Frame Relay

Frame Relay is a wide area networking protocol.

Frame Relay Basics

With PPP, you would need a separate physical wire for each connection. So let's say you had a router in the Los Angeles headquarters, and three branch offices in Chicago, New York, and Atlanta. If using PPP, the Los Angeles router would need three physical serial interfaces. Assuming the three branch offices all need to connect not just to LA but to each other as well, these would each need three serial interfaces as well:

PPPMULTI

As the business grows, the number of serial connections could grow exponentially. A better protocol is therefore needed for situations like this. Enter frame relay. Frame relay allows you to set up your router as if it has multiple connections to other routers, even though it only has one physical connection to your service provider. For example, physically, you could have this set up:

PHYSICAL FRAME RELAY

And then you could set up virtual connections, called Virtual Circuits or VCs, between each branch, making it act just like multiple independent connections:

VIRTUAL FRAME RELAY

In this case, the physical serial interface is broken into multiple virtual sub-interfaces. For example, s0/0 becomes s0/0.1 and s0/0.2, which then connect to the branch offices. Each sub-interfaces would be in its own sub-network, just as if they were separate physical interfaces.

DLCI

Each virtual circuit has an identifier called a Data Link Connection Identifier (DLCI). Think of this in the way you would think of a bus or train number. The DLCI address to get to another location is locally significant, unlike an IP address which is the address of the far computer. The far router in this case may not even be aware of the number of the DLCI used to get to it. For example, in the following illustration, the DLCI to reach the New York branch office is numbered 101, in effect saying, "Any packets that want to go to NY need to get on DLCI 101. But note that New York doesn't know or care what number the DLCI is. In fact, from it's end, the DLCI has a different number: 205. And it uses DLCI 205 to send packets to Los Angeles.

DLCI DEMO

So keep in mind that DLCIs are a bit different than addresses, as they are only important for the local router to know about.

LMI

Each virtual circuit also has a Link Management Interfae (LMI), which maintains the status of the virtual circuit to make sure it is still up. The LMI type needs to be the same on both ends, and there are three types of LMI: ansi, cisco, and q933a.

Encapsulation

The encapsulation type is obviously frame relay, but this can be of two types: cisco and ietf.

FECN, BECN, and DE

A frame relay service is provided by your service provider, and your router then connects to the service provider. The provider will provide a guarantee of a certain amount of bandwidth, but many times you can go over that amount. However, any data that is over the amount cannot be guaranteed and can be marked with a Discard Eligibility bit (DE), which tags it as "OK to be discarded if over bandwidth."

Now, let's consider a situation where the frame relay network is getting congested with traffic. You may want to be able to tell the routers to slow down their transmission rate if this occurs. The service provider's frame relay switch can notify your receiving router to expect a delay in traffic, and it can notify your sending router that there is congestion in order to give it the option to slow down. The frame relay switch can set the Forward Explicit Congestion Notifier, which notifies the receiving router of the congestion:

FECN

The frame relay switch can also set the Backward Explicit Congestion Notifier, which tells the receiving router to notify the sending router of the congestion when it sends any packets (for example, confirmation notifications) to the recieving router:

BECN

The FECN and BECN bits can then be used for shaping traffic: causing routers to slow down or speed up in response to congestion, in order to avoid too much data being tagged with a DE bit and discarded.

Inverse ARP

Remember Address Resolution Protocol? When a computer needs to know the MAC (physical) address of a local computer, it broadcasts an ARP message, in effect asking, "Hey, whoever 192.168.3.101 is, please give me your MAC address so I can send you some data!"

Inverse ARP is used in frame relay to get the IP address of a DLCI. The router will broadcast, "Hey, DLCI 101! Give me the IP address that you connect to on the other end!" In this way, it will automatically map DLCIs to IP addresses so that it knows which DLCI to put packets on.

Configuration

The simplest way to configure frame relay is to simply enter the interface mode, and then the command "encap frame relay."