Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 252 VTP Modes VTP-enabled switches can operate in one of three modes: • Server • Client • Transparent Only VTP Servers can create, modify or delete entries in the shared VLAN database. Servers advertise their VLAN database to all other switches on the network, including other VTP servers. This is the default mode for Cisco Catalyst switches. VTP servers can only advertise VLANs 1 - 1005. VTP Clients cannot make modifications to the VLAN database, and will receive all of their VLAN information from VTP servers. A client will also forward an update from a server to other clients out its trunk port(s). Remember, VTP switches must be in the same VTP Domain to share/accept updates to the VLAN database. A VTP Transparent switch maintains its own separate VLAN database, and will neither advertise nor accept any VLAN database information from other switches, even a server. However, transparent switches will forward VTP updates from servers to clients, thus acting as a pass-through. Transparent switches handle this pass-through differently depending on the VTP version: • VTP Version 1 – the transparent switch will only pass updates from the same VTP domain. • VTP Version 2 – the transparent switch will pass updates from any VTP domain. As a best practice, a new switch should be configured as a VTP client in the VTP domain, and have its configuration revision number (described in the next section) set back to zero before being installed into a production network. There is a specific reason for this: if by some circumstance a new switch’s configuration revision number is higher than that of the existing production switches, a new VTP switch could conceivably advertise a blank or incorrect VLAN database to all other switches. This could result in a significant network outage. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 253 VTP Updates VTP updates contain a 32-bit configuration revision number, to ensure that all devices have the most current VLAN database. Every change to the VLAN database increments the configuration revision number by 1. A VTP switch will only accept or synchronize an update if the revision number is higher (and thus more recent) than that of the currently installed VLAN database. This is true even if the advertising switch is a VTP Client. Updates with a lower revision number are ignored. REMEMBER: a VTP client can update other clients and VTP servers in the VTP domain, if its revision number is higher. The simplest way to reset the configuration revision on a VTP switch is to change the VTP domain name, and then change it back to the original name. VTP utilizes three message types: • Summary Advertisement – sent out every 300 seconds, informing all VTP switches of the current configuration revision number. • Subset Advertisement – sent out when there is a change to the VLAN database. The subset advertisement actually contains the updated VLAN database. • Advertisement Request – sent out when a switch requires the most current copy of the VLAN database. A switch that is newly joined to the VTP domain will send out an Advertisement Request. A switch will also send out an Advertisement Request if it receives a Summary Advertisement with a configuration revision number higher than its current VLAN database. A Subset Advertisement will then be sent to that switch, so that it can synchronize the latest VLAN database. A Subset Advertisement will contain the following fields: • VTP Version • VTP Domain • VTP Configuration Revision • VLAN IDs for each VLAN in the database • VLAN-specific information, such as the VLAN name and MTU (Reference: http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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