisements” are following the Summary Advertisement.

Subset Advertisements – Sent by VTP Servers when the VLAN configuration is changed, this Ad type is triggered by and focused more on specific VLAN information, such as a created / delete / suspended (not yet covered) / changed. Changes can include the VLAN type such as Ethernet / Token Ring / FDDI, also VLAN Name or MTU changes.

Client Advertisement Requests – These requests can be triggered by Clients to Servers for VLAN information for several reason including if a switch is reset, if its database becomes corrupt, the Client sends this request and will receive back a Summary and Subset Ads back from the Server to rebuild it VLAN Database.

What triggers a Client Ad Request specifically? Good question, I don’t know!

I know that just wiping out the config using Cisco’s methods from my previous post worked fine, to just unplug the switches Trunks / roll Config Revision # back to 0 / plug Trunk ports back in, however I want to test when it triggers on a completely reset switch.

So I did a “wr er” / “delete flash:vlan.dat” / “reload” / unplugged any cables to other switches, and will walk through every step until it gets a synchronized VTP config from the VTP server – The only thing configured will be the hostname SW1 after the reset.

I will describe the step and show output, so it may be a bit output heavy here:

Plugging in cables on NON Trunk ports on all switches:

SW1#
*Mar 1 00:06:58.121: %LINK-3-UPDOWN: Interface FastEthernet1/0/16, changed state to up
*Mar 1 00:06:59.128: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/16, changed state to up
*Mar 1 00:06:59.447: %LINK-3-UPDOWN: Interface FastEthernet1/0/14, changed state to up
*Mar 1 00:07:00.453: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/14, changed state to up
SW1#sh vtp status
VTP Version capable : 1 to 3
VTP version running : 1
VTP Domain Name :
VTP Pruning Mode : Disabled
VTP Traps Generation : Disabled
Device ID : 1ce6.c7c1.c800
Configuration last modified by 0.0.0.0 at 0-0-00 00:00:00
Local updater ID is 0.0.0.0 (no valid interface found)

Feature VLAN:
————–
VTP Operating Mode : Server
Maximum VLANs supported locally : 1005
Number of existing VLANs : 5
Configuration Revision : 0
MD5 digest : 0x57 0xCD 0x40 0x65 0x63 0x59 0x47 0xBD
0x56 0x9D 0x4A 0x3E 0xA5 0x69 0x35 0xBC
SW1#

Moving cables on SW2 / SW3 to their configured Trunk ports:

SW1#
*Mar 1 00:09:58.686: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/16, changed state to down
*Mar 1 00:09:59.693: %LINK-3-UPDOWN: Interface FastEthernet1/0/16, changed state to down
*Mar 1 00:10:05.447: %LINK-3-UPDOWN: Interface FastEthernet1/0/16, changed state to up
*Mar 1 00:10:07.209: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/14, changed state to down
*Mar 1 00:10:08.216: %LINK-3-UPDOWN: Interface FastEthernet1/0/14, changed state to down
*Mar 1 00:10:08.467: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/16, changed state to up
*Mar 1 00:10:13.962: %LINK-3-UPDOWN: Interface FastEthernet1/0/14, changed state to up
*Mar 1 00:10:16.009: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/14, changed state to up
SW1#sh vtp status
VTP Version capable : 1 to 3
VTP version running : 2
VTP Domain Name : CCNP
VTP Pruning Mode : Disabled
VTP Traps Generation : Disabled
Device ID : 1ce6.c7c1.c800
Configuration last modified by 0.0.0.0 at 3-1-93 00:06:20
Local updater ID is 0.0.0.0 (no valid interface found)

Feature VLAN:
————–
VTP Operating Mode : Server
Maximum VLANs supported locally : 1005
Number of existing VLANs : 8
Configuration Revision : 4
MD5 digest : 0xED 0xA4 0x2C 0x01 0x71 0x62 0x26 0x93
0x14 0xB7 0xA9 0x29 0x3D 0xE4 0xA3 0x33
SW1#

So much for output heavy, that was pretty fast. This demonstrates that if two random access ports are connected it won’t Advertise VTP info, but as soon as I moved the cables on SW2 / SW3 to their Trunk ports, they immediately formed a Trunk because they are in Trunk Mode (actively trying to Trunk) and SW1 ports are in Auto mode by default so it accepted the invitation to form a trunk:

SW1#sh int trunk

Port          Mode     Encapsulation    Status     Native vlan
Fa1/0/14    auto           n-802.1q         trunking    1
Fa1/0/16    auto           n-802.1q         trunking    1

It also demonstrates that you can tell this side is in Auto mode just from the output, as the “n-802.1q” encapsulation type means this side “negotiated” the encap type, so if you are shown this output on exam day you know these ports were asked to Trunk by the switch on the remote end of the cable!

So a couple of highlights from this and I am slacking off for the night is:

• Summary Ads are both sent every 5 minutes and triggered by VLAN DB changes
• Subset Ads are only triggered by changes to the VLAN Database
• Client Request Ads are sent by Client or New switches on the network once a Trunk is formed with another switch in a VTP domain
• Both Summary and Subset Ads are sent to the Client requesting VTP info

I will be getting into VTP Versions and finally some password configuration for VTP Domains coming up, I am not sure this would work the same (or at all) with a password configured for the domain, I would assume not but I will save that info for another post!