Frame Relay(Solving Rechability Issues)


Split Horizon:
By default, a Frame Relay network provides NBMA connectivity between remote sites. NBMA clouds usually use a hub-and-spoke topology. Unfortunately, a basic routing operation based on the split horizon principle can cause reachability issues on a Frame Relay NBMA network.
Recall that split horizon is a technique used to prevent a routing loop in networks using distance vector routing protocols. Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface.
Frame Relay Subinterfaces
Frame Relay can partition a physical interface into multiple virtual interfaces called subinterfaces. A subinterface is simply a logical interface that is directly associated with a physical interface. Therefore, a Frame Relay subinterface can be configured for each of the PVCs coming into a physical serial interface.
To enable the forwarding of broadcast routing updates in a Frame Relay network, you can configure the router with logically assigned subinterfaces. A partially meshed network can be divided into a number of smaller, fully meshed, point-to-point networks. Each point-to-point subnetwork can be assigned a unique network address, which allows packets received on a physical interface to be sent out the same physical interface because the packets are forwarded on VCs in different subinterfaces.
Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode:
Point-to-point - A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router. In this case, each pair of the point-to-point routers is on its own subnet, and each point-to-point subinterface has a single DLCI. In a point-to-point environment, each subinterface is acting like a point-to-point interface. Typically, there is a separate subnet for each point-to-point VC. Therefore, routing update traffic is not subject to the split horizon rule.
Multipoint - A single multipoint subinterface establishes multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All the participating interfaces are in the same subnet. The subinterface acts like an NBMA Frame Relay interface, so routing update traffic is subject to the split horizon rule. Typically, all multipoint VCs belong to the same subnet.
In split horizon routing environments, routing updates received on one subinterface can be sent out another subinterface. In a subinterface configuration, each VC can be configured as a point-to-point connection. This allows each subinterface to act similarly to a leased line. Using a Frame Relay point-to-point subinterface, each pair of the point-to-point routers is on its own subnet.
The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network layer address and DLCIs, are assigned to the subinterface.
You can use multipoint configurations to conserve addresses. This can be especially helpful if Variable Length Subnet Masking (VLSM) is not being used. However, multipoint configurations may not work properly given the broadcast traffic and split horizon considerations. The point-to-point subinterface option was created to avoid these issues.
Paying For Frame Relay:
Key Terminology
Service providers build Frame Relay networks using very large and very powerful switches, but as a customer, your devices only see the switch interface of the service provider. Customers are usually not exposed to the inner workings of the network, which may be built on very high-speed technologies, such as T1, T3, SONET, or ATM.
From a customer's point of view then, Frame Relay is an interface and one or more PVCs. Customers simply buy Frame Relay services from a service provider. However, before considering how to pay for Frame Relay services, there are some key terms and concepts to learn, as illustrated in the figure:
Access rate or port speed - From a customer's point of view, the service provider provides a serial connection or access link to the Frame Relay network over a leased line. The speed of the line is the access speed or port speed. Access rate is the rate at which your access circuits join the Frame Relay network. These are typically at 56 kb/s, T1 (1.536 Mb/s), or Fractional T1 (a multiple of 56 kb/s or 64 kb/s). Port speeds are clocked on the Frame Relay switch. It is not possible to send data at higher than port speed.
Committed Information Rate (CIR) - Customers negotiate CIRs with service providers for each PVC. The CIR is the amount of data that the network receives from the access circuit. The service provider guarantees that the customer can send data at the CIR. All frames received at or below the CIR are accepted.
A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge. This allows customers to "burst" over their CIR as a bonus.
Access or port speed: The cost of the access line from the DTE to the DCE (customer to service provider). This line is charged based on the port speed that has been negotiated and installed.
PVC: This cost component is based on the PVCs. Once a PVC is established, the additional cost to increase CIR is typically small and can be done in small (4 kb/s) increments.
CIR: Customers normally choose a CIR lower than the port speed or access rate. This allows them to take advantage of bursts.
Oversubscription
Service providers sometimes sell more capacity than they have on the assumption that not everyone will demand their entitled capacity all of the time. This oversubscription is analogous to airlines selling more seats than they have in the expectation that some of the booked customers will not show up. Because of oversubscription, there will be instances when the sum of CIRs from multiple PVCs to a given location is higher than the port or access channel rate. This can cause traffic issues, such as congestion and dropped traffic.
Bursting
A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge.
Using the previous example, the figure shows an access rate on serial port S0/0/0 of router R1 to be 64 kb/s. This is higher than the combined CIRs of the two PVCs. Under normal circumstances, the two PVCs should not transmit more than 32 kb/s and 16 kb/s, respectively. As long as the amount of data the two PVCs are sending does not exceed its CIR, it should get through the network.
Because the physical circuits of the Frame Relay network are shared between subscribers, there will often be time where there is excess bandwidth available. Frame Relay can allow customers to dynamically access this extra bandwidth and "burst" over their CIR for free.
Bursting allows devices that temporarily need additional bandwidth to borrow it at no extra cost from other devices not using it. For example, if PVC 102 is transferring a large file, it could use any of the 16 kb/s not being used by PVC 103. A device can burst up to the access rate and still expect the data to get through. The duration of a burst transmission should be short, less than three or four seconds.
Various terms are used to describe burst rates including the Committed Burst Information Rate (CBIR) and Excess Burst Size (BE).
The CBIR is a negotiated rate above the CIR which the customer can use to transmit for short burst. It allows traffic to burst to higher speeds, as available network bandwidth permits. However, it cannot exceed the port speed of the link. A device can burst up to the CBIR and still expect the data to get through. The duration of a burst transmission should be short, less than three or four seconds. If long bursts persist, then a higher CIR should be purchased.
For example, DLCI 102 has a CIR of 32 kb/s with an additional CBIR of 16 kb/s for a total of up to 48 kb/s. Frames submitted at this level are marked as Discard Eligible (DE) in the frame header, indicating that they may be dropped if there is congestion or there is not enough capacity in the network. Frames within the negotiated CIR are not eligible for discard (DE = 0). Frames above the CIR have the DE bit set to 1, marking it as eligible to be discarded, should the network be congested.
The BE is the term used to describe the bandwidth available above the CBIR up to the access rate of the link. Unlike the CBIR, it is not negotiated. Frames may be transmitted at this level but will most likely be dropped.

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