Route Lookup Process Terminology The Internetwork Operating System (IOS) internally classifies routes into different types. Routes are classified as follows: Level One Routes (Technically Level One Final Routes) Parent Routes (Technically Parent Routes) Level Two Routes (Technically Level Two Final Routes and also refered as Child Routes) Level One Final Routes: These routes are differentiated by the subnet mask. Subnet masks on Level One Final Routes are higher or equal of its default class. See examples of Level One Final Routes: 172.16.1.0/16 192.168.1.0/24 0.0.0.0/0 192.168.0.0/15 As you see these types of routes are Level One Final Routes because they follow the above description, but note that there is an empty space yet: Why they are Final Routes ? The reason why is because these routes have an exit interface when they are installed in the routing table. See the following example: Configuring a Level One Final Route: R1>enable R1#configure terminal R1(config)#interface loopback 192 R1(config-int)#ip address 192.168.1.1 255.255.255.0 First Table note that the configured IP address have a subnet mask of its own default class, in this case is a class C network. See the show ip route output: R1(config-int)#do show ip route <outupt ommitted> C 192.168.1.0/24 is directly connected, Loopback192 Parent Routes: These type of routes are automatically created inside the routing table in order to further classify other types of routes. Parent Routes doesnt have any exit point interface, in fact, they dont even have an indentification tag to reference the type of route inside the routing table, i.e. a C to refer to the connected type of route. Parent Routes are created when Level Two Final Routes are added. See example below: R1>enable R1#configure terminal R1(config)#interface loopback 172 R1(config-int)#ip address 172.16.1.1 255.255.255.0 Note that this route has a subnet mask higher than its default class. The network is technically class B, but we configure a subnet mask higher than its default, in this case a /24 subnet mask. See the show ip route output: R1(config-int)#do show ip route <output ommitted> 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Loopback172 <output ommitted> This route is a general route. Note that this route says that there is an additional subnetted route which is the route we explicitly added the 172.16.1.0/24 route, the address configured on the 172 loopback interface. Level Two Final Routes: These types of routes are identified when the subnet mask its higher than the default mask of its class. Level Two Final Routes are the most specific routes inside the routing table., in fact, these are the router process first in order to execute the longest-match rule on IOS. Example of a Level Two Final Route: R1>enable R1#configure terminal R1(config)#interface loopback 172 R1(config-int)#ip address 172.16.1.1 255.255.255.0 <output ommitted> 172.16.0.0/24 is subnetted, 1 subnets C 172.16.1.0 is directly connected, Loopback172 <output ommitted> Note that this is the route we configured earlier on the Parent Route example. This is the route the Parent Route is refering to when described: ... is subnetted, 1 subnets). The subnet is refering to is this route. Note that Level Two are always Final Routes, so there are not such thing as Level Two Parent Routes, since they always have an exit interface. Elvin Route Lookup Process The router examines level 1 routes, including network routes and supernet routes, for the best match with the destination address of the IP packet. If the best match is a level 1 ultimate route – a classful network, supernet, or default route – this route is used to forward the packet. If the best match is a level 1 parent route, proceed to Step 2. The router examines child routes (the subnet routes) of the parent route for a best match. If there is a match with a level 2 child route, that subnet will be used to forward the packet. If there is not a match with any of the level 2 child routes, proceed to Step 3. Is the router implementing classful or classless routing behavior? If classful routing behavior is in effect, terminate the lookup process and drop the packet. If classless routing behavior is in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one. If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet. If there is not a match with any route in the routing table, the router drops the packet. Reference: beautbelsblog.wordpress/2010/04/14/the-route-lookup-process/ https://learningnetwork.cisco/thread/46423 ITnotes.in
Posted on: Fri, 31 Oct 2014 07:23:31 +0000
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