frame relay router configurations - DLCI configuration on partially meshed network

ryan80
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I am studying for the CCNA and have a question about setting the DLCI for a VC. I was looking at a configuration where there are a total of 4 routers that are connected. There is one main router that has 3 others connected to it via one serial interface. Now my question is about the command for frame-relay interface-dlci. the DLCI that is given, is that the local DLCI for the interface on that router? inverse arp is then used to discover the other router?

Here is the configuration:

R1(config)# interface serial0/0/0
R1(config)# encapsulation frame-relay

R1(config-if)#interface serial 0/0/0.1 point-to-point
R1(config-subif)#ip address 140.1.1.1 255.255.255.0
R1(config-subif)#frame-relay interface-dlci 52

R1(config-if)#interface serial 0/0/0.2 point-to-point
R1(config-subif)#ip address 140.1.2.1 255.255.255.0
R1(config-subif)#frame-relay interface-dlci 53

R1(config-if)#interface serial 0/0/0.3 point-to-point
R1(config-subif)#ip address 140.1.3.1 255.255.255.0
R1(config-subif)#frame-relay interface-dlci 54

R1(config-fr-dlci)#interface fastethernet 0/0
R1(config-if)#ip address 140.1.11.1 255.255.255.0


R2:
interface serial0/0/0
   encapsultation frame-relay

interface serial 0/0/0.1 point-to-point
   ip addresss 140.1.1.2 255.255.255.0
   frame-relay interface-dlci 51

interface fastethernet 0/0
   ip address 140.1.12.2 255.255.255.0

R3
interface serial0/0/0
   encapsultation frame-relay

interface serial 0/0/0.2 point-to-point
   ip addresss 140.1.2.3 255.255.255.0
   frame-relay interface-dlci 51

interface fastethernet 0/0
   ip address 140.1.13.3 255.255.255.0

interface serial0/0/0
   encapsultation frame-relay

interface serial 0/0/0.3 point-to-point
   ip addresss 140.1.3.4 255.255.255.0
   frame-relay interface-dlci 51

interface fastethernet 0/0
   ip address 140.1.14.4 255.255.255.0

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I am even further confused on this. I though that the frame-relay interface-dlci command assigned that dlci to the router that it was configured on, but now i am thinking that it is just telling it what the dlci is of the router that it will be sending to? If this is the case, how is the ip discovered of the other router?
and if this is the case, how does the router know what DLCI it is and what frames to recieve?

I know this is a point to point example, but wouldnt the same fundamentals apply to a multipoint as well?
Don JohnstonInstructor
Top Expert 2015

Commented:
>There is one main router that has 3 others connected to it via one serial interface.

How are you connecting three routers to one serial interface?

Please explain the topology of your network.
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This is just an example from a book. They dont define how it is connected, just that it is a point-to-point PVC between each router.
Don JohnstonInstructor
Top Expert 2015

Commented:
>the DLCI that is given, is that the local DLCI for the interface on that router?

Correct.

>inverse arp is then used to discover the other router

Nope. Inverse ARP isn't necessary. It's a point-to-point interface. There's only one router reachable through this interface.

The interface DLCI command is how you associate a (local) DLCI with a sub-interface.

>I know this is a point to point example, but wouldnt the same fundamentals apply to a multipoint as well?

No. With a mulitpoint interface, you would create a map for each PVC.
here is a diagram of the network. I have labeled what I think each local DLCI will be.Do these local DLCI's look correct for the given configuration? ( I see that I made a ommision in the configuration and didnt put in the label of R4 for the last serial interface)

I am still a little confused however about the point to point frame relay. as an example, lets use the link between R1 and R2 (lets assume that the routing is set statically to make it easier). R1 is assigned a local DLCI of 52 in the config along with the IP address of 140.1.1.1. R2 is assigned the local DLCI of 51 through the configuration and an IP of 140.1.1.2.

1. How does each router know the DLCI of the router on the opposite end?(is there a up status sent when the routers are brought online?)

2. How does R1 know to route to R2 if it does not know the IP address of R2's serial interface? (the routing table is statically entered, but how does R1 know that DLCI 52 is 140.1.1.2?)




Thanks again for your help.


network.png
Don JohnstonInstructor
Top Expert 2015

Commented:
1) It doesn't know the DLCI of the other end of the PVC. Nor does it need to.

2) Two ways. A) your static router could be: "192.168.1.0 255.255.255.0 int s1/0.1" in which case R1 knows that to get to the 192.168.1.0 network, it would have to send out the s1/0.1 interface. B) You have a typical route of "192.168.1.0 255.255.255.0 140.1.1.2". When R1 needs to get to the 192.168.1.0 network it looks at the next hop address. It then checks it's interfaces to determine which one has an ip address on the same network as 140.1.1.2. In this case, the interface with an IP address of 140.1.1.1 would be the one.

ok,

so frame relay can operate without the need for a DLCI if it is a point to point link. And it just forwards the frames on blindly expecting that the router on the other end will accept the frames and be configured correctly.

Can there be a frame relay switch between the 2 routers if it sends the frames out without a DLCI?
Don JohnstonInstructor
Top Expert 2015

Commented:
>so frame relay can operate without the need for a DLCI if it is a point to point link

No. The DLCI is required so that the Frame-Relay edge switch (the provider) will know what PVC to send the frame over.

So how does R1 discover what DLCI to put as the address when sending to R2?

Don JohnstonInstructor
Top Expert 2015

Commented:
It doesn't discover what DLCI. You configured it.

R1(config-if)#interface serial 0/0/0.1 point-to-point
R1(config-subif)#ip address 140.1.1.1 255.255.255.0
R1(config-subif)#frame-relay interface-dlci 52
I thought that was the local dlci of the router that you were configuring it on.

Resulting in R1 having a local dlci of 52, and then R2 having a local dlci of 51.

So if this is the case, how does R1 know what dlci it is itself?
thanks for your patience
Don JohnstonInstructor
Top Expert 2015

Commented:
The DLCI (effectively) identifies one end of a PVC. So for any PVC, there will be two DLCI's (one for each end). These DLCI's are defined by the Frame-Relay provider. If you have multiple PVC's coming in to your location, the DLCI is how you communicate to the provider which PVC you want the frame to be sent over.

Now in your case where you've got three PVC terminating on R1, you have decided to create point-to-point sub-interfaces. Each sub-interface will have a PVC that is associated with it. This association is achieved with the "interface-dlci" command on the sub-interface which tells the router that.anything coming in with DLCI 52 will be treated as if it came in on the sub-interface s0/0/0.1. Likewise, anything transmitted by the router on sub-interface s0/0/0.1 will go out with DLCI 52 in the frame header.


OK, so does this mean:

1. A frame-relay router interface will accept any frames that are sent to it?

2. The providers frame-relay switch already knows what DLCI's are where and it is responsible for taking the frame, and then forwarding it onto the correct router. That router then just accepts the frame.
 
3. I guess this makes sense since the frame is stripped of the destination DLCI and has the source DLCI put in instead at the switch, so there is nothing identifying the destination DLCI when the frame is received only the source DLCI?

4. There is always a frame-relay switch inbetween that is not shown in the diagram?

I think this is beginning to make sense to me now. I got confused because I was thinking that the configuration identified the DLCI of the router that it was configured on. I think this clears things up.
Don JohnstonInstructor
Top Expert 2015

Commented:
1. Only if they have a DLCI that the router is aware of (configured or learned).
2. No. See #1
3. No. When you receive a frame, it will have the DLCI (local... your end) of the PVC is arrived on.
4. Yes. In the real world, your traffic goes through numerous frame-relay switches.
Let me try this again.

1. On R1 the DLCI of 52 is configured. So when it sends out a frame it goes out with the DLCI of 52 as the destination.

2. The frame relay switch at the provider takes that frame and then switches the DLCI to that of the sending router, and now sends the frame to R2. So in our example, the frame now has a DLCI of 51.

3. The frame is now received by R2 which has the DLCI of 51 configured for the interface so it accepts the frame.

Is this correct?

Don JohnstonInstructor
Top Expert 2015

Commented:
1) Yes and No. There is no "destination" in frame-relay. The DLCI tells the frame-relay switch what PVC to send the frame over. The switch doesn't even know where the PVC goes.

2) Kinda... The frame-relay switch doesn't know anything about routers. Granted, in our world, we always have a router connected to the frame-relay provider but the switch doesn't know that. But at the end of the whole process, the frame arrives at R2 with a DLCI of 51.

3) Correct.
OK,

So I think that takes care of the general idea of the process.

Just a few thing to clarify. so the DLCI really is more of an identification of the PVC between the router and the switch rather than the router itslef?

The switch knows what the PVC is that the frame comes in on, so that is the DLCI that gets put into the frame by whatever the process is once it reaches the provider?
Instructor
Top Expert 2015
Commented:
>Just a few thing to clarify. so the DLCI really is more of an identification of the PVC between the router and the switch rather than the router itslef?

I "think" you've got the idea. But I'm not nuts about the way you phrased it. Let's just say the DLCI identifies the PVC.

>Just a few thing to clarify. so the DLCI really is more of an identification of the PVC between the router and the switch rather than the router itslef?

I'm not following you.
Ok, that clears things up a lot. I can work out the rest from here.

Thanks for you time and patience.

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