http://rekrowten.wordpress.com/2011/08/22/cisco-routing-table-structure/
Ultimate route - route that contains either a next-hop IPv4 address or an exit interface.
Level1 - route with a subnet mask equal to or less than the classful mask of the network address.
10.0.0.0/8 - network route
10.0.0.0/7 - supernet
0.0.0.0/0 - default
Level1 parent - L1 route that is subnetted. A parent route can never be an ultimate route.
Level2 child - route that is a subnet of a classful network address.
Remember that the routing table hierarchy in Cisco IOS has a CLASSFULL routing scheme.
What happens if the router does not have a route? Then it discards the packet. (in classfull behavior)
Remember that classful and classless routing behaviors (ip classless) are independent from classful and classless routing protocols.
The routing table hierarchy in Cisco IOS has a classful routing scheme.
A level 1 parent route is the classful network address of the subnet route. This is the case even if a classless routing protocol is the source of the subnet route.
Routing table consists of route entries from the following sources:
- 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.
The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol.
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. 192.168.0.0/22
- Network route - A network route is a route that has a subnet mask equal to that of the classful mask. 192.168.1.0/24.A network route can also be a parent route.
Ultimate route - Level 1 route in RT that include next hop addr AND/OR an outgoing interface
[ C 192.168.1.0/24 is directly connected, Serial0/0/1 ]
Load-Balancing - when 2 or more route have save mask, AD and metric are added all in RIB and router balance traffic between them
Level 1 parent route is a level 1 network route that is subnetted does not contain any next-hop IP address or exit interface information.
A parent route can never be an ultimate route. Created automatically when you subnetting.
(23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks)
A level 2 child route is a route that is a subnet of a classful network address. 172.16.1.0/24
The source of a level 2 route can be a directly connected network, a static route, or a dynamically learned route.
Level 2 child routes are also ultimate routes. (Level 2 routes are also called child routes.)
Null Zero (null0) route - router send traffic to bitbucket (nowhere), but if there are more specific routes, null0 not used. prevent loops
172.16.0.0/24 is subnetted - if ALL child have subnets=/24
172.16.0.0/16 is subnetted - if child subnets are different
R1#sh ip route
.....
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets <- Parent
C 1.1.1.0 is directly connected, Loopback0
23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks <-Parent
B 23.23.0.0/18 [20/0] via 12.12.12.2, 00:10:20
B 23.23.23.0/24 [20/0] via 12.12.12.2, 00:10:20
12.0.0.0/24 is subnetted, 1 subnets
C 12.12.12.0 is directly connected, FastEthernet0/0
The route with the most number of equivalent left-most bits, or the longest match, is always the preferred route.
[ default-information originate ] - distribute a default route (network 0.0.0.0)
[ ip classless ] - the route lookup process uses classless routing table lookups (behavior).
Routing Process
When a packet arrives at the router, the router makes its path decision in this order.
1. Search Layer 1 Ultimate Routes
2. Search Layer 1 Parent Routes
a. Search Layer 2 Ultimate Routes of Parent
3. If classful routing
a. Discard
4. Else search Layer 1 Ultimate Routes without subnets.
5. Discard
The router when presented with multiple route possibilities must choose the best match.
The best match is also known as the longest match. The route with the most number of equivalent left-most bits is always the preferred route and known as the longest match. Prior to IOS 11.3 the default behavior for Cisco routers was no ip classless. When no ip classless is set default routes are ignored. All routers running IOS 11.3 or more current are configured as ip classless.
This setting can be changed inside the global configurations.
Note: A router will not choose the default route if the router is not searching in the ip classless mode.
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