How are DLCI numbers assigned in Frame Relay ?

[polm]

the basic config for frame relay requires only an interface with an IP and the encapsulation set to frame relay.

Now, if I don't define a local DLCI, how does it get assigned ?

I don't understand how InARP assigns the DLCI.

 

[spidey07]

the DLCI is sent from the frame switch.

LMI Full Status packets from the frame switch contain all the DLCIs for each virtual circuit and are normally sent every 60 seconds.

Normally when you configure frame relay you use point-to-point subinterfaces and manully specify the DLCI.

When using point-to-multipoint (NOT recommended, too many issues to deal with) frame-realy inverse arp will map the IP addresses to the DLCIs.

 

[polm]

Originally posted by: spidey07
the DLCI is sent from the frame switch.

LMI Full Status packets from the frame switch contain all the DLCIs for each virtual circuit and are normally sent every 60 seconds.

Normally when you configure frame relay you use point-to-point subinterfaces and manully specify the DLCI.

When using point-to-multipoint (NOT recommended, too many issues to deal with) frame-realy inverse arp will map the IP addresses to the DLCIs.

Where does the frame switch get the numbers from ? Some pool of DLCI numbers ?

 

[spidey07]

Originally posted by: polm

Originally posted by: spidey07
the DLCI is sent from the frame switch.

LMI Full Status packets from the frame switch contain all the DLCIs for each virtual circuit and are normally sent every 60 seconds.

Normally when you configure frame relay you use point-to-point subinterfaces and manully specify the DLCI.

When using point-to-multipoint (NOT recommended, too many issues to deal with) frame-realy inverse arp will map the IP addresses to the DLCIs.

Where does the frame switch get the numbers from ? Some pool of DLCI numbers ?

Provisioned. ie, setup by the provider.

you can also simulate a frame-realy switch with a cisco router and two serial ports. It really is a very simple protocol. Layer2 only has destination DLCI. That's why its called a "frame switched network", it switches frames at layer 2 using the virtual circuit indetifiers (DLCI)

 

[polm]

Originally posted by: spidey07

Originally posted by: polm

Originally posted by: spidey07
the DLCI is sent from the frame switch.

LMI Full Status packets from the frame switch contain all the DLCIs for each virtual circuit and are normally sent every 60 seconds.

Normally when you configure frame relay you use point-to-point subinterfaces and manully specify the DLCI.

When using point-to-multipoint (NOT recommended, too many issues to deal with) frame-realy inverse arp will map the IP addresses to the DLCIs.

Where does the frame switch get the numbers from ? Some pool of DLCI numbers ?

Provisioned. ie, setup by the provider.

you can also simulate a frame-realy switch with a cisco router and two serial ports. It really is a very simple protocol. Layer2 only has destination DLCI. That's why its called a "frame switched network", it switches frames at layer 2 using the virtual circuit indetifiers (DLCI)

gotcha, thanks once again Senior Spidey !

 

[spidey07]

heh, ask away. Frame-Relay is one of my specialties.

 

[polm]

Originally posted by: spidey07
heh, ask away. Frame-Relay is one of my specialties.

Is InverseArp the same mechanism used by the FrameSwitch to provide the provisioned DLCI back to the router ?

 

[spidey07]

inverse-arp simply maps IP addresses to DLCIs and is a function of the router.

think of a DLCI as a MAC address (which is really what it is - a layer 2 address)

Here's a simple frame-realy circuit, with one PVC.

routerA (DLCI 500)---frame switch---cloud---frame-switch---routerB (DLCI 600)

routerA will receive LMI from the frame switch and withing the full status LMI frames it will say "hey, I've got a PVC with number 600 and his CIR is 384000 bits/sec"

routerA then goes "gotcha, I'll map my circuit to that DLCI 600 because it is configured on my subinterface"

int s0/0
encap frame
int s0/0.600 point-to-point
interface dlci 600
ip address

more info...
http://www.cisco.com/en/US/tec...09186a008009457a.shtml

 

[polm]

So I must manually configure the DLCI always with the "interface dlci <number>" command ?

the book left me with the impression that somehow my router would determine the local DLCI number automatically. I just couldn't figure out how the DLCI would get autoconfigured.

 

[spidey07]

well you only program the remote DLCIs. What your saying is "use this virtual circuit to get to this IP address", the routing protocol or static routes take care of it from there.

Your LOCAL DLCI is only significant to circuits that want to talk to you. It identifies you.

 

[polm]

Originally posted by: spidey07
well you only program the remote DLCIs. What your saying is "use this virtual circuit to get to this IP address", the routing protocol or static routes take care of it from there.

Your LOCAL DLCI is only significant to circuits that want to talk to you. It identifies you.

ahh..so the local DLCI is defined at the Frame Switch.

any remote IP --> DLCI mappings would be done either via InvARP or Map statements ath the Router.