Hi and welcome to this CertificationKits CCNA Training Video on getting to know your good friend Frame Relay. And we’ll talk about what Frame Relay is, we’re going to go over all the terms associated with Frame Relay. You’re going to have to know all those terms. Addressing concerns with Frame Relay IP addressing, data link connection identifiers and global DLCI maps as well as how Frame Relay can handle broadcast messages.
First thing for your CCNA exam, I want to do is talk about what Frame Relay is and go over some of the terminology. Frame Relay is a long distance non-persistent connection or that’s what it’s recommended for through Cisco. And one of the nice benefits with Frame Relay if I have one router connection and one line to a service provider I can connect to multiple different destinations. They can have connections between each other or they can all route in between me however I want to set it up, obviously the more connections I have the more money it’s going to cost. And it’s a very flexible, very useful connection. There are certain terms associated with Frame Relay of virtual circuit, and what a Virtual Circuit is it’s a logical connection between endpoints so DTE is a Data Terminal Equipment which is my stuff, so that’s the stuff I’m going to have on my premises. The DCE is a Data Communications Equipment, meaning the service providers stuff, if I work for a service provider then this maybe technically called my stuff. Anyway, the virtual circuit is the connection that’s established between the DTEs from one end to another; let me get rid of that so that’s a virtual circuit.
There are two types of Virtual Circuits switch, established on demand or permanent which is kind of like lease line always on. And access-link is my link here in my local loop link here between DTE and the DCE, we’ve got an access rate that’s the speed that I’m promised when I’m sending information out to my service provider. A DLCI Data Link Connection Identifier, what that is it’s an identifier simply a number to indentify connection or a virtual circuit between my router and another destination. So if I had a router and I had connections to multiple different destinations, even though I had one actual line out to my service provider I will have two virtual circuits, one going to – let’s call this router A, one go in to router B. I have to label these virtual circuits to something so what we do is we just use a number it might be 10, it might be 20, it might be 200 whatever. This number that represents the circuit is a DLCI, and also compare it a DLCI to a phone in my office, if I have a phone in my office I’ve got five lines on my phone and I can number them one, two, three, four, five doesn’t matter what the number is, I could number 900, 233, 3999 as long as each line on my phone had a different number I would still be able to associate a line with a name, basically the name is the number and that’s what a DLCI is, is just a numerical representation of virtual circuit or a conversation.
NBMA Non Broadcast Multi-access let me clean up the slide to explain this one a little bit. This is much cleaner, so Non Broadcast Multi-access NBMA, let me start if we just had a switch and some computers, so here is the switch, the switch is the rectangular thing the computer all these things with the smiley faces on them. And what happens with these computers is they have direct access to multiple other devices, so this is a multi-access environment. This computer right here has access to two other devices without having to get routed or anything like that, direct access to two other devices, that’s the part of the multi-access. The switch environment is called broadcast multi-access or BMA if you want to be lazy, because this computer can send a true broadcast message, he sends a message that the switch receives and that message and the header has a bunch of S. The switch floods that message out to all other devices, so that’s a true broadcast message, in the router environment it’s non-broadcast it can’t send a true broadcast out. Because what happens is in this multi-access environment this is the actual physical link right here. So there’s one physical link going into the router, what happens is on that physical link there are multiple virtual circuits. There’s this green virtual circuit which takes us over to this green router over here, I should say a blue router with a green square on it. And we’ve got this red virtual circuit which takes us over to the blue router with the red square on it. I apologies if you are color blind. Now each virtual circuit is its own connection, so for this router to send the message to both of these destination routers he can’t send a true broadcast, if you’re to send – he can’t send one message that the Frame Relay switch would take and send out to both different destinations. What he has to do is he has to fake it, he makes a unicast message out of any broadcast that must go out of this interface and sends two messages he’ll send one out the green virtual circuit to the green square router, he’ll send the other one out the red virtual circuit to the red square router. That’s not a true broadcast, a true broadcast is one message that goes to everybody so this is non-broadcast but it is multi-access because with one line he has access to multiple different devices and really can have access to as many device as he wants to pay for, so that’s where we get the term non-broadcast multi-access.
Let’s look at some of those other CCNA Frame-relay terms. LMI, Local Management Interface it’s basically keep alive status information between our data terminal equipment our stuff and the data communications equipment the service provider stuff. So important messages that tell us information about our virtual circuits as well as perform keep alive functions. LAPF, this is a long one, Link Access Procedure Frame Bearer Services, let me just type that out, all right here is what we started with Link Access and this is much, much cleaner. Link Access Procedure Frame Bearer Services, what it is, it’s a layer two encapsulation for our traffic going across our WAN links, particular type of – encapsulation, for a Frame Relay line. Link Access Procedure Frame Bearer Services.
Now I want to spend some more time talking about the LMI types on the CCNA exam as well as LAPF. Let’s start with LAPF, Link Access Procedure Frame Bearer Services, now LAPF is a standard for a layer two and again I’m talking about the OSI model. Layer two down here so out IP packet so this would be for sending IP traffic, this would be out IP packet, this term packet at layer three it’s called a packet layer two of frame layer four segment. So our IP packet gets encapsulated with the LAPF standard, and that has a number associated with it, it’s q922 , q922 is the LAPF standard and again that’s just our ANCRI to you information standard and it’s protocol q922 that is for LAPF. What it does, it specifies the type of header and type of trailer that’s going to go on the IP packet, but there’s something else that needs to go on this packet. Remember layer two of the OSI model has two sub layers LLC Logical Link Control and the MAC sub layer, this is where LAPF is hanging out down here the second part of the layer two encapsulation. If you notice something right bud it up next to the packet is what’s called a little type field there. And this is the big difference between the two types of Frame Relay encapsulation now they’re going to off across our Frame Relay Lines there’s two standards for this. There’s one standard design by IETF and it is called IETF Internet Engineering Task Force, they have a multi vendor standard for this type field, multi vendor meaning more than just Cisco, and then you’ve got the Cisco type field. That one’s going to work with Cisco routers. So the LAPF very important understand that, that is the header and the trailer but it’s the second half of that data link layer of the OSI model, logical link control that puts that type field in there. And what this type filed is, it’s for a multiple upper layer protocol support like IPX it can also support IPX, IP it’s just an indicator that tells the receiving device what layer three protocol was used to package the information up until this point right here, so very important. And this arose later on down the road as devices started to need – the need or they got the need to build to have multiple upper layer protocol support like Novell started supporting IP instead of just IPX, so very important.
And again LAPF down here and then there’s two types of that type field Cisco and IETF so when we type in on our interface when we’re getting ready to set up Frame Relay we type in the command encapsulation Frame Relay, I’m just going to cut it short, we would – we could either do a carriage return and our Cisco router will simply use the Cisco type field if we have a multi vendor environment instead of just getting carriage return we would specify IETF as that type field. The IETF type field there is specified in two RFCs request for comments, original RFC 1490 followed by RFC 2427 and specified in the protocol q933 so you must know these numbers here 1490, 2427 on their request for comments that’s specify the standards for this IETF multi vendor type field. Let me clean up this slide and we’re going to go in and take a closer look at LMI, Local Management Instrumentation. It is very important not to confuse layer two encapsulation with the LMI type. LMI type performs a keep alive function letting us know that everything is cool between our DTE and the DCE and if we had multiple connections on the other end let’s say multiple virtual circuits that also lets us know the status of our virtual circuits. If we’re checking on our virtual circuits on our router we will see active, inactive or deleted state. So those are three statuses we’ll see and our LMIs let us know the status of these virtual circuits so it’s a very important function.
There are three different types of local management instrumentation on the CCNA exam. Cisco — proprietor is Cisco. So, if you got Cisco stuff going on, we’re good. ITU also known as Q933A. The A is the big difference here. Because I also said the IETF specified in a protocol Q933, but that is Annex E. A different part of the Q933 protocol. And the LMI type, Q933 is Annex A. And that’s what’s the A stands for there. So, there is a difference, but they are both supported underneath the Q933 protocol. Again, protocol set of rules. And then ANSI. So, Cisco ITU and ANSI. So, it’s important that we have matching LMI types on our hands here. So, we have appropriate communication. Otherwise, our device will think the link is there, and in fact we have problems communicating. So, very important, LMI is a status message and keep alive function. LAPF is a layer 2 encapsulation. There is a Cisco LMI type, and there is a Cisco layer 2 encapsulation. Do not confuse them. Even though the name is Cisco for both, they are two totally separate things. So, it’s very important not to confuse them. Let’s go in and take a look at these DLCIs’ a little bit further, and how they get those numbers. I brought up a slide here for the global DLCIs.’ And what’s going to happen is our service provider is going to come up with a global map of our DLCIs’. And basically the DLCI will represent the destination. So, 10 represents the destination, we’ll call as Palaestra1 router. Twenty represents the destination of Palaestra2 router. Thirty represents the destination of Palaestra3 router. Now there are two, let’s say we have connectivity between all of the devices here. There is full connectivity going on. So, paying some money for this full mesh connectivity. Now I color coated everything to make it easy. Blue represents this router. Red represents P3. Green represents P2.
So, what’s going to happen is the virtual circuit from P1 going out to P2 is going to have a number on P1 of virtual circuit 20. Now we’ll follow this along. And what happens’ that DLCI field in the encapsulation for Frame Relay. Actually gets swat in transit. So, what’s going to happen, this virtual circuits ends up being this virtual circuit right here, blue. But the number is not 20 anymore. The number is 10. How that works is our service provider actually swaps the DLCI out in transit. So, when we’re sending the 20. Our service provider knows, it’s going all the way over here and going to P2. What he does is? He gets that 20 out of the header and removes it, and before he passes it on to P2, he says, “Hey, this is from router 10”. So, the DLCI is one field and it’s actually the source and destination. It just gets swapped in transit by our service provider. So, this guy when he receives information on the same virtual circuit, he’ll be seeing a source DLCI of 10, and this guy will see a DLCI of 20. Just like when he wants to send the P3. He’ll go out DLCI 30. Whose is all the way along there, and it goes out here, and what he is going to see is the source DLCI as a DLCI of 10. So, here are the DLCI of 10, and a DLCI of 20. DLCI10 goes to router P1, Palaestra1. DLCI 20, goes router Palaestra2. P2 of DLCI of 30. Thirty goes to Palaestra3, and 10 goes to Palaestra1. So, again to make that clear, or confuse you more. Palaestra2 when he sends out of DLCI30 over the destination DLCI 30. Remember it’s all one physical.
The DLCI is what tells our service provider, which virtual circuit it’s going to be heading on, when it goes to that WAN Cloud. So, DLCI30, is “Oh! that’s got to go over here to Palaestra3. He’ll take that DLCI number, the 30 odd and put in 10 as the source. And that way it’ll come in on this virtual circuit to Palaestra3. Don’t know where it came from. So, its global map and our service provider will help us lay that out. Let’s go back and recap all these terms and everything we want to remember them. We talked about the virtual circuits and again that is that logical path between our destinations. So, we have one router, Router A, Router B, Router C, he has one line, but he has a connection to B, and a connection to C simultaneously. Those connections are called virtual circuits. DTE, my stuff. DCE service provider stuff. We got the Access link, which is right here, also local loop. You can call it Access rate or speed that they promised us. DLCI data link connection identifier. And again, that’s the number that represents the virtual circuit. Service provider will tell us what that number is going to be. NBMA, non-broadcast multi-access. Multi-access meaning, just one device, right here, can have access to multiple different destinations through one link. Non-broadcast as he can’t send just one message to get to both devices. He’d actually have to send one Unicast over each virtual circuit to get to the destination, and that’s not a broadcast message.
LMI Local Management Interface. The CCNA exam covers three types, Cisco, ANSI, and Q933A. Very important remember do not confuse this LMI type with LAPF. LMI type is a keep alive. Also notifies us of a status of a virtual circuit. LAPF is a layer2. Layer2 meaning OSI model encapsulation. Do not confuse the two of those. Last thing I want to talk about is IP addressing. And it depends on the links we have. Couple of different situations. Because we have these virtual circuits, what we can do is? We can create a sub-interface for each virtual circuit. So, we’d have our serial interface. This is Serial 0 and what we would do is we chop it up into sub-interfaces. We could call it Serial 0.1 would be one sub-interface. Point 2 another, point 3 another. We get into this. And for the detail when we get into configuration or frame relay. But we do want to be aware of the IP address and concerns, depending on how we have this configured. If we have a full mesh environment. Meaning everybody can connect to everybody. It’s not necessary to create the sub-interfaces. We can just configure Serial 0 with frame relay encapsulation. Give it one IP, 172.16 I want to use that 2.1., 172.16.2.2, 172.16.2.3. We got to have one subnet here and everybody sharing IP in that one subnet. If we have a full mesh environment happening. Let me clear this off and we’ll take a look at what will happen if we don’t have a full mesh. Without the full mesh, if we just had a central router and this is very common. We might have Palaestra1 being the central router. Palaestra2 and Palaestra3 only communicating directly with Palaestra1.
In that situation we would create sub-interfaces out of Serial0. So, we’d give Serial 0.1, Serial 0.2. This would be considered a point to point connection. Each sub-interface would need its own subnet. And our sub-interfaces act like completely separate physical interfaces. And we would assign a DLCI to each interface or virtual circuit. So, sub-interface1 that might have a subnet of 172.16.1.0. Sub interface2, 172.16.2.0. So, this IP address would be 1.1, 1.2 over here, 1.1, 1.2 but for completely different subnets so if a system on the far side of Palaestra3, 1 and tied to a system on the far side of Palaestra2 would have to route through Palaestra1. One other scenario you might run into — let me clean this off one last time here — you might run into an environment, let me draw another switch and router in there. Can you tell which one I drew? So, what will happen is we might have a full-mesh environment going on right here, so between Palaestra1 — we will call this one Palaestra3, this will be Palaestra2, this is Palaestra4 down here. So, we might have communication between Palaestra1, 3 and 4 and then Palaestra2 will only have direct communication with Palaestra1.
So, in this situation we have a CCNA environment where there are three routers that have Mesh access to each 3 to 4, 3 to 1 so on and so forth. And 1 where it’s only in point-to-point Palaestra2 can only directly connect to Palaestra1. So, in that environment we would have serial 0 and we only chop it up in two sub-interfaces as well, Serial 0.1 could go to Palaestra2, and that would be a point-to-point interface. And this would be 1 subnet and that could be subnet 172.16.1.0 subnet. The second sub-interface would be multipoint, and what would happen is all three of these routers interfaces here would share one subnet themselves so it would be 172.16.2.0. Even though there is multiple virtual circuits going on they can still share the same IP because they have a simulated broadcast environment, it’s Non-Broadcast — this is the Non-Broadcast Multi-Access, this environment isn’t really multi-access it’s of by itself so it needs its own subnet. In this multi-access environment they can fake the broadcast messages send in or out multiple virtual circuits and simulate that broadcast environment so they can all share one IP subnet.
So, we have talked about Frame Relay much friendlier with it now; what it is – terminology – addressing concerns with Frame Relay, the point-to-point connections and multi-point connections we just went over – global DLCI maps and broadcast handling what NBMA is and how that differs from our broadcast multi-access environment with the switch. I hope you have enjoyed this CertificationKits Training Video on Getting to Know Frame Relay.