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Frame Relay is a technology that covers both the physical and datalink layers of the OSI model As such, it has its own physical addressing structure, which is quite different from the Media Access Control (MAC) addressing structure A Frame Relay address is called a Data Link Connection Identifier (DLCI) Unlike MAC addresses, DLCIs do not specify the physical port; rather, they specify the logical link between two systems (virtual circuit) In this manner, each physical Frame Relay port can have multiple DLCIs because multiple VCs may be associated with that port For example, Figure 3-4 shows both the logical and physical layout of a Frame Relay implementation
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Figure 3-4: Logical and physical layouts of one Frame Relay network In all of these Frame Relay implementations, you may be wondering what the "cloud" is Put simply, the Frame Relay cloud is your telco It is drawn as a cloud because it is "voodoo" As long as the packet enters where we want it to and leaves where we want it to, we don't care how it works Most likely, if you call your telco and have them try to explain it to you, they will say "Uguh Buguh Alacazam" and hang up OK, that last part was a joke but honestly, we don't really care from an implementation perspective However, I will shed some light on this voodoo for you just so you fully understand the technology The Frame Relay cloud is really just a huge bank of Frame Relay switches In Frame Relay terminology, two types of devices exist: the Data Communications Equipment or Data Circuit-Switching Equipment (DCE), and the Data Terminal Equipment (DTE) The DCEs are the frame switches in the cloud The DTEs are the routers The DCEs operate on the same
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basic principles as the frame routers they are just built to handle a lot of VCs simultaneously In addition, the DCEs provide what's called a clocking signal to the DTEs The clocking signal is needed because Frame Relay is a synchronous protocol: the frames are synchronized with the clocking signal, so no start bit and stop bit are needed As a result, Frame Relay becomes a little more efficient and, subsequently, faster To put these concepts into perspective, the cloud represents a fragmented path to the final destination In other words, you do not have a dedicated piece of cable supporting each of your PVCs Instead, you have a VC that may be switched through ten, hundreds, or even thousands of smaller lines on its way to its final destination This keeps the telco from having to build a new trunk for every client they add Rather, they are just reselling their current infrastructure Figure 3-5 shows an example of a simple frame cloud and how these VCs are switched
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Figure 3-5: A simple Frame Relay cloud However, this depiction is a bit oversimplified In truth, another aspect of Frame Relay addressing comes into play: local significance In reality, very few Frame Relay DLCIs are globally significant, as shown in Figure 3-5 Instead, most of them are locally significant Therefore, it doesn't matter if the DLCI changes throughout the cloud, as long as your DTE and its corresponding DCE both use the same DLCI number What your DCE uses as the DLCI for your connection with another DCE in the cloud doesn't matter to your DTE All your DTE is concerned with is what the DLCI means to it For example, let's take a simple Frame Relay implementation and show DLCI-switching in the cloud In Figure 3-6, two routers (our DTEs) are communicating with each other through a Frame Relay cloud composed of three switches At Router A, DLCI 50 identifies the VC to Router B At Router B, DLCI 240 identifies the connection to Router A Throughout the frame cloud, mappings take place to associate these DLCIs This example is mapping DLCIs (layer 2 addresses) to other DLCIs; but, in reality, the DLCIs are mapped to layer 3 (such as IP) addresses, and routing takes place to send the packet down the correct path However, because we have yet to discuss IP addressing, this example suffices One way or another, the principle is the same
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Figure 3-6: Frame Relay DLCI-switching through the cloud The telco gives you your DLCIs, which will most likely not be the same on either side of the connection, and the telco sorts out how the DLCI-switching occurs Note that the reason locally significant DLCIs are needed in the first place is because no structure exists to assure uniqueness of DLCIs In fact, global uniqueness of DLCIs would bring the whole frame infrastructure to its knees because most providers use a frame structure that provides for only 10-bit DLCI numbers This means that only 1,024 DLCIs could exist at any given time (because only 1,024 values can be represented with ten bits), and there are a lot more frame VCs than 1,024 With the current locally significant structure, each DCE can have up to 1,024 VCs (minus a few reserved DLCIs), and these VCs can be different from the VCs present on every other DCE, which makes a locally significant DLCI structure much more scalable than a globally significant DLCI structure
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Frame Relay uses an interesting concept for link management called local management interface (LMI) The LMI is an extension of Frame Relay that provides a number of benefits, as described in the following sections Globally Significant DLCIs Your telco can assign certain links DLCIs of global significance This strategy allows you to use Frame Relay just like a big LAN, with each individual PVC having its own static address Unfortunately, it also limits the scalability of the frame cloud
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Status Inquiries Possibly the most useful benefit of LMI is that status messages can inform a DTE of a PVC going down inside the cloud, preventing the DTE from forwarding frames into a black hole Status inquiries work like this: Once every so often, the DTE contacts its directly connected DCE and asks for an update on its PVCs How often the LMI does this is called a heartbeat After a certain number of heartbeats have passed, the LMI also requests a full update on all PVCs that terminate at the LMI This process allows a DTE to remove any PVCs for which the status inquiry comes back negative This function is extremely useful in a frame environment in which more than one path or route exists to any given destination Without this function, the router may take a significant amount of time to determine that its primary link is down During this time, it continues to send frames through the dead PVC, resulting in lost data However, if LMI status inquiries are used, the router will know that the PVC is dead and will instead send packets down the secondary path For example, take a look at the network shown in Figure 3-7 Let's see what would happen if a PVC failure occurred between Networks 1 and 2
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Figure 3-7: Link going down If the DTE was not receiving periodic LMI messages, as shown in Figure 3-8, it wouldn't yet realize that the link between Network 1 and Network 2 is down The DTE would proceed as normal, sending packets down DLCI 200, but the packets would never reach Network 2 due to the PVC failure The packets would be lost for all eternity This is known as "routing into a black hole"
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Figure 3-8: Frame Relay sending frames into a black hole However, if the DTE was using LMI, it would notice that its periodic status messages were not arriving, and would assume the link was down DLCI 200 would be removed from the routing table due to the LMI status message failure An alternate path, via DLCI 75, would be entered in the routing table, allowing the DTE to route packets down DLCI 75, as shown in Figure 3-9
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Figure 3-9: Frame Relay sending frames down a secondary path due to LMI status messages DLCI Autoconfiguration This benefit is useful for automatically configuring Frame Relay DLCIs in a small frame environment The telco simply sets the DLCIs on a DCE, and the directly connected DTEs automatically initialize the correct DLCI numbers Multicasting LMI also allows multicasting over Frame Relay by allowing multicast groups to be established and multicast status inquiries to be sent over the PVCs Multicasting is discussed in more detail in 6
Now that you know what the LMI does, we can look at the different LMI types Unfortunately, as with most aspects of networking, no one could seem to agree on one single standard As a result, we now have three distinct, separate LMI types They all perform basically the same tasks, but they just accomplish them differently And, of course, they are not compatible with one another The three LMI types are Cisco (sometimes called Gang of Four after the four companies who created it), ITU Q933-A (sometimes called Annex-A), and ANSI T1617-D (sometimes called Annex-D) Basically, the point to remember about LMI types is that you do not have to have the same LMI type on two DTEs in a given VC, but they need to be the same from the DCE to the DTE on each side Figure 3-10 illustrates this point Luckily, this limitation isn't much of an issue today because Cisco routers running IOS 112 or later can automatically detect and configure the LMI type (and most providers use Annex-D, anyway)
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