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:
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 NodetoDevice A? - Does a DLCI(102) define the link from
Sending NodetoReceiving 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?
Best Answer
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.