Frame Relay DLCI question

I'm a bit confused on the DLCI information in the books. My cisco press books seems to concentrate more on the "global DLCI" where each DTE to FR switch has a specific DLCI. And when router A wants to send to router b it uses router b's known DLCI and then the frame relay switch changes that the DLCI to routerA's when forwarding to routerb. However in my sybex book it seemed a bit more confusing stressing the local signifigance of the DLCI.

Could someone please take the time to fill me in. Thank you

John

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Comments

keenon

DLCIs are locally significant ro each router..

routerA- DLCI 301 Frameswitch<> Frameswitch- DLCI 201- routerB

jbkm

so does router a use router b's dlci to communicate with router b? The fr switch will then change the dlci to router a's prior to finishing the forward to router b, so that router b will know the source dlci as a's? Or does it even matter what the source dlci is? will router b just pay attention to the source IP and know where to forward the responces?

steve-o87

No. The DLCI is only important between your router and the frame switch. Your router might use DLCI 120 and the remote router could communicate back with your router on DLCI 210.

It is locally only significant!

mzinz

After reading this post, I realized that my understanding of the DLCI may be a little bit shady as well.

I understand that a DLCI is a Layer 2 address in Frame Relay networks.
I understand that it is only *locally* significant.
I understand that it is used between the router and Frame-Relay switch.

Now, what I don't understand:
Lets say that there are three Frame relay routers set up, all of which connect to a single Frame Relay switch (hub/spoke). Each of the routers would have its own DLCI, right?

If this is true, then, for example:
Router_A

> FR_Switch: DLCI=101
Router_B

> FR_Switch: DLCI=202
Router_B

> FR_Switch: DLCI=303

If Router_A is trying to send packets to Router_B, how would it address it?
I believe this is what someone else was getting at earlier, but I don't think the question was completely answered.

I realize my understanding of FrameRelay may be a little bit "not perfect", so feel free to edit anything I say. I just want to understand a bit better.

EdTheLad

You would need 2 dlci's on RTA
Hub & Spoke example:
Router_A

> FR_Switch: DLCI=101,102
Router_B

> FR_Switch: DLCI=202
Router_C

> FR_Switch: DLCI=303

Frame switch would have dlci's x-connected like
ser0 101 -> ser 1 202
ser0 102 -> ser 2 303
ser 1 202 -> ser 0 101
ser 2 303 -> ser 0 102

mzinz

So each router has an individual DLCI for each VC?

So Router_A would send DLCI 101 if its communicating with Router_B, and itw ould send DLCI 102 if it was communicating with Router_C.

Then the FR switch would see that it was sending DLCI 101 and address that to DLCI202 on its interface leading to Router_B?

EdTheLad

Yep,view the dlci like a mac address,rta would need to know the mac address of rtb and rtc if it were an eth segment,this is fr so it needs to know the devides dlci.

mzinz

After it is switched by the FRSwitch, will the Source DLCI be equal to the DLCI for RTB

> RTA?

For instance:

RTA ---> RTB // DLCI= 101

RTB ---> RTA // DLCI= 299

RTA wants to send packet to RTB and get a reply:

RTA ---> FRSwitch @ DLCI 101.
FRSwitch ---> RTB @ DLCI 299.
RTB receives packet. RTB wants to send reply:
RTB ---> FRSwitch @ DLCI 299.
FRSwitch ---> RTA @ DLCI 101.

//Sorry I'm dragging this out. I just REALLY want to make sure I understand.

EdTheLad

I really dont see what the fuss is all about.
Rta wants to send a packet to Rtb,It will look at its local fr mapping to map the destination ip address of rtb to a local dlci.The packet will be encapsulated in a frame with a fr header indicating the dlci number.The frame is sent to the frame relay switch,the frame relay checks the frame relay header of the received frame and see's a dlci, it then checks its internal switching table to find a destination port and dlci that are mapped to the source port and dlci of the received packet.The switch creates a new head with new dlci and sends it out the port to Rtb.

mp3spy

ed_the_lad wrote:

I really dont see what the fuss is all about.
Rta wants to send a packet to Rtb,It will look at its local fr mapping to map the destination ip address of rtb to a local dlci.The packet will be encapsulated in a frame with a fr header indicating the dlci number.The frame is sent to the frame relay switch,the frame relay checks the frame relay header of the received frame and see's a dlci, it then checks its internal switching table to find a destination port and dlci that are mapped to the source port and dlci of the received packet.The switch creates a new head with new dlci and sends it out the port to Rtb.

I know this thread was started a while ago, but I use the search feature on this forum for my studies, its like a big o'l library

Anyways,

So the DLCI is locally significant got that part...

Why wouldnt you utilize the map option when mapping your remote router's ip to your local DLCI all the time?? Whats the big deal if you do that for point to point? Routing Loops? Why would you do inverse arp anyway?? I don't understand how inverse arp plays a roll in this concept...why not just map everything statically?

Rt A DLCI 101 10.0.0.4
Rt B DLCI 201 10.0.0.5

rta#frame-relay map 10.0.0.5 DLC 101 (points it to DCLI 101/Fr sw and it gets handled from there)

rtb#frame-relay map 10.0.0.4 DLC 201 (points it to DCLI 201/Fr sw and it gets handled from there)

point to point set the DLCI and be done with....
maybe the whole point to point and multi-point concept is throwing me off..

cjdock

we use frame where I work. We don't use the map command though. A config on my router may look like this:

interface Serial0/0/0
description WAN
no ip address
encapsulation frame-relay
ip route-cache flow
service-module t1 timeslots 1-4
frame-relay lmi-type ansi
!
interface Serial0/0/0.1 point-to-point
description main_office
bandwidth 256
ip address 10.123.456.1 255.255.255.0
no cdp enable
frame-relay interface-dlci 123

This router is not dlci 123, it is configured to point at dlci 123. Dlci 123 is the far end this serial interface connects to. The far end has this routers dlci in it's configs. Thats how the two are able to communicate.

(ip's and dlci changed to bogus info)

!

rossonieri#1

hello,
DLCI is more like your MAC addr significan to your L2 switch.
your L2 switch doesnt care anything but "this port has this MAC".
this goes for FR-pvc in the FRS also - "this router has this DLCI".

FR map ip comand is only used when you disable inverse-ARP.

cheers...

mp3spy

cjdock wrote:

we use frame where I work. We don't use the map command though. A config on my router may look like this:

interface Serial0/0/0
description WAN
no ip address
encapsulation frame-relay
ip route-cache flow
service-module t1 timeslots 1-4
frame-relay lmi-type ansi
!
interface Serial0/0/0.1 point-to-point
description main_office
bandwidth 256
ip address 10.123.456.1 255.255.255.0
no cdp enable
frame-relay interface-dlci 123

This router is not dlci 123, it is configured to point at dlci 123. Dlci 123 is the far end this serial interface connects to. The far end has this routers dlci in it's configs. Thats how the two are able to communicate.

(ip's and dlci changed to bogus info)

!

Yea but what happened to the concept of DCLI's are locally significant in all cases???