What is Frame Relay and how is work

Evaluation of Layer 2 Technologies:

1. X.25
2. Frame Relay
3. ATM
4. MPLS (New)

The Foundation concepts of all technologies are same. The all about are connected through shared bandwidth cloud. The connections are defined by PVC (Privet Virtual Circuit). Each one of them has bandwidth limitation, know as CIR (Committed Information Rate).

Frame Relay: It’s a Standardized WAN technology. Is a second generation technology in packet switching world. That Specifies the Physical and logical link layer.  In the scenario, frame relay service provider sells you cheaper connection (Shared bandwidth), cause they don’t have dedicated bandwidth to you. Example - Frame relay provider has total bandwidth 2Gbps bandwidth and they sell like 3.5Gbps to their customer to meet their customer requirement. Cause they monitoring, testing their network to determine that not everybody going to use all their bandwidth at the same time.  (this example like as bank)

Frame Relay Terminology: Frame Relay is not like Ethernet or even leased line, so how this connection work to using some terminology.

• CIR (Committed Information Rate) – The minimum bandwidth the service provider provides you, like 500Kbps. Some time always your network use (Bosted) bandwidth above CIR, the service provider always monitor their network, so they sagest you to get paid and also upgrade your CIR.

• LAR (Local Access Rate) – LAR is how fast the circuit can go. This is one of the big different with Ethernet. Example – in LAN environment you plug-in cable, that cable speed like 100Mbps, if the computer can send 100Mbps data than the cable can handle it.Now in Frame Relay, the link speed like 10Mbps, and your CIR is 512Kbps. So you should be configure the router with 512Kbps as agreement, So here is the different Logical and Physical speed mismatch.

• LMI (Local Management Interface) – LMI is a language you speak between your router to Frame Relay switch. It is a signaling protocol. The service provider send you statistic to use the LMI, like link status, quality of transmission, about dropping packet and also send about DLCI information

• DLCI (Data Link Connection Identifier) – on frame relay network DLCI take place instead of MAC Address. DLCIs are locally significant (Means you can duplicate your DLCI number, same DLCI placed in both site, and also in different link, but you can’t add same DLCI in single link where the link act as multiple access). Example - In frame relay network you send data to DLCI (200) as your destination, the service provider take the data through the cloud and comes on DLCI (300) other site. 

• PVC (Privet Virtual Circuit) – Logical connection between one point to another point in frame relay network. Every single one of PVC has specific CIR. So you pay for each PVC.

Type of Frame Relay Network: 

• Hub-And-Spoke:  Much cheap/inexpensive, one act as hub and all other may spoke, if Hub goes down than all spoke may down. Spoke communicate with other spoke by using hub.   

• Full Mesh: Much expensive $$. Fully redundant topology. All routers were connected each other. There low possibility of goes down. Exe – Voip, MPLS
• Partial Mesh: Some part using hub-and-spoke and other may redundant.

Interface Configuration in Frame Relay:

About “Split Horizon” - Is a loop prevention technique, “Never sent an update back in the same direction that receives from the same interface.”

• Multipoint – Same subnet use for all routers. Multiple DLCI is mapped to single interface. Split Horizon problem (Disable on Physical Interface, Enabled on Sub-Interface)
• Point to Point – Different subnets is needed to connect to each connection. Point to Point sub-interface created for each peer. No problem with split horizon.

How to Configuration FrameRelay:

Multipoint Frame-Relay configuration Command: (How to Config Multipoint Example)

Router(config)#interface s1/0

Router (config-if)#encapsulation frame-relay >> turn on frame relay encapsulation

Router (config-if)#ip address 192.168.100.1 255.255.255.0  >> specify ip  address

Router (config-if)#frame-relay map ip 192.168.100.2 102 broadcast >> for multipoint interface.

Or may create Sub interface to configure Frame-Relay:

Router (config)#interface s1/0.1 multipoint

Router (config-subif)#encapsulation frame-relay >> turn on frame relay encapsulation

Router HQ(config-subif)#ip address 192.168.100.1 255.255.255.0  >> specify ip  address

Router (config-subif)#frame-relay map ip 192.168.100.2 102 broadcast >> Frame Relay map for multipoint interface.

What the “broadcast” does?

Router(config-subif)#frame-relay map ip 192.168.100.2 201 broadcast 

The broadcast is initiate broadcast to another site router through frame relay. RIP, OSPF, EIGRP either use broadcast or multicast messages, by default the frame relay routers deny those broadcast messages from being sent through the link. So if you do not put the broadcast keyword with the command, you routing protocol wouldn’t work through frame relay network.

Point –to-Point Frame-Relay configuration Command: (Point-to-Point Configure Example) the point to point frame relay mode is only configured on sub interfaces.  

Step 1: Turn on Frame-relay on Physical Interface

Router(config)#interface serial 1/0

Router (config-if)#encapsulation frame-relay

Router (config-if)#exit

Router (config-if)#no shutdown  >> put this command after create all sub interface.

Step 2: Create point to point Sub Interfaces under the Physical Interfaces to support point to point frame relay.

Router (config)#interface serial 1/0.102 point-to-point

Router (config-subif)#ip address 192.168.1.1 255.255.255.252

Router (config-subif)#frame-relay interface-dlci 102

Router (config-fr-dlci)#exit

#frame-relay interface-dlci 102 >> what the command does?

If any time you (Traffic) use this sub interface, make sure that you use this (102 (Local DLCI)) DLCI to go out. No map necessary, no broadcast keyword as like multipoint configuration.

Frame Relay Troubleshoot command:

Router#show frame-relay lmi  >> check (Num status megs send rcvd number)

Router#sho frame-relay pvc  >> Check out PVC status.

Router#show frame-relay map

You check detail about frame relay status. Like your remote IP address, your local DLCI, “Static” (Configured by administrator as static), “Broadcast” may active, “Cisco” (Other site must be cisco device), and status active/inactive/deleted.

• Active – both site correctly setup and operational
• Inactive – your site is configured but other site not
• Deleted – you may put an DLCI “345” but the frame relay switch don’t know about this DLCI or does not exist , so this status will be shown as Deleted.

Some Note and Tips:

Router(config-if)##frame-relay lmi-type cisco/ansi/q9eea >> Frame Relay encapsulation for older router. Now on new router able to auto detect LMI using #encapsulation frame-relay

Router(config-subif)#frame-relay map ip 192.168.100.3 301 ietf  broadcast >> if you communicate with non cisco router on other site (Industry Standard). By default is cisco proprietary.