200-301 prerequisite

200-301 prerequisite.. Manual route summarization.. Static routing.Incorrectly designed subnet plans result in overlapping address ranges between subnets.In some cases, overlapping routes can cause problems; in other cases, this is simply a normal result of using certain features. This section will focus on how the router chooses which overlapping routes to use, and please also ignore for now whether the overlapping routes are wrong5.2 Basic Content 115of the problem. The section that follows this chapter, "Routing Problems Due to Incorrect Addressing Planning," will discuss some problematic use cases.So now we describe how the router matches the routing table, even if there are overlapping routes in the table. If only one route matches a given packet, then the router uses that route. However, when there are multiple routes that match the destination address of a packet, the router uses the "best" route, as defined below. Broadly speaking, when a specific destination IP address matches multiple routes in the router's IPv4 routing table, the router uses the most specific route.SS by a In other words, the route with the longest prefix length mask. Using show iproute and subnetting to find the best route There are a number of ways we can find out how the router should choose the best route. One way is to use the show ip route command, along with some subnetting arithmetic, to determine the route that the router should choose. Let's look at how to use this option; Example 5-4 lists a series of overlapping routes. Example 5-4 Overlapping Routes Displayed by the show ip route Command Rl# show ip route ospf Codes:L -local, C -connected, S -static, R -RIP, M -mobile, B -BGP D -EIGRP, EX -EIGRP external, 0 -OSPF, IA -OSPF inter area N1 -OSPF NSSA external type 1, N2 -OSPF NSSA external type 2 El -OSPF external type 1, E2 -OSPF external type 2 i-IS-IS, su -IS-IS summary, LI -IS-IS level-1, L2 -IS-IS level-2 ia -IS-IS inter area, *-candidatedefault, U-per-user static route o -ODR, P -periodic downloaded static route, H -NHRP, 1 -LISP + -replicated route, % -next hop override 172.16.0.0/16is variablysubnetted,9subnets,5masks 172.16.1.1/32 [110/50] via 172.16.25.2, 00:00:04, Serial0/l/l [110/100] via 172.16.25.129, 172.16.1.0/24 00:00:09, Serial0/l/0 172.16.0.0/22 [110/65] via 172.16.25.2, 00:00:04, Serial0/l/l [110/65] via 172.16.25.129, 172.16.0.0/16 00: 00:09, Serial0/l/0 Gatewayof lastresort is 172.16.25.129 to network 0.0.0.0 0 0.0.0.0/0 [110/129] via 172.16.25.129,00:00:09, Serial0/l/0 Tip: By the way, the showiprouteospf command will only list routes known to OSPF, but the statistics for the number of subnets and masks (9 and 5, respectively, in the example) are for all routes, not just those known to OSPF. Two pieces of information are needed to predict which route the router will match: the IP address of the packet and the contents of the router's routing table. The route's subnet ID and mask define the range of addresses that this route can match. With only a partial subnetting operation, the network engineer can find the range of addresses that each route matches. For example, Table 5-2 lists the five subnets in Example 5-4, along with the address ranges covered by each subnet. Table 5-2 Analysis of Subnet Address Ranges in Example 5.4 Subnet / Prefix Address Range 172.16.1.1/32 172.16.1.1 (only this address) 172.16.1.0/24 172.16.1.0-172.16.1.255 172.16.0.0/22 172.16.0.0-172.16.3.255 172.16.0.0/16 172.16.0.0-172.16.255.255 0.0.0.0/0 0.0.0.0-255.255.255.255 (all addresses) Tip: The route at 0.0.0.0/0 is the default route. As you can see in these ranges, some of the routing address ranges overlap. When multiple routes are matched, the prefix length is used 116 Chapter 4, Part 1 Troubleshooting IPv4 Routing routes that are longer in degree. That is, a route with a /16 is better than a route with a /10, a route with a /25 prefix is better than a route with a /20 prefix, and so on. For example, a packet destined for 172.16.1.1 actually matches all five routes listed in the routing table in Example 5-4. The various prefixes range in length from /0 to /32. The longest prefix (the maximum /P value, meaning the best and most specific route) is /32. Then packets sent to 172.16.1.1 will use route 172.16 ? 1/32, and no other route. The following list gives some examples of destination IP addresses. For each address, the list describes the routes in Table 5-2, which route the router will match, and which specific route it will use. 172.16.1.1: matches all 5 routes,the longest prefix is /32,route 172.16.1.1/32 O .172.16.1.2: matches the next 4 routes, the longest prefix is /24, route 172.16.1.0/24. 172.16.2.3: 3 routes after match, longest prefix is /22,route 172.16.0.0/22. .172.16.4.3: 2 routes after match; longest prefix is /16,route 172.16.0.0/16. Use showiproute address, to find the best route The second way to identify which route a router will use is the show ip route address command, which does not require subnetting operations. The last parameter in this command is the IP address of an assumed IP packet. The router lists the routes used to send the packet to that address in reply. For example, Example 5-5 lists the input to the show ip route172.16.4.3 command on a router that is the same router used in Example 5-4. The first line of output (highlighted) lists the matching route: 172.16.0.0/16. The rest of the output lists the details of this particular route, such as the output interface of S0/1/0 and the next-hop router 172.16.25.129 0 Example 5-5 The show ip route Command for Overlapping Routes Rl# show ip route 172.16.4.3 Routing entry for 172.16.0.0/16 Known via ospf 1", distance 110, metric 65, type intra area Last update from 10.2.2.5on Serial0/l/0, 14:22:06 ago Routing Descriptor Blocks:* 172.16.25.129, from 172.16.25.129, 14:22:05 ago, via Serial0/l/0Route metric is 65, traffic share count is 1