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I am currently studying for my CCNA exam and I am stuck at the concept of DLCIs in a network using frame relay encapsulation. This may be because I am a non native english speaker and misinterpret the material I have got.

This is the text in question:

VCs provide a bidirectional communication path from one device to another. VCs are identified by DLCIs, as shown in Figure 2. DLCI values typically are assigned by the Frame Relay service provider. Frame Relay DLCIs have local significance, which means that the values themselves are not unique in the Frame Relay WAN. A DLCI identifies a VC to the equipment at an endpoint. A DLCI has no significance beyond the single link. Two devices connected by a VC may use a different DLCI value to refer to the same connection.

Locally significant DLCIs have become the primary method of addressing, because the same address can be used in several different locations while still referring to different connections. Local addressing prevents a customer from running out of DLCIs as the network grows.

Which has a picture for reference:

frame relay

The VC (virtual circuit) spans from the Sending node (DLCI 102) to the receiving node (DLCI 201). It then states that DLCIs have 'local significance', and are not unique in the Frame Relay WAN.

My question is: Since a DLCI is local, what device is meant by 'endpoint'?

  • Does a DLCI(102) define a link from Sending Node to Device A?
  • Does a DLCI(102) define the link from Sending Node to Receiving Node (SN -> A -> B -> C-> D -> Receiving Node) ?

In the end it boils down to: Is the single link mentioned in the text, the single physical line between the router and the switch or the line between the 2 ultimate endpoints?

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Your first answer is the right one, DLCI (102) define a link from Sending Node to Device A.

Without local significance, then every customer FR PVC will need to have the same DLCI ID value at both endpoints.

Now, assume we are connecting a new customer, what if a DLCI is available on one switch, but the same DLCI is already taken on the 2nd switch by another customer ?

Without local significance, the provider will have to check all the available matching DLCI Pairs on both endpoint switches to connect any new customer, and might not even available matching DLCIs.

With local significance, the provider will just connect the PVC using any free DLCI on the the 1st switch, and any free DLCI on the 2nd switch, even if they do not match.

Note also that the DLCI ID is locally significant to each FR Port on the switch, i.e. DLCI 102 can be used on more than one FR Port on the same switch.

  • So this would mean, device A is a frame-relay switch, which would mean, it is a layer 3 switch, because it must have the ability to map DLCIs to IP addresses via inverse ARP. Alternatively it could be a router, is this correct? – Marco Jan 3 '14 at 14:06
  • No, a frame relay switch does not care about the IP Address, it only routes the PVC from one endpoint Port on Switch A up to the other endpoint Port on Switch D. It is up to the layer 3 router configuration at customer site to map each DLCI to the destination IP address correctly. – aseaudi Jan 3 '14 at 14:17
  • I do hope there some more in depth material to the principles of frame-relay switching later on. Anyway: Your answer makes total sense. Thank you. – Marco Jan 3 '14 at 14:20
  • Check Cisco Comprehensive Frame-Relay configuration and troubleshooting guide at cisco.com/en/US/tech/tk713/tk237/…. Instead of delving deeper into the frame-relay protocol itself, you will find that this document is about mapping the layer 3 ip to layer 2 frame relay pvcs. After going through this document, you will see that frame-relay pvcs are "pipes", and that routers select which traffic to put into each pipe. – aseaudi Jan 3 '14 at 14:52
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"local significance" simply means a DLCI (number) is only unique per hop. For example, DLCI 16 on my link is not the same DLCI 16 on your link, even from the same switch. Put another way, DLCI 16 is not a global reference across the entire frame network, or even a single frame switch.

In your example, DLCI 102 defines the link between SN and A. Likewise 201 is only between RN and D. How the VC traverses the Frame WAN is our of scope. (unless you're building that WAN you don't need to know.) From either end, the DLCI may not be 102 and 201. (imagine both ends being 16, from each end, the other is not 16.)

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