Introduction to Frame-Relay for CCNA Students

Lessons Discussion
25

Rene,

Can u explained the steps involved after u completed and order for a VC.
What is the service provider obligation/responsibilities to get the VC up and running for u?
What equipment will they installed on ur premise? What cables will they run to ur premise?

26

Hi Raymond,

To be honest, I never worked with frame-relay on a production network here :slight_smile: Itā€™s pretty old and here in the Netherlands a lot of companies switched to DSL / fibre / cable around 2000.

The installation is similar to any other link that you would get today (Fibre perhaps). The ISP takes care of the cabling and normally supplies the customer with a router for each site. These routers are normally managed by the ISP and you are not allowed to change the configuration.

Routers nowadays have WIC modules with an integrated CSU/DSU, older WIC modules so you needed a separate modem that was connected to your router. About the cabling, frame-relay supports a lot of different cabling optionsā€¦copper and fibre.

Rene

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27

Hi,

Someone can explain me what is the relationship between the DE and the CIR in Frame Relay?

Dominik

28

Hi Dominik,

The CIR rate is the committed rate, in other words the rate that the ISP guarantees that you will get.

The DE bit indicates that the frame has a lower importance. When there is congestion, the frames with the DE-bit set can be dropped.

Normally, all frames that exceed the CIR rate will have their DE-bit set so thatā€™s the relationship between the two.

Rene

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29

hello

when HQ and sites are connected through physical serial interface. what is the use of virtual circuit(logical).
is it just to provide DLCI num

30

Hello Dheeraj!

As you mentioned correctly, the virtual circuit is a logical connection between two DTE (Data Terminal Equipment) devices within the framework of a Frame Relay implementation.

Regardless of how many physical serial interfaces you have connected to the frame relay network, you still require a virtual circuit for each logical connection. In other words, if you only have one physical serial interface, and you only have one virtual circuit connecting to a remote device via a frame relay connection, then you still require the virtual circuit. The virtual circuit is what defines the connection (via the DLCI as you mentioned) and the physical interface just provides the ā€œpipeā€ through which the DTE connects to the network.

I hope this has been helpful!

Laz

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31

19 posts were merged into an existing topic: Introduction to Frame-Relay for CCNA Students

32

Hi,
Is it possible to do the lab in unetlab
Thanks

33

Hi,
Can you explain the below

If you use point-to-point it will solve your split-horizon problem but youā€™ll need to use a different IP subnet per PVC. Point-to-multipoint means you have the split-horizon problem but you can use a single IP subnet for all PVCs.
Thanks

34

Hello Sims.

When designing frame-relay topologies, there are a couple of options.

First you can create a point-to-multipoint topology where each router on the frame-relay network has an IP address within the same subnet. Letā€™s say the subnet is 10.10.10.0/24. Take a look at the following diagram and assume that each S0/0 interface of each router has an IP address of 10.10.10.X/24:

Keep in mind that point-to-multipoint means that Spoke1 cannot communicate directly with Spoke2. All communication between them MUST go through the Hub router. The Hub is the ā€œpointā€ and the Spokes are the ā€œmultipoints.ā€

So, using a routing protocol like EIGRP or OSPF, Spoke1 sends a routing update to the Hub router. Because of the split horizon rule, Spoke 2 will NEVER receive this update.

(Split horizon rule states that no routing update will be sent out of the same interface that it was received on. This is used to prevent routing loops.)

So because the Hub router received the routing update on S0/0, it will not send it out from there.

To solve this split-horizon problem, you can create point-to-point links between the routers such that each router has a direct path to every other router using subinterfaces. Take a look at the following figure:

Here you can see that there are subinterfaces configured on each of the serial links and each subinterface connects to a single subinterface on another router. The point to multipoint links are replaced by three point to point links, each with its own subnet. So now, for example, any advertisements that are received via subinterface S0/0/1.102 on R1 can be readvertised via S0/0/1.103 on the same router without violating the split-horizon rule.

Note that even though the readvertisement is exiting the same PHYSICAL interface, it is in essence exiting a different LOGICAL (sub)interface, thus it is not violating the split-horizon rule.

I hope this has been helpful!

Laz

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35

Hi,

Is there any reason why you canā€™t use the same DLCI for both routers at either end of the VC?

Thanks,

Sam

36

Hello Samir

The DLCI is locally significant, which means it must be unique only within the scope of the specific device. So the hub must have a unique DLCI for each of its VCs to remote frame relay routers. This means you can indeed have the same DLCI number on both ends of a VC.

I hope this has been helpful!

Laz

37

Hi,

Thanks very much, I guess itā€™s just best practice not to do it that way.

Regards,

Sam

38

Hello Samir

Actually, some administrators prefer to use the same DLCI on both ends to indicate which circuit is connected to which. Others choose to make them unique on both ends. Itā€™s a matter of personal preference. It makes no difference to the performance of the topology, so it depends on what is more convenient for each individual administrator. I tend to agree with you, Iā€™d prefer to have the same DLCI on both ends, just to keep things organized in my mind.

Laz

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