The routers are typically responsible for directing traffic across multiple networks. Each router maintains a list of known network 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 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 a 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 router interface is configured with IP address and the interface state is up and up, the directly connected route is added to the router’s routing table automatically. Connected routes always take precedence over 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– This is the entry, from where the route has learned. C and L are two route source codes for directly connected routes. C identifies a directly connected network automatically created when an interface activated and configured with an IP address. L identifies the local route created whenever an interface configured with an IP address and activated. The L entry did not appear in routing table entries prior to IOS resale 15.
- Destination network– This is the address of the destination network also showing the connectivity.
- Outgoing interface– This showing 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 Identify the static route created by an administrator to reach a specific network.
- D– This code Identify the dynamically learned route using the EIGRP routing protocol.
- O– This code Identify the dynamically learned route using the OSPF routing protocol.
- R– This code Identify route learned dynamically from another router using the RIP routing protocol.
Remote Network Routing Table Entries
The figure below displays an IPv4 routing table entries 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.