Confused about Frame-Relay DLCI.

[brave_heart]

Hi, would you please clear the following confusion for me? In the Frame-Relay's frame format, the q.922 address means the local DLCI or the destination DLCI?

In the command "frame-relay interface-dlci 140", the 140 is the local DLCI or the destination DLCI?

Thank you very much in advance!!!

[deilert]

This is going to be the DLCI that is advertised to you by the directly connected FR switch

turn on debug frame-relay lmi

you will get it shows the DLCI it is advertising and the CIR

Serial0/0(in): Status, myseq 235

RT IE 1, length 1, type 0

KA IE 3, length 2, yourseq 238, myseq 235

PVC IE 0x7 , length 0x6 , dlci 138, status 0x2 , bw 384000

Serial0/0(out): StEnq, myseq 236, yourseen 238, DTE up

datagramstart = 0x3568574, datagramsize = 13

FR encap = 0xFCF10309

00 75 01 01 01 03 02 EC EE

[brave_heart]

Thank you, deilert.

Does that mean if the router does not use subinterface, then there is no need to use the command "frame-relay interface-dlci"?

[Dragan Mickovic]

DLCI has local significance, so "frame-relay interface-dlci 140", 140 is dlci that is local to the router.

[brave_heart]

Thank you, dmickovic.

Then, does it mean the source router does not need to know the DLCI on the other end? And the frame-relay switch use the source DLCI to route the frames? Is it the truth that before exit the final switch, the q.922 address in the frame header is the source DLCI, after exit the final switch, the q.922 address will be changed to destination DLCI by the final switch, to indicate to the end router of which pvc the frame comes from?

[Dragan Mickovic]

The router needs to know about the DLCI on the other end, if you are not using sub-interfaces then you need to setup "frame-relay map". I can not remember about the how the packet transits/changes through the FR network, but you can read about at the following link: http://www.cisco.com/en/US/tech/tk713/tk237/tech_protocol_family_home.html

[pkhatri]

Hi,

Yeah, the source router does not need to know the DLCI at the destination side. It only cares about the DLCI at its end (hence, local significance).

An example follows:

16 39 63 48

RtrA ---- SwA ----- SwB ----- SwC ---- RtrB

In the above figure, RtrA, RtrB are routers, SwA, SwB, SwC are frame-relay switches and the number on top of the dotted lines is the DLCI. RtrA sends out q922 frames with DLCI = 16. SwA gets the frame and sends it out to SwB with a new DLCI of 39. SwB gets the frame and sends it out to SwB with a new DLCI of 63. Finally, SwC gets the frame and sends it out to the destination, RtrB with a final DLCI of 48.

Only the frame-relay switches need to know about the intervening DLCIs as the frame goes from RtrA to RtrB. The routers only need to know the DLCI that they see.

Hope that helps,

Paresh

[pkhatri]

Hi,

It looks like my drawing get somewhat mangled. Here is how it should read...

16 is the DLCI between RtrA and SwA

39 is the DLCI between SwA and SwB

63 is the DLCI between SwB and SwC

48 is the DLCI between SwC and RtrB

Cheers,

Paresh

[brave_heart]

Thank you, dmickovic

Since the source DLCI identifies the same PVC as the destination DLCI, so why you say the source router needs to know the DLCI of the other end?

I have read a lots of materials, but they seem to contradicated with each other, so I get really confused!