I think it is because is an easy layer 2 technology who uses packet commutation, uses PVC, and it is a NBMA network. It is evident that it is an end of life technology, but at least is a easy to emulate technology.
Basic frame relay configuration:
R1(config)# int s1/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#ip add 10.0.0.1 255.255.255.0
R1(config-if)#no shut
R2(config)# int s1/0
R2(config-if)#encapsulation frame-relay
R2(config-if)#ip add 10.0.0.2 255.255.255.0
R2(config-if)#no shut
Inverse ARP is enabled by default, and you are able to ping one router to the other.
Inverse ARP
It is a layer 3 to layer 2 conversion mechanism. Inverse ARP communicates the router with the FR switch and gives to you the DLCI numer automatically and allowing your interface to send frames using the right DLCI number.
Life without Inverse ARP
You must use static frame relay maps.
The broadcast command allows the interface send broadcast, and multicast traffic using this DLCI, an important detail for CCIE lab.R1(config-if)#frame-relay map ip 10.0.0.2 102 broadcast
Test Broadcast traffic over frame relay:
Configure a frame relay map with broadcast in both sides and make a ping to 255.255.255.255, the other side must reply.
Configure a frame relay multipoint subinterface:
R1(config)#int s1/0.X multi <--- Select the number that you want
R1(config-if)#frame-relay interface-dlci X <----the number of the DLCI, like vlans in router on a stick, but only if you use inverse arp, not required for static mapings.
Point to point subinterfaces:
R1(config)#int s1/0.X point <--- Select the number that you want
R1(config-if)#frame-relay interface-dlci X <----the number of the DLCI, like vlans in router on a stick
Useful commands:
show frame-relay map
clear frame-relay inarp
show frame-relay pvc
show frame-relay pvc X <-shows you where is configurated a pvc(the subinterface)
no frame-relay inverse-arp
no frame-relay inverse-arp ip x <----- the number of the DLCI
