cisco 200-301 videos

cisco 200-301 videos.18.1.4 Default Routing and Distance Vectoring ProtocolsChapter 18 Basic Routing 457Default routes can cause problems when using some routing protocols. Routing protocols fall into two categories: classified and classless. Classified routing protocols include RIPvl and IGRP (IGRP is no longer supported by Cisco); classless routing protocols include RIPv2, Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System Intermediate System (IS-IS), and Border Gateway Protocol (BGP) OClassification routing protocols can only identify classification subnets. For example, if the routing update contains 192.168.1.0/23, the categorical routing protocol will not understand it because the subnet mask for a class C network must be /24. This can cause problems for default routes with a subnet mask of /0.In addition, routers that use the categorical routing protocol will not include a subnet mask when advertising routes out through their interfaces. For example, you might configure the router's interface with the IP address 192.168.1.1/26, and the router might receive a routing update that contains 192.168.1.0. If you are using a categorical routing protocol, the router will only understand the subnet mask configured on its interface for the network number. In this example, the router believes that for 192.168.1.0, the only legal subnet mask is /26; therefore the router treats the network number as 192.168.1.0/26, but if it is actually 192.168.1.0/27, this will cause many routing problems.Classless protocols can accept routing updates that use any subnet mask, and to have classification protocols accept routes that use a non-default subnet mask (such as the default route), you must configure the following command.Router(config)# ip classlessIn networks that use classification protocols, this command is also used to handle subnet separation: subnets that have another, different classification network between them. For example, suppose you have class B subnets 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24, but there is another classified network between the first two subnets and the last one - 192.168.1.0/24. In this case, connecting subnets 172.16.1.0/24 and 172.16.2.0/24 will be ignored when the router receiving route 172.16.0.0 from network 192.168.1.0/24.Don't forget that in classification protocols, routes are propagated across classification network boundaries where the network number is the classification network number. Therefore, routers connected to 192.168.1.0/24 and 172.16.3.0/24 advertise updates about 172.16.3.0/24 outside of subnet 192.168.1.0/24 with 172.16.0.0.0, not the actual subnet number 172.16.3.0/24. Since routers connected to 172.16.1.0/24 and 172.16.2.0/24 advertise updates about 172.16.0.0/24, they advertise 172.16.0.0, not the actual subnet number 172.16.3.0/24. Also, even if you configure a default route, routers connected to 172.16.1.0/24 and 172.16.2.0/24 will assume that 172.16.3.0 is also directly connected to it. If there is no route to that subnet in the routing table, it cannot travel to that subnet. This was discussed in Chapter 7.This behavior can be overridden by using the command ip classless,which allows a router using a categorical routing protocol to use the default route to a separated subnet. Note that this design is not recommended, but it does allow you to solve the reachability problem for detached subnets.The RIPvl classification protocol can only understand classified subnets-you can only use a subnet mask in a classified network; RIPv2, EIGRPxOSPF, and IS.IS classless protocols do not have this limitation. The command ip classless lets the classification protocol use the default route; if you do not configure this command, the router drops packets that do not match any specific route in the routing table.