Frame Relay DLCI confusion (local versus global)

I'm currently reading the ODOM ICND2 book in Chapter 13 - the Frame Relay section. Can someone please explain the local versus global DLCI's and clarify this some - or point me to some doc that explains this better? I've read the section a couple of times and it seems to be confusing as to why the DLCI is local, but then I have a global DLCI as well - and are these usually the same? It just seemed to be very unclear to me.

Thanks

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Comments

pogue

As I understand it, the local addressing section is explaining how DLCI's are only locally significant on the local frame relay switch. It is explaining how the actual encapsulation works to put the locally significant DLCI in the frame relay header, i.e., Figure 13-7.

"Global Addressing" as I understand it, is simply a different way to think about Figure 13-7.

Imagine if all routers with a PVC to Router A used the same locally significant DLCI.. 40. This would look like a global address on a diagram. I.e. Figure 13-8.

It is just an ISP's way of making the concept easier for the configuring engineer.

You understand Figure 13-7, right? Router A uses locally significant DLCIs 41 and 42 when sending to Routers B and C, respectively. Both B and C use 40 to get back to A.

All Figure 13-8 does is change the visualization. The ISP puts the Global DLCI "recipient" addresses as a label on each router, effectively telling each local engineer what DLCI to set as a label on the PVC connecting to each distant router.

Global addressing is simply a different way to visualize "local addressing", with one caveat: The ISP needs to make sure each distant router uses the same DLCI # to get back to the hub router. (Because technically, there is no technical requirement to do so.)

Hope this helps. Let me know if this is unclear, and I will try to help. And you're right. Odom didn't really make this clear.

Russ

Todd Burrell

This does help some. I think I am just trying to make this more difficult than it really is. And the Figures were fairly clear, but I was not very clear if the DLCI's were local or global? Again, I think I am trying to make something easy much harder than it really is. This WAN stuff just all seems sort of fuzzy, but it is getting somewhat clearer. I need some labbing to probably bring it home.

Thanks for the info.

pogue

This might clarify the concept.

Think of each PVC between routers as a tube.

Each tube has a label on each end. That label is a number that only has "local" significance.

When configuring the tube, all that matters to you is the local label. You have zero idea what the ISP is doing, you have zero idea what the other end is doing.

When you draw this out on a diagram, you have a tube with two numbers on it.

To send to a remote router, you insert a label into the packet with the local number of the tube you want it to enter.

Straightforward enuff, right?

Now, think about global addressing this way:

Instead of it being a tube with only a locally significant #, the tube is a mailbox and the other router simply has an address (global #).

Do you see how it will not change your configuration of the local router at all? You still label the DLCI locally. You are just changing the way you think about the overall circuit in a way that makes it more intuitive.

"If I want to send to Router B, label the packet with this # (address)."

Russ

Todd Burrell

This description helps a great deal. Thanks for this clarification. I think I have it now.

This stuff is about as confusing as some of the NAT global/local indside/outside stuff... But that's another story. I'll tackle that in another couple chapters.

CodeBlox

I think wendell should have left the whole global thing out of the chapter... It really confused me until I realized that it was simply a way to think about the DLCIs. It allows you to 'pretend' that the DLCI is somewhat of an address for the different hubs/spokes. In reality, the DLCI is locally significant and defines the PVC on a particular access link.

pogue

CodeBlox wrote: »

I think wendell should have left the whole global thing out of the chapter...

I agree. It confused me as well, and I am still not sure if I am right about what he was attempting to communicate.

Russ

4_lom

Yeah, there is no point (other than confusion) to learning the global dlci concept. Just remember that they are locally significant (meaning from the router to the FR switch) and you should be in good shape.

Todd Burrell

Thanks for all the replies. After reading Chapters 13 and 14 in the Odom2 book today, I have figured out that the global stuff is pretty much insignificant. I have no idea why this was in the book, as all it did was confuse me.. Frame Relay is still a challenge, but it is clearing up some...

I will be very glad when I have the CCNA behind me... Target date for the test is January 19th.