How to configure Frame-Relay Point-to-Point

Lessons Discussion
42

Hello NetworkLessoms Team.
Which statement below about Frame-Relay Point-to-Point connections is true?

  1. they use two DLCIs to communicate with multiple endpoints over Frame Relay cloud;
  2. they can operate normally without DLCI map.

It seems both answers are true. But I think the best answer is 2. What does it mean “multiple endpoints” in 1st answer? Help please.

Thanks.

43

Hello Boris

You’re right, answer 1. is not very clear. However, if you have multiple endpoints in a point to point configuration, you will require a pair of DLCIs for each point to point link. Let’s say you have one hub and four spokes. You will need four pairs of DLCIs to communicate with all four spokes, so 1. seems incorrect.

The second statement is definitely true as you don’t need to configure DLCI mappings in order for point to point frame-relay to function.

I hope this has been helpful!

Laz

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44

Thank you very much.

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45

I’m still confused about Frame-Relay point-to-point and point-to-multipoint.
The article stated that the difference for Point-to-point is the IP subnet per PVC.
From my understanding, point-to-multipoint is for one same subnet. On the other hand, point-to-point is for different subnets. Is that correct?
I’ve compared two configurations and noticed two different things.

  1. In Point-to-multipoint, it’s using frame-relay map ip IP DLCI broadcast whereas in point-to-point, it’s using sub-interface. Is this just for personal preference as I thought we can config sub-interface in point-to-multipoint too.
  2. In Point-to-Point, there is a setting for RIP. Is that just for testing?

Thank you in advance.

46

Hello Po

Yes, this is correct. You’ll notice that point to point has two PVCs, and for each PVC, you have a different subnet. So for Hub to Spoke 1, you’re using 192.168.12.0/24 subnet and for Spoke 2 you’re using 192.168.13.0/24 subnet. Each one of those subnets corresponds to a different subinterface on the S0/0 interface of the Hub.

For point to multipoint on the other hand, all three devices are in the 192.168.123.0/24 subnet as if they are on the same broadcast domain or network segment.

Now in order to make this work, because Frame-Relay doesn’t intrinsically support all related features, additional configurations must be made, as you correctly pointed out. For example:

  1. Point to multipoint must have inverse ARP enabled by default. This allows Frame-relay to dynamically learn the DLCI of the spoke that belongs to the IP address we’re looking for. Point to point doesn’t need this to function. Alternatively, Frame-relay mappings of IP addresses to DLCIs must be manually configured.
  2. Similarly, the broadcast must be included in order to tell Frame-Relay to simulate broadcasts and multicast so that all devices in the subnet will receive them. When inverse ARP is enabled, this too is enabled by default. If inverse ARP is removed, then the broadcast keyword must be used when creating the frame-relay mappings.
  3. For point to multipoint, subinterfaces are used on the hub simply because only a single serial connection comes into the premises. Subinterfaces eliminate the need for multiple physical channels. Both point to point and point to multipoint can easily use subinterfaces, or physical interfaces to function, as long as there are enough physical circuits to accommodate the requirements in each case.

In the lesson this is simply demonstrating how you can enable a routing protocol over the topology so that you can route traffic between the subnets that each of the routers serves. One of these is represented by the loopback address on Spoke 1. This RIP configuration could be implemented on the point to multipoint topology as well.

I hope this has been helpful!

Laz

47

Hi guys,

With regards to your point-to-point example. For Spoke 1 or Spoke 2, does the interface require it be a sub-interface?

I can understand why the Hub requires this as it has mappings from two different PVCs on one interface. But Spoke and 1 and Spoke 2 only have one PVC.

48

Hello Joseph

The interface configuration on the spokes doesn’t have to be a subinterface, it can be configured directly on the physical interface itself.

I hope this has been helpful!

Laz

49

Just to quote:

“We don’t use the frame-relay map command for point-to-point sub-interfaces but you have to use the frame-relay interface-dlci command here.”

Is Inverse ARP disabled in this instance? If not and we were to disable it would it require the frame-relay map command then?

50

Hello Joseph

When configuring Frame-Relay in a point-to-point arrangement, that is, when you implement the interface serial 0/0.1 point-to-point command, the router will expect the frame-relay interface-dlci command within the subinterface. It will not use a mapping or Inverse ARP. Cisco describes it like so in this Cisco documentation:

A Frame Relay point-to-point subinterface is connected to an end-to-end virtual circuit provisioned between two locations. Therefore, a point-to-point subinterface can only accept one DLCI and the next hop protocol address is automatically linked to the sole outgoing local DLCI. Hence, the frame-relay map command is applicable only to multipoint subinterfaces and physical interfaces (physical interfaces are multipoint interfaces by default).

So for a point to point link, Inverse ARP, nor Frame-Relay mapping are not applicable. Inverse Arp and frame-relay mapping is only applicable for multipoint configurations.

Is inverse ARP actually disabled? Not unless you explicitly disable it using the no frame-relay inverse-arp command on the interface. But even so, it is simply not used in a point to point situation.

I hope this has been helpful!

Laz

51

Hi Laz,

When ever I see reference to Point-to-Point it’s always in the context of being in a sub-interface.

Is Frame Relay Point-to-Point only achievable from the sub-interface?

52

Hello Joseph

It is possible to employ a point-to-point frame-relay configuration on a physical interface instead of the subinterface. This could be done simply by using the command:

Router(config)#interface serial 0/0/0 point-to-point 
Router(config-if)#

The disadvantage here is that if you do this, you can only connect a single spoke router to this physical interface. When you have a multi-site topology, it usually has a hub router at the headquarters, and several spoke routers that connect via point-to-point subinterfaces. This is why you so commonly see this topology implemented on subinterfaces at the hub.

I hope this has been helpful!

Laz

53

100%, Thanks Laz, much appreciated

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54

Is the reason we don’t create mapping while doing frame-relay point to point -Inverse ARP ?

55

Hello Saicharan

Yes indeed, it is because of inverse ARP that you don’t require mapping. Inverse ARP is enabled by default. As seen in the Frame-Relay point to multipoint lesson below, you can see an example of where inverse ARP is disabled, and mapping can be used instead.

I hope this has been helpful!

Laz