Frame Relay Basic Concepts Overview

FRAME RELAY CONCEPTS

• Frame relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points. This speeds up overall data transmission. Alot of the Wan technologies like T1 for example are stuck to fixed frame size + di extensive error checking.So this use of variable size in the wan made frame relay attractive
• 2 types of frame-relay encapsulation cisco or ietf can be enabled on per vc basis
• Lmi is the signaling protocol between the dte router and dce which is the fr switch
• lmi types supported are cisco (dlci 1023) ansi annex d (dlci 0) and q933a (dlci0)
• from the output of show frame-relay pvc status  if you see active-connection is good between router + fr switch inactive- dlci number is incorrect + problem with connection deleted-dcli number is incorrect it is not what the sp or fr switch is advertising
• When encap frame relay is issued the router will learn all dlci associated with that interface. Then when ip is configured inverse arp request will be sent on these dlci other configured sides will replay
• Inverse arp is a dynamic way of mapping layer 3 address to layer 2 dlci
• if you statically configure a frame-relay mapping it will disable inverse arp from sending but not replying
• if you statically configuring do not forget 'broadcast' to enable pseudo broadcast accross the line
• if you disable frame-relay inverse arp on the main interface it does not disable it on the sub interfaces
• inverse arp is disabled by default on frame-relay multipoint interfaces
• static frame-realy mapping override dynamic inverse arp mappings
• by default split horizon is off on multipoint main interface but not on sub interfaces 
• frame relay can have issue with autoinstall if you see a mapping for 0.0.0.0  in output for sh frame-relay map command save the config + restart.
• If we are in a partial mesh scenerio using the multipoint interface or multipoint sub interface we need to manual map ip for each spoke address we will learn the hub ip address but not the spokes ip address as we have no direct layer 2 connectivity
• P2P does not need manually mapping as presumed all traffic goes out configured dlci as it is the only one

NBMA Interface Types Requirement for Layer 2 to Layer 3 resolution

Multipoint Interfaces

• These type interface can be assigned + terminate multiple layer 2 ciruits. Effictevly the interface has multiple layer 2 addresses.
• These interfaces require layer 2 to layer 3 resolution.
• An example in frame relay would be having multiple dlci assigned to frame relay main interface

frame-relay map "protocol"  "ip address" dlci 'dlcino' - for static mappings

Point-to-Point

• Can terminate only 1 layer 2 circuit. Only one possible layer 2 address. So does not require resolution of layer 2 to layer 3

frame-relay interface dlci "no" - no ip required as only one way out as p2p

Frame relay has a quite a simple packet structure

The dlci is the layer 2 virtual cirtuit that it will use. The FECN and BECN + DE bits are all to do QOS parameters.

Commands

on interface
encap frame-relay 'cisco' 'ietf' default is cisco

frame-relay lmi-type 'cisco|ansi|q933a) the lmi protocol type is auto detected if for some reason it has changed it can be changed back cisco is default

show frame-relay lmi- will give you the lmi type detected + stats on frame errors etc
The frame-relay dlci will only be in a up up state if it is recieving lmi keepalives from the frame-relay switch so if you see up line down it typically either the lmi type is misconfigured or one side is not running encap frame-relay


sh frame-relay pvc  lmi will advertise the vc on the line + there status

status can be
active- the connection is good between the router + fr switch
inactive- dlci number is incorrect+ problem with communication
deleted-  dlci number is incorrect it is not what the service provider advertised
static-  lmi has been disabled

sh frame-relay map - see dlci to ip resolutions

sh frame-relay lmi- see what version of lmi been used and stats on errors etc

debug frame-relay packet - view all packets coming

debug frame-relay events - can be used to view inverse arp packets

IF you see in the debugs encap failed it means the router does not know how to build the l2 frame most likely there is not map statement