DLCI once and for all

can someone explain the difference between

DLCI is locally significant but yet can be configured as globally significant.
(which I understand)

INverse arp dynamically creates mapping between the layer 3 address and the layer 2 address(DLCI).
(which I do not)

Maybe I don't understand the DLCI enough.
DLCI is just identifier between DTC and DCE so that when multiple VCs use the same access link, FR switches know how to foward the frames to the correct remote sites....
So how does page 397 on cisco press ccna icnd can configure frame relay with no DLCI on? if it's locallly significant, how does DTE learns DLCI from inverse arp? I don't understand this concept at all and I would appreciate if someone can really explain this.
I thought DLCI is some number yourself just make up and put on the router's interface for the sake of identifying the circuit. If we put no DLCI on, how does router learn the DLCI from inverse arp(from DCE? so it picks up DLCI itself?) can someone either explain in detail or point me to where I should be reading? I spent too much time on this .. please help

router1
int s 0
encap frame-relay
ip address xxx.xxx.xxxx.xxx

router2
int s 0
encap frame-relay
ip address xxxx.xxxx.xxxx.xxx

router3
int s 0
encap frame-relay
ip address xxxx.xxxx.xxxx.xxxx

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Comments

rakem

In that example that you mentioned Frame Relay is configured on the physical interface which means that there is only one VC to the destination.
The router will use Inverse ARP to map the DLCI to the remote IP address

DLCIs are learned through the LMI messages so the DLCI would probably be assigned to the DCE which is configured at the service providers end.

You only really asign DLCIs when you use subinterfaces (point-to-point and multipoint) because then the router needs to decide which subinterface to send the data down, and it decides this by using the DLCI.

Inverse arp will map the remote IP address to the local DLCI address through the exchange of LMI messages. But can be turned off useing the no frame-relay inverse arp command

if Inverse ARP is disabled you then use the frame-relay map ip [remote IP [local DLCI] command, so something like:

frame-relay map ip 10.1.1.1 102 broadcast

You use the broadcast command so routing protocols can send updates.

For the CCNA exam most of your questions will be surrounding subinterfaces thou because its the best way to do thing with frame relay.

Hope this helped...

agustinchernitsky

Well, imagine you have routerA and routerB connected thru FR.

in the FR switch you configure the PVC 101 -> 201 and 201->101.

in your routers you should have something like this:

#int s0/0
#encap frame
#ip address 192.168.0.1 255.255.255.252
#no shut

here you didn't configure a DLCI with the frame interface-dlci. Still if you ping the other ip (192.168.0.2) it works... why?

Here's where IARP comes into action... the router sends an IARP request thru its PVC to find out what's the ip address of routerB on the other side.

Now, if IARP is disabled on the S0/0 int, this won't happen... and you won't be able to ping. This is when you have to use the frame-relay map cmd... to create static mapping.

convenientstore

ok.. so then, lets take this example, (below is the output from boson, I just can't get frame relay switch lab at the home just yet so please bear with me)

#R1
!
interface Serial0
ip address 10.1.1.1 255.255.255.0
encapsulation frame-relay
frame-relay interface-dlci 102
!

#R2
interface Serial0
ip address 10.1.1.2 255.255.255.0
encapsulation frame-relay
frame-relay interface-dlci 201
!

R2#sh frame-relay map

Serial0 (up): ip 10.1.1.1 dlci 201(0x66,0x1860), dynamic,
broadcast,CISCO, status defined, active

BUT according to cisco press book, when I did frame-relay map, it should have shown R1's IP with R1's dlci.... Is boson wrong?
page 402 in cisco press CCNA ICND

rakem

the frame-relay map command will show the mappings between the remote IP and the Local DLCI, so that output is correct

convenientstore

so you are saying it maps destination IP with local(its own) DLCI?

rakem

Yep.

Remote IP to Local DLCI.

AND if your using point-to-point subinterfaces the IP address doesnt even show up!

agustinchernitsky

Always the destination DLCI is your local DLCI... think of the OUTPUT PVC or something like that.

A --- B --- C --- D

A -> B 201
B -> C 301
c -> D 401

To get from A to B, the packet has to go out to PVC 201. Then the switch to get it to Switch C, has to put the packet thru PVC 301... so on, so forth.

A and D never know or use the other DLCIs.

From the sh frame map cmd, you get the Dynamic word, stating that the DLCI was resolved with IARP. In english: To get to ip 10.1.1.1, send the packet out s0/x thru DLCI 201.

convenientstore

so you are saying

#R1
!
interface Serial0
ip address 10.1.1.1 255.255.255.0
encapsulation frame-relay
frame-relay interface-dlci 102

102 that's configured is actually DLCI for R2?

rakem

yea i think so haha...

convenientstore

that's very confusing.
Can you guys kindly point me to any URL that explains frame relay really in detail?

rakem

sorry its local, DLCI is always local

so

frame-relay interface-dlci 102

102 ia the local DLCI

convenientstore

THIS is very bad and confusing.

http://www.mcmcse.com/cisco/guides/frame_relay_dlci.shtml

The locally significant DLCI must be mapped to the destination router's IP address. There are two options for this, Inverse ARP and static mapping.

This is really bad... so cisco press ICND book has error???

rakem

convenientstore wrote:

The locally significant DLCI must be mapped to the destination router's IP address. There are two options for this, Inverse ARP and static mapping.

thats exactly right. You only use static mapping when Inverse ARP is not is disabled. The definition if Inverse ARP is that it maps the Remote IP to the Local DLCI.

loboernesto

One way I got to understand it better was configuring DLCIs to match the interfaces in the FR switch:

Interface Serial1
x
x
x
frame-relay route 122 interface serial2 221
! I configure the DLCI so it "tells" you where it goes whit its name: 122 (one to two) and 221 (two to one). does it make any sense??

then when you are on router 1 and you see DLCI 122 you know that it is the DLCI that will route the traffic from router 1 to two....
This works better if you match the FR switch's interface numbers with your router name (router 1 to interface Serial 1 on FR switch)

I better stops because it starts to get consufing for me again haha. Hope that helps.