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frame relay question for learning DLCIs

Posted on 2010-11-09
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Last Modified: 2012-05-10
I am studying for my CCNA and have a few questions regarding frame relay DLCI's. I am trying to understand how they work on a router and what they refer to.

For an example we have the following setup:


R1---------Frame Relay Switch--------------R2


Now on R1 I use the commands for a point-to-point connection

ip address 192.168.1.1 255.255.255.0
frame-relay interface-dlci 100

On R2 i use

ip address 192.168.1.2 255.255.255.0
frame-relay interface-dlci 200

First thing that I need to be sure about, is that there is only one PVC in this example. It is my understanting that a PVC refers to a router to router connection, in this example from R1 to R2. Is this correct?

Secondly am I correct in saying that the DLCI refers to section of the PVC between the router and the frame relay switch?

Lastly when you configure the command "frame-relay interface-dlci 100" on R1, the DLCI 100 is refering to the link between itself (R1) and the frame relay switch. It is then the frame relay switch's job to pass the frame received from R1 along the DLCI 200, then reaching R2 (after switching the DLCI identifier). Does this sound correct?


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Question by:ryan80
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by:moon_blue69
moon_blue69 earned 100 total points
ID: 34091586
Permanent Virtual Circuit

A permanent virtual circuit (PVC) is a permanently established circuit through the Frame
Relay service provider network. It enables the routers at each end to communicate with each
other without any setup process. A PVC closely emulates a leased-line connection between
your devices. So you are right the PVC is between R1 and R2

Data Link Connection Identifier

It is a little known fact that router serial interfaces do not have MAC addresses. MAC addresses
are related only to LAN connections. WAN connections therefore all have different Layer
2 addresses that they use to identify the other side of the connection. In the case of Frame
Relay, this is known as a Data Link Connection Identifier (DLCI). Communicating through
DLCIs is unlike most other network communication that you’ve seen thus far. Rather than
access a destination DLCI number to reach the remote router, you leave on a local DLCI number
to reach a remote router. This is very similar to the way airline travel works. When you
want to fly to a remote destination, say North Dakota, you might leave on flight #4513. When
you want to return from North Dakota to your original location, you might leave on flight
#4839. This means that the DLCI numbers you use are locally significant; the DLCIs you have
in Arizona matter only to the router in Arizona. The DLCIs you have in North Dakota matter
only to the router in North Dakota.

Please refer the attached image

Figure shows a full mesh design with three locations: Arizona, California, and Michigan.
At each location, the routers have two virtual circuits that enable them to reach the other locations.
Now remember, the way DLCIs work is you leave on a local DLCI number rather thanaccess a remote DLCI, so if Arizona wants to reach California, it uses DLCI number 507 to
get there. If California wants to get back to Arizona, it uses DLCI 705. Likewise, California
uses DLCI 401 to reach the router in Michigan.

Because DLCIs are locally significant, service providers can do whatever they want with them.
Take a look at the DLCI connection between Arizona and Michigan. It’s not a mistype!
Arizona is assigned to use DLCI 508 to reach Michigan. Michigan also uses DLCI 508 to
reach Arizona. This seems to make no sense until you realize that the DLCI number 508
means something totally different if it is received in Arizona than if it is received in Michigan.
Remember: locally significant. The service provider probably has DLCI 508 in use in many
locations for many different customers. As long as the set of DLCIs is unique at each location,
everything is fine with the Frame Relay standards.

Hope this will answer all your questions
frame-relay.jpg
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Don Johnston earned 400 total points
ID: 34092675
>It is my understanting that a PVC refers to a router to router connection, in this example from R1 to R2. Is this correct?

Close enough. :-) It's actually a circuit from one edge of the providers cloud to the other edge.

>Secondly am I correct in saying that the DLCI refers to section of the PVC between the router and the frame relay switch?

Yes

>Lastly when you  configure the command "frame-relay interface-dlci 100" on R1, the DLCI  100 is refering to the link between itself (R1) and the frame relay  switch. It is then the  frame relay switch's job to pass the frame received from R1 along the  DLCI 200, then reaching R2 (after switching the DLCI identifier). Does  this sound correct?

Pretty close. DLCI 100 is used by the edge switch so it knows where to send the frame. Another way to think about it is "which PVC to use".

Also, none of the "interface-dlci" commands are necessary on current version Cisco routers if you are not using sub-interfaces.  
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by:ryan80
ID: 34093322
Thanks donjohnston.

Thank for letting me know that you dont need to specify the dlci if there are no subinterfaces.

I am pretty sure that I understand the theory now.

Looking at it in a layman's view, a PVC seems like it consists of 2 DLCI's in my simple configuration. The first DLCI refers to the connection between R1 and the Frame Relay switch and the second DLCI refers to the connection between the Frame Relay Switch and R2.
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Expert Comment

by:Don Johnston
ID: 34093547
> The first DLCI refers to the connection between R1 and the Frame Relay  switch and the second DLCI refers to the connection between the Frame  Relay Switch and R2.

This is a problematic description. In many situations, a router may have access to multiple PVC using multiple DLCIs. Which means a connection between R1 and the Frame-Relay edge switch could be referred to by multiple DLCIs.

So a better approach would be to consider the DLCI as a means to utilize a specific PVC.
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Author Closing Comment

by:ryan80
ID: 34093598
Yeah, i guess that i didnt word it the best, but I understand exactly what you are saying thank you for the help and answering my questions.
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Expert Comment

by:Don Johnston
ID: 34093744
Don't sweat it. Frame-Relay is really an antiquated technology. Sadly, you really only need to know for CCNA and for the routing protocols section of the CCNP.
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