routing explained

The routers are typically responsible for directing traffic across multiple networks. Each router maintains a list of known networks and directions calling routing table. The router performs a routing table entries lookup to find the proper interface that leads to the destination address. Each entry in a routing table is called a “route entry: or “route”.

The route identifies the destination network to which traffic can be forwarded. The destination network, in the form of an IP address and netmask, can be an IP network, subnetwork, supernet, or host. Routing table entries can originate from the following sources:

• Directly connected networks
• Dynamic routing protocols, such as EIGRP, OSPF, and RIP
• Routes imported from other routers or virtual routers
• Statically configured routes

Directly Connected Entries

When the router interface is configured with an IP address and the interface state is up and up, the directly connected route is automatically added to the router’s routing table.

Connected routes always precede static or dynamically discovered routes because they have the administrative distance value of 0 (the lowest possible value). The directly connected routes contain the following information.

• Route source is the entry from where the route has been learned. C and L are two route source codes for directly connected routes. C identifies a directly connected network automatically created when an interface is activated and configured with an IP address. L identifies the local route created whenever an interface is configured with an IP address and activated. The L entry did not appear in routing table entries before IOS resale 15.
• Destination network– This is the address of the destination network that also shows the connectivity.
• The outgoing interface shows the exit interface for packet forwarding to the destination network.

Remote Route Source

A router stores information about both directly connected and remote routes. As with directly connected networks, the route source identifies how the route learned. Common codes for remote networks include:

• S– This code Identifies an administrator’s static route to reach a specific network.
• D– This code Identifies the dynamically learned route using the EIGRP routing protocol.
• O– This code Identifies the dynamically learned route using the OSPF routing protocol.
• R– This code Identifies the route learned dynamically from another router using the RIP routing protocol.

Remote Network Routing Table Entries

The figure below displays an IPv4 routing table entry for the route to remote network 192.168.0.0. We can identify the following information from this entry:

• Route source– discussed earlier in this lesson.
• Destination network– discussed earlier in this lesson.
• Administrative distance– Identifies the trustworthiness of the route.
• Metric– This is the value assigned to reach the remote network. Lower values indicate preferred routes.
• Next hop– Identifies the IPv4 address of the next router.
• Route timestamp– Identifies from when the route was last heard.
• Outgoing interface– This is the outgoing interface towards the destination.