200-301 questions answers

200-301 questions answers.The presence of overlapping routes in a router's routing table does not necessarily mean that a problem exists. Both automatic and manual automatic aggregation of routes can result in overlapping routes on a router that do not cause any problems. However, some overlaps (especially those related to addressing errors) can cause problems with user traffic. Therefore, when troubleshooting overlapping routes, if they exist, the engineer should also look for the actual cause of the problem caused by the overlapping routes.IP addressing plans, or simple errors in implementing this plan, can cause overlaps that can also lead to problems. In these cases, one router claims to be connected to a subnet in a certain address range, while other routers claim to be connected to other subnets, but there is overlap between the two, which breaks the rules of IP addressing. Its symptom is that the router sometimes sends packets to the correct host, and sometimes it does not.This problem can occur whether or not VLSM is used. However, when VLSM is used, the problem is much harder to detect. In this section we will review VLSM, give some examples of problems with and without VLSM, and discuss the configuration and verification commands associated with these problems. Identifying when VLSM is usedAn interconnected network is usually considered to be using VLSMO when multiple subnet masks are used for different subnets of a single classful network For example, if an interconnected network has all subnets from network 10.0.0.0, and uses masks /24, /26, and /30, then the interconnected network is using VLSM.Sometimes people fall into the misconception that any interconnected network using multiple masks must be using VLSM, but this is not always the case. For example, an interconnected network using a subnet of network 10.0.0.0 and a subnet of network 172.16.0.0, which use mask 255.255.240.0 and mask 255.255.255.0, respectively, is not designed to use VLSM. only one mask is used in any of the classful networks, despite the fact that two different masks are used. VLSM is used only if multiple masks are used in a subnet of a single classful network.Only classless routing protocols can support VLSM. the current CCENT and CCNA routing and switching certifications cover only classless routing118 Chapter 5, Part 2 Troubleshooting IPv4 Routingrouting protocols (OSPF and EIGRP), all of the routing protocols discussed in this book, should support VLSM. but note that in real life, RIPv2 (as a classless routing protocol) can also support VLSM, while the classful routing protocols RIPvl and InteriorGateway Routing Protocol (IGRP) cannot. Overlap when VLSM is not usedAddressing errors that cause overlapping subnets can occur even when VLSM is not used. For example, Figure 5-12 shows an example network, along with information about the router LAN IP address/mask. There is overlap in this example, but it may not be obvious at first glance.If overlap exists when all subnets use the same mask, then the overlapping subnets have the exact same sub IDs, and the exact same range of IP addresses in the subnets. To find the overlap, all you have to do is calculate the subnet IDs for each subnet, and then compare those numbers. For example, Figure 5-13 shows an updated version of Figure 5-12 with the subnet IDs, and the same subnet IDs for the LANs outside R3 and R4.