cisco ccna in 60 days pdf

cisco ccna in 60 days pdf.Two burst rates allow you to occasionally exceed the CIR, provided that the carrier's network has sufficient bandwidth to support the burst traffic. Be allows the VC to exceed the CIR for the average burst traffic, but the time to calculate the average burst traffic is shorter than the time to calculate the CIR. If you send a message faster than the CIR, but lower than the Be, the carrier will allow your frame to enter its network.BE refers to the maximum rate at which you can send traffic to the carrier's network via VC, and frames above that rate will be discarded. If you send traffic at a rate between Be and BE, the carrier switch will mark the frame as discardable-setting the discardable (DE) field in the 1-bit-long Frame Relay frame header. By marking this field, the operator conveys the idea that these frames can enter the network, but will be dropped first whenever there is congestion on the operator's network. In the operator's view, frames sent between Bc and BE at a rate violate the rules, but they will be transmitted as long as sufficient bandwidth is available.It is important to note that each VC has CIR, Be, and BE, but may not use Be and BE, depending on the carrier's Frame Relay implementation and how you purchased the VC. In some cases, the BE is by default equal to the access rate-the speed of the physical connection from the Frame Relay DTE to the Frame Relay DCE. The physical connection may be a partial T1 connection (e.g. 256kbit/s), or a full T1 connection (1.544Mbit/s) oRegardless of the number of VCs, and regardless of the CIR values of these VCs, you are always constrained by the access rate: it is not possible to exceed the speed of the physical connection. Oversubscription beyond the speed of the physical connection is common; this means that the sum of the CIRs of all VCs exceeds the access rate, and it is based on the assumption that there will not be a situation where all VCs are running at their CIRs and therefore require less total bandwidth than the speed of the connection to the carrier. The Frame Relay cost has two components: the cost of the physical connection to the Frame Relay switch and the VC cost, the latter of which usually depends on its rate parameter.Chapter 25 Frame Relay 689Figure 25.7 illustrates the effect of these Frame Relay traffic parameters on the VC data rate with an example. In this figure, the frames leaving the router interface via the VC increase linearly. From this figure, it is clear that the Frame Relay switch will allow frames to enter Frame Relay as long as the VC data rate is below CIR/Bc. However, for those frames after Be (frames 4 and 5), their DE bits are set, allowing the operator to drop them when its network is sending congestion. In addition, frames after BE (here Frame 6 and Frame 7) are discarded.Usually, the operator will set the DE bit of the frame after the Be is exceeded.Some carriers do not support Be and BE, and mark all frames after the CIR as discardable. This means that you can send frames to the carrier's network at the access rate and the carrier will allow them in (but mark the DE bit). The options and implementations are so numerous that it can be confusing when you are looking for the right Frame Relay solution for your network. For example, some carriers may sell VCs with a CIR of 0 kbit/s, meaning that the carrier allows all the traffic you send into the network, but marks it all as discardable. As long as there is no congestion on the carrier's network, you will get very good service. Of course, if the carrier's network is frequently congested, you will get a terrible service because some or most of the frames you send will be discarded.Figure 25.7 VC traffic parametersIf a VC with a guaranteed rate is required, it is available from most carriers, but at a higher cost than a VC with a CIR of 0 kbit/s. The greater the bandwidth requested, the more expensive the circuit is to purchase, as the operator must reserve this bandwidth in its network to meet your traffic delivery rate requirements. The situation would be further complicated by the need to both meet the traffic demands of all connections and to make the most of every penny spent. Considering that not all VCs send traffic at their CIR at the same time, some network administrators oversubscribe access rates. Given how Frame Relay works and the changing traffic delivery requirements, it is difficult to select the right Frame Relay service for the network.