As highlighted in Figure 1, the routing table of R1 contains three directly connected networks. Notice that two routing table entries are automatically created when an active router interface is configured with an IP address and subnet mask.
Figure 2 displays one of the routing table entries on R1 for the directly connected network 172.16.1.0. These entries were automatically added to the routing table when the GigabitEthernet 0/0 interface was configured and activated. The entries contain the following information:
- Route source - Identifies how the route was learned. Directly connected interfaces have two route source codes. C identifies a directly connected network. Directly connected networks are automatically created whenever an interface is configured with an IP address and activated. L identifies that this is a local route. Local routes are automatically created whenever an interface is configured with an IP address and activated.
- Destination network - The address of the remote network and how that network is connected.
- Outgoing interface - Identifies the exit interface to use when forwarding packets to the destination network.
Note: Local routing table entries did not appear in routing tables prior to IOS release 15.
A router typically has multiple interfaces configured. The routing table stores information about both directly connected and remote routes. As with directly connected networks, the route source identifies how the route was learned. For instance, common codes for remote networks include:
- S - Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route.
- D - Identifies that the route was learned dynamically from another router using the EIGRP routing protocol.
- O - Identifies that the route was learned dynamically from another router using the OSPF routing protocol.
- R - Identifies that the route was learned dynamically from another router using the RIP routing protocol.