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Invisible Networks (No Kidding), the World of Virtual LANs (VLANS), Part III

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 7, 2011 by jjrinehart

Oops...I Forgot to Turn Off VTP

The history of technology and innovation is full of attempts to create greater efficiencies and automate tasks, a good example is DHCP; this protocol hands out IP address information with little intervention needed.  Any engineer/technician that has had to renumber a network can certainly appreciate this particular task being simplified.  On the other hand, if someone brings in a home router and plugs it in, it can wreak havoc for every other user.

The focus of this post centers on a switching technology that also had good intentions but can create outages that turn an engineer’s hair gray or loose, namely, Virtual Trunking Protocol, or VTP.  The intention of VTP was to simplify the configuration of VLANs across multiple switches.  In a network of less than a dozen devices, manually configuring VLANs is not that big of a deal, but in a large campus environment of a hundred or more switches this becomes ridiculously difficult.  Why not just configure this once and let it propagate automatically?  Seems like a win-win right?  Hold that thought while we go through some specifics.

There are three operational modes of VTP on a Cisco switch (yes, this is a Cisco proprietary protocol), as follows:

  1. Server: This switch serves in a master operational mode, where all the changes are made and then passed out to switches.  To ensure that the latest data is propagated accurately, each time the database changes, the revision number is incremented.
  2. Client: This switch does not store any VLAN information locally, nor can any changes be made to the information it contains.  Think of it as a “read only” mode of operation.
  3. Transparent: A switch in transparent mode operates independently, just as it would if VTP didn’t even exist.  It ignores all updates, though it does pass those updates to all other switches it has trunk links too.  Since VTP cannot be shut off, this is about the closest you can get to off.  If all switches are in a network are configured in this mode it effectively negates any effect it could have on the environment.

Now that you have an idea of how VTP operates, you can appreciate the “gotchas” that come with using it.  If only one switch operates as the server and the rest are all clients, then life is good.  But what happens when you add a new switch in server mode?  Hopefully nothing, unless the configuration revision number just so happens to be higher than the one that all the other switches have.  At that point, “poof” (see picture above), the database on every single switch is immediately erased.  The technical term for this is RGE–Resume Generating Event.  Best practices recommend just operating in transparent mode.

Talk to you later…

– Joe