Cisco CCNA And CCNP BCRAN Exam Training: Frame Relay PVC, SVC, DLCI, and Map Statements

In today's CCNP certification tutorial, we'll concentrate on the BCRAN exam and Frame Relay PVCs, SVC, and DLCIs in particular.

There are actually two kinds of virtual circuits - Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs).  An SVC doesn't have anything to do with an L2 switch - it's a VC that is built only under predefined circumstances, similar to an ISDN connection that is only built when interesting traffic hits the line.  SVCs go through a basic four-step process, and it's a lot like ISDN:

Call Setup -- Just what it sounds like, the circuit between the two DTEs is created.

Data Transfer -- Again, just what it sounds like.

Idle -- The connection is active, but no data is going across the line.  As with an ISDN call, if the call stays idle too long, the call will be torn down.

Call Termination - The call is torn down.

A PVC is just what the name says - it's permanent, and is already built when traffic needs to be sent.  PVCs have two steps, data transfer and idle.  Unlike an SVC, when a PVC goes to idle, the PVC will not be torn down.

The DLCI (pronounced "del-see") is simply an identifier for the connection between the DTE and the closest DCE in the frame switch.  You don't just make the DLCI number up - the Frame provider will provide you with the DLCIs you should use.

The key to a successful Frame configuration is remembering that the local DLCI must be mapped to a remote IP address.  There are two ways to do this, one being dynamic and the other involving a static configuration.  Those of you who have read my previous books know that I'm all for the router and the switch using dynamic methods as often as possible, since dynamically learned information is the easiest information to keep current.  However, in this case, the static method of DLCI - IP address mapping is considered more reliable than the dynamic method, Inverse Address Resolution Protocol (IARP).

To allow IARP to perform dynamic mappings, just enable Frame Relay with “encapsulation frame-relay” and open the interface.  When you see the word dynamic in the output of show frame map, that particular mapping was performed by Inverse ARP.

It's highly recommended that you run either static or dynamic DLCI mappings - don't use them both on the same interface.  This can result in incomplete mappings if and when the router is reloaded.