Routers add directly connected subnets to their routing table if the interface has an IP address in the up-up state. Static routes can be added to a router's table to learn routes to non-connected networks. Dynamic routing protocols allow routers to automatically exchange route information so that routes are learned without manual configuration.

2.  Subnets directly connected to a router interface are added to
the router's routing table.
 Interface has to have an IP address configured and both
interface status codes must be in the "up and up" state.
 A router will be able to route all packets destined for all
hosts in subnets directly connected to its active interfaces.
 Consider the following example.
 The router has two active interfaces, Fe0/0 and Fe0/1. Each
interface has been configured with an IP address and is
currently in the up-up state, so the router adds these subnets
to its routing table.

4.  As you can see from the output above, the router has two
directly connected routes to the subnets 10.0.0.0/8 and
192.168.0.0/24.
 The character „C“ in the routing table indicates that a route
is a directly connected route.
 TIP - you can see only connected routes in a router's routing table
by typing the show ip route connected command.

5.  By adding static routes, a router can learn a route to a
remote network that is not directly connected to one of its
interfaces.
 Static routes are configured manually by typing the global
configuration mode command ip route
DESTINATION_NETWORK SUBNET_MASK
NEXT_HOP_IP_ADDRESS.
 This type of configuration is usually used in smaller
networks because of scalability reasons (you have to
configure each route on each router).

6. Router A is directly connected to router B.
Router B is directly connected to the subnet 10.0.1.0/24. Since
that subnet is not directly connected to Router A, the router
doesn't know how to route packets destined for that subnet.
You have to configure that route manually.

9.  TIP - Another version of the ip route command exists.
You don't have to specify the next-hop IP address.
You can rather specify the exit interface of the local router.
In the example above you could have typed the ip route
DEST_NETWORK NEXT_HOP_INTERFACE command
to instruct router A to send all traffic destined for the subnet
out the right interface.

10.  A router can learn dynamic routes if a routing protocol is enabled.
 A routing protocol is used by routers to exchange routing
information with each other.
 Every router in a network can then use information to build its
routing table.
 A routing protocol can dynamically choose a different route if a
link goes down, so this type of routing is fault-tolerant.
 Also, unlike with static routing, there is no need to manually
configure every route on every router, which greatly reduces the
administrative overhead.
 You only need to define which routes will be advertised on a
router that connect directly to the corresponding subnets, routing
protocols take care of the rest.
 One of the disadvantages of dynamic routing is that it increases
memory and CPU usage on a router, because every router has to
process received routing information and calculate its routing
table.

11. Both routers are running a routing protocol, namely EIGRP.
There is no static routes on Router A, so R1 doesn't know
how to reach the subnet 10.0.0.0/24 that is directly
connected to Router B. Router B then advertises the subnet
to Router A using EIGRP. Now Router A has the route to
reach the subnet. This can be verified by typing the show ip
route command:

12. You can see that Router A has learned the subnet from EIGRP. The
letter „D“ in front of the route indicates that the route has been
learned through EIGRP. If the subnet 10.0.0.0/24 fails, Router B can
immediately inform Router A that the subnet is no longer reachable.

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