Frame Relay

Frame Relay is a network technology that runs on layer 2. Frame Relay is a NBMA (Non-Broadcast Multiple Access) network. Frame Relay usually used as an option for WAN connection.

when connecting more than two sites, a topology must be chosen to create the connection between each sites. the Frame Relay topologies are star, full-mesh and partial-mesh.

Star (hub and spoke)
a central site acts as a hub and other remote sites act as spokes. each remote site has an access link to the central site.

Full Mesh
full mesh topology connects every site to every other.

DLCI is the identifier of each VC in frame relay. DLCI is 10-bits long and there for can hold up to 1024 (0 - 1023). some DLCI number are reserved and cannot be used. those are

1-15 : reserved for future use
992-1007 : CLLM
1008-1022 : reserved for future use (ANSI, ITU)
1019-1020 : multicasting (cisco)
1023 : LMI (cisco)

DLCI is local significance which means that it's only unique to the local. at other hop, the same DLCI number can be used to identify different VC.

Inverse ARP
ARP is used to find out the layer 2 address from a layer 3 address available. inverse ARP does the opposite.

used primarily in frame relay and ATM.

on cisco routers, inverse ARP is on by default.

Inverse ARP works as much like ARP. the frame relay router will sends an inverse ARP requests on its PVC to dscover the layer 3 adress of the remote device connected to the frame relay network.

to clear dynamically created frame relay maps that are created using inverse ARP, use the command
clear frame-relay inarp

Local Management Interface, a protocol developed by Cisco, digital equipment corporation (DEC), Northern Telecom and StrataCom. enable the DTE to communicate with the DCE and acquire information about the status of the network.

LMI is basically a keepalive mechanism. every 10 seconds or so, the end device polls the network, either requesting a dumb sequenced response or channel status information. if the network does not respong width the requested information, the user device may consider the connection to be down.

there are 3 LMI types:
- cisco
- Ansi
- Q933a (ITU standard)
starting from 11.2 IOS version, the LMI is autosensed. if it happen to manually configure the lmi type, do it with the command
frame-relay lmi-type [cisco | ansi | q933a]

if you configure the lmi type manually, you need to set the keepalive interval too. if the keepalive interval is mismatch too big. the switch can declare that the router is dead. configure the keepalive interval with the command "keepalive second". default keepalive is 10 seconds on cisco serial interface.

Address Mapping
dynamic mapping, dynamic mapping in frame relay is achieved from the combination of LMI and inverse ARP. LMI will acquire the active DLCI for each data link, so the router knows what DLCI that's exist. then the router will send inverse ARP request for each DLCI to acquire the corresponding network layer address.

static mapping, you can enter a static mapping for a DLCI on cisco routers. enter the command (at interface level configuration)

frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco]

protocol-address : layer 3 address
dlci : DLCI number associated for the layer 3 address
ietf : use this keyword instead of cisco when connecting a non-cisco device

you cannot use both dynamic mapping and static mapping for the same dlci and protocol. you can disable the inverse ARP protocol for an interface by entering the command

no frame-relay inverse-arp

used in frame relay to enable forwarding routing updates through a physical interface which has multiple connections (split horizon rule).

point-to-point subinterface
establish one PVC connection to another physical interface / subinterface on a remote router. acts as a point-to-point connection. has a single DLCI, each point-to-point subinterface is on its own subnet. typically, there is a seperate subnet for each point-to-point subinterface. solve the split horizon issue.

multipoint subinterface
establich multiple PVC. acts as a NBMA network. doesn't solve the split horizon issue. all the participating interface are in the same subnet.

when you use the subinterface, usuall you must assign the subinterface with a DLCI number to differentiate it from the physical interface (LMI does not know about subinterfaces).

do not assign network address to the physical interface. if the physical interface has an address, frames are not received by the local subinterfaces. the required command for the physical interface is to specify the encapsulation to be used (ALL SUBINTERFACES WILL BE USING THE SAME ENCAPSULATION AS SPECIFIED AT THE PHYSICAL INTERFACE ENCAPSULATION).

Flow Control
frame relay switched use the FECN and BECN bit to control the flow of frames going through the network

SW1 ----------------------------------------- SW2

assume that congestion occure at the link between SW1 and SW2.

FECN bit is set for every frame going from SW1 to SW2.
BECN bit is set for every frame coming to SW1 from SW2 and is leaving on another interface.

DTE can set the DE (Discard Eligible) flag to 1. usually less important frames are marked with the DE 1. this is used to avoid more critical data to be discarded should congestion happened.

Configuring Frame Relay

needed configuration for frame-relay to work
- encapsulation frame-relay
- assign ip address to the interface

Cisco routers autosense the LMI type used. recall that there are three LMI types: Cisco, ANSI Annex D and Q933-A Annex A. the default LMI type for cisco routers is cisco.

you can configure subinterfaces with the command
interface serial number.sub-number multipoint | point-to-point

number = the physical port number
sub-number = subinterface number. to make troubleshooting easier, use the DLCI as the subinterface number.
multipoint | point-to-point = specify the type of the subinterface

to assign a subinterface its DLCI use the following command
frame-relay interface-dlci dlci-number

dlci-number = the number of the DLCI you wished to be associated with the subinterface.