frame relay "map" vs "interface-dlci" and "Inverse ARP"

I remember when I was first getting into Frame Relay I had much trouble getting the concept of DLCIs, LMIs and how it all works, because it´s so much different then other L2-L3 protocols. It took me a while to realize that the concept is quite simple, you just need someone to tell you from which side you should "attack" this technology.

Three main concepts will be explained within this post:
 - Frame Relay configuration on a Physical Interface + Inverse ARP
 - Frame Relay configuration on a Point-to-Point Sub-interface
 - Frame Relay configuration on a Multi-Point Sub-interface


DTE-DCE

There are two possible interface plugs in the Frame Relay cabling: DCE and DTE. I´m not going to go into details here, but what you do need to know as a Network Engineer are two things:
- If the interface is DCE you do need to provide the clocking on that side. Check the type using:
#show controller Serial0/0 | inc Clocks

and if you see "DTE V.35 Tx and Rx clocks detected" you are safe


If you want to FORCE the DCE and provide the clocking:
(config-if) #frame-relay intf-type dce

Inverse ARP

Like a regular ARP (Address Resolution Protocol) the Inverse-ARP is used to map the Layer 2 (DLCI in the case of Frame Relay) and the Layer 3 (IP Address).

When frame-relay is configured in a point-to-point manner there is no need to configure the mapping and the inverseARP is disabled by default.

When you configure the L2-L3 mapping manually, like in the Physical Interface configuration case (Case 1 of this post), you already have the mapping that the ARP provides, so you need to manually disable the inverse ARP.

1. Frame Relay configuration on a Physical Interface

When the Frame Relay configuration is done on a Physical interface, you need to do the following:
- Assign the IP to the interface
(config-if)#ip address 4.2.2.1 255.255.255.0

- Define the encapsulation using the Interface command:
(config-if)#encapsulation frame-relay

 - Map the IP Address you want to access to the DLCI you have defined for that particular IP:
(config-if)#frame-relay map ip 4.2.2.2 201 broadcast

* Have in mind that DLCIs are only LOCALLY significant to the Router (DLCI value 201 will mean one thing to one router, and a totally different thing to another)
* "broadcast" is added because the local router will not be sending Broadcasts. The optional broadcast keyword in the frame-relay map command is required if broadcast and multicast traffic are to be sent over the specified dlci. Without the broadcast option, dynamic routing protocols such as EIGRP, OSPF and RIPv2 would not be able to advertise multicast route updates over the specified dlci.

Check the mappings using the following Privileged Mode command:
#show frame-relay map

Disable the inverse ARP (for reasons explained above)
(config-if)#no frame-relay inverse-arp

2. Frame Relay configuration on a Point-to-Point Sub-interface

For both of the following cases Frame-Relay encapsulation needs to be defined first on a Physical Interface bases, and the interface needs to be Administratively "UP". Create the Point to Point sub-interface and assign the IP address:
(config)#interface Serial0/0.12 point-to-point
(config-if)#ip address 4.2.2.1 255.255.255.0

Have in mind that there is no need to create the mapping L2-L3 because there is only one router on the other side of the PVC, so only one DLCI needs to be defined on the interface. For the same reason (L2-L3 mapping already defined) the inverse-ARP is disabled on this kind of sub-interfaces by default.

(config-if)#frame-relay interface-dlci 201

3. Frame Relay configuration on a Multi-Point Sub-interface

Create the Multipoint sub-interface and assign the IP address:
(config)#interface Serial0/0.123 multipoint
(config-if)#ip address 4.2.2.1 255.255.255.0

As there can be various routers on the other side of the PVC, the IP-DLCI mapping needs to be defined:
(config-if)#frame-relay map ip 4.2.2.2 102
(config-if)#frame-relay map ip 4.2.2.3 103

Unlike on the per-physical-interface basis, the inverse ARP doesn't need to be disabled manually, as it is automatically disabled once the sub-interface is configured.

Also have in mind that the SPOKEs will be able to ping each other without additional configuration (remember that additional configuration IS required in the first to cases)