Frame-Relay DLCI

Unanswered Question
Feb 7th, 2008

Hi All,

If DLCI are locally significant as defined by Frame-relay studies, how then would my local DLCI knows where the traffic should be directed?. Considering that we have hundreds of DLCIs in the frame-relay cloud.

I am not talking of p-2-mltpt but on p-2-p subinterface because its not map to any remote address.

Thanking you all in advance,

Regards

I have this problem too.
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mohammedmahmoud Thu, 02/07/2008 - 04:53

Hi,

The DLCI is agreed between the local router and the facing FR Switch, local significant means that as if we are saying to the router use this local DLCI to reach this remote IP address (DLCI is local significant between the local router and the facing FR Switch), after the local router knows which DLCI to use to reach which remote address it sends the traffic to the facing FR Switch with this DLCI, which has a configured PVC (all over the FR cloud, over all the FR Switches) to take the traffic to its required destination, and so on.

I hope that i've been informative.

BR,

Mohammed Mahmoud.

Richard Burts Thu, 02/07/2008 - 11:34

Olu

Perhaps there are aspects of your question that I do not understand well. So let me try an answer and if that does not address your issue than perhaps you can clarify your question.

For Frame Relay point to point subinterface there is an automatic mapping of the local subnet of the interface to the DLCI active on that interface. You do not have to do anything or configure any mapping and the router creates the mapping automatically.

So you send something on the point to point subinterface on its DLCI. This comes into the Frame Relay switch on an inbound port and inbound DLCI (which is the same DLCI as your router). And the Frame Relay switch will switch the frame to an outbound port and outbound DLCI.

We say that the DLCI is locally significant because we know what the DLCI is on our interface (perhaps it is DLCI 100) but we do not know (and do not need to know) what the DLCI is on the remote router to which our PVC connects.

If this does not address your issue then please clarify the question.

HTH

Rick

microcisconet Thu, 02/07/2008 - 12:07

Rick,

The later paragraph of your response actually answer my question in part;

"We say that the DLCI is locally significant because we know what the DLCI is on our interface (perhaps it is DLCI 100) but we do not know (and do not need to know) what the DLCI is on the remote router to which our PVC connects"

Going by this statement of yours, how then would our DLCI knows which remote end to talk to since as you know we have hundreds or thousand of PVCs within the FRSW.

I hope you can take the pain to elaborate further?.

Regards

mohammedmahmoud Thu, 02/07/2008 - 12:30

Hi Olu,

Lets assume that you have a topology of a hub and 2 spokes, your provider will assign you 2 DLCIs at the hub side, one for each spoke, on your side you'll have to either dynamically or statically map the DLCIs to the remote IP addresses, on the provider side the provider will configure the FR Switch attached to your hub router with 2 PVCs, one for each spoke using one of the DLCIs, and thus when your router uses the DLCI for a specific spoke the FR Switch will forward the traffic on its particular PVC.

I hope that this addresses your query.

BR,

Mohammed Mahmoud.

Richard Burts Thu, 02/07/2008 - 20:14

Olu

You ask:

Going by this statement of yours, how then would our DLCI knows which remote end to talk to since as you know we have hundreds or thousand of PVCs within the FRSW.

I attempted to address this in the third paragraph of my response where I talk about Frame Relay switching. Since that does not seem to have resolved your question let me try again a little differently. Let me first emphasize that a PCV is a virtual circuit. Similar to a physical circuit (like a point to point T1) which originates on a particular device and terminates on a particular device and may go through several data switches on its path, the virtual circuit for Frame Relay starts on a particular device and terminates on a particular device and will go through 1 or more Frame Relay switches to get to its destination. The DLCI on your point to point subinterface represents a point to point connection to a particular destination. Even though there are hundreds of DLCIs on the Frame Relay switch, the DLCI from your router (perhaps DLCI 100) connects to a unique Frame Relay switch. And that Frame Relay switch will take incoming DLCI 100 and will switch it to an outbound DLCI (perhaps 476) on an outbound interface. DLCI 476 may connect to the far end router.

We say the DLCI is locally significant because you only need to know about DLCI 100 (you do not need to know about 476). And the remote only needs to know about 476 (they do not need to know about 100). And it is the responsibility of the Frame Relay provider to know about both DLCIs and to establish the correct connection between them.

It may help to think of an analogy. Lets think of a router that has 3 point to point serial interfaces and of another router that has Frame Relay with 3 point to point subinterfaces (and 3 DLCIs). Each router connects to 3 destinations. Just like serial1/1 and serial1/2 and serial1/3 connect to 3 unique destinations, serial 2/1.100, serial2/1.200, and serial2/1.300 connect to 3 unique destinations. When you choose serial1/2 as the outbound interface you have chosen which destination you will communicate with. And when you choose serial2/1.200 you have chosen which destination you will communicate with.

Just like the provider of the T1 circuit engineers the circuit to select certain components and combines them to provide the end to end connectivity, the Frame Relay provider is engineering DLCIs and specifying Frame Relay switching so that your DLCI 100 builds a virtual circuit that connects to a certain destination and so that your DLCI 100 builds a virtual circuit that connects to a certain destination.

HTH

Rick

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