CCNA Exploration 2.
Routing protocols and Consepts.
Chapter 8. Resume. The Routing table. A closer look
- Directly connected networks
- Static routes
- Dynamic routing protocols
A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address.
A level 1 route can function as a:
~ Default route - A default route is a static route with the address 0.0.0.0/0.
~Supernet route - A supernet route is a network address with a mask less than the classful mask.
~Network route - A network route is a route that has a subnet mask equal to that of the classful mask. A network route can also be a parent route.
An ultimate route is a route that includes:
~ either a next-hop IP address (another path)
~ and/or an exit interface
A level 1 parent route is a network route that does not contain a next-hop IP address or exit interface for any network. A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes.
Level 2 child routes are also considered ultimate routes because they will contain the next-hop IP address and/or exit interface.
Although the parent/child relationship uses a classful structure to display networks and their subnets, this format can be used with both classful and classless addressing. Regardless of the addressing scheme used by the network (classless or classful), the routing table will use a classful scheme.
The parent route states that the child routes are "variably subnetted".
The Route Lookup Process
Step 1.
The router examines level 1 routes, including network routes and supernet routes, for the best match with the destination address of the IP packet.
Step 1a.
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.
Step 1b.
If the best match is a level 1 parent route, proceed to Step 2.
Step 2.
The router examines child routes (the subnet routes) of the parent route for a best match.
Step 2a.
If there is a match with a level 2 child route, that subnet will be used to forward the packet.
Step 2b.
If there is not a match with any of the level 2 child routes, proceed to Step 3.Click Step 3.Is the router implementing classful or classless routing behavior?
Step 3a.
Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.
Step 3b.
Classless routing behavior: 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.
Step 4.
If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.
Step 5.
If there is not a match with any route in the routing table, the router drops the packet.
Classful and classless routing behavior will be discussed in more detail in a later section.
Note: A route referencing only a next-hop IP address and not an exit interface must be resolved to a route with an exit interface. A recursive lookup is performed on the next-hop IP address until the route is resolved to an exit interface.
The route with the most number of equivalent left-most bits, or the longest match, is always the preferred route.
If the router is using classful routing behavior, no other routes will be searched and the packet will be discarded. Classful routing behavior was the default routing behavior on Cisco routers prior to IOS 11.3. Classful routing behavior can be implemented using the no ip classless command.
Starting with IOS 11.3 classless routing behavior became the default. If there is a match with a parent route but none of the child routes, the routing table process will continue to search other routes in the routing table including a default route should one exist. Classless routing behavior is implemented by using the ip classless command.
The parent route states that the child routes are "variably subnetted".
The Route Lookup Process
Step 1.
The router examines level 1 routes, including network routes and supernet routes, for the best match with the destination address of the IP packet.
Step 1a.
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.
Step 1b.
If the best match is a level 1 parent route, proceed to Step 2.
Step 2.
The router examines child routes (the subnet routes) of the parent route for a best match.
Step 2a.
If there is a match with a level 2 child route, that subnet will be used to forward the packet.
Step 2b.
If there is not a match with any of the level 2 child routes, proceed to Step 3.Click Step 3.Is the router implementing classful or classless routing behavior?
Step 3a.
Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.
Step 3b.
Classless routing behavior: 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.
Step 4.
If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.
Step 5.
If there is not a match with any route in the routing table, the router drops the packet.
Classful and classless routing behavior will be discussed in more detail in a later section.
Note: A route referencing only a next-hop IP address and not an exit interface must be resolved to a route with an exit interface. A recursive lookup is performed on the next-hop IP address until the route is resolved to an exit interface.
The route with the most number of equivalent left-most bits, or the longest match, is always the preferred route.
If the router is using classful routing behavior, no other routes will be searched and the packet will be discarded. Classful routing behavior was the default routing behavior on Cisco routers prior to IOS 11.3. Classful routing behavior can be implemented using the no ip classless command.
Starting with IOS 11.3 classless routing behavior became the default. If there is a match with a parent route but none of the child routes, the routing table process will continue to search other routes in the routing table including a default route should one exist. Classless routing behavior is implemented by using the ip classless command.
