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11.4 Routing Table
Routing Table Section 11: Exploring the Functions of Routing As discussed in a previous topic, a routing table contains a list of all networks that are known to the router as well as information about how to reach those networks. Each line, or entry, of the routing table lists a destination network and the interface or nexthop address by which that destination network can be reached. A routing table may contain the following types of entries: Directly connected networks: All directly connected networks are added to the routing table automatically. A directly connected network is a network that is directly connected to one of the interfaces on the local router. If the interface fails or is administratively shut down, the entry for that network is removed from the routing table. Static routes: Static routes are entries that you manually enter directly into the configuration of the router. Static routes can be effective for small, simple networks that do not change frequently. However, statically populating routing tables does not scale well and can lead to problems if the network topology changes. Default routes: A default route is an optional entry that is used when no explicit path to a destination is found in the routing table. The default route can be manually configured as a static route, or it can be entered by a routing protocol. Dynamic routes: The router learns dynamic routes automatically when a routing protocol is configured and a neighbor relationship to other routers is established. The information is updated when changes in the network occur. Larger networks require the dynamic routing method because there are usually many addresses and constant changes. These changes require updates to routing tables across all routers in the network, to prevent connectivity loss. RouterA#show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, + - replicated route Gateway of last resort is 10.1.1.1 to network 0.0.0.0 C 10.1.1.0/24 is directly connected, GigabitEthernet0/0 L 10.1.1.2/32 is directly connected, GigabitEthernet0/0 O 172.16.1.0/24 [110/2] via 192.168.10.2, 00:01:08, GigabitEthernet0/1 D 192.168.20.0/24 [90/156160] via 10.1.1.1, 00:01:23, GigabitEthernet0/0 S 192.168.30.0/24 [1/0] via 192.168.10.2 C 192.168.10.0/24 is directly connected, GigabitEthernet0/1 L 192.168.10.1/32 is directly connected, GigabitEthernet0/1 S* 0.0.0.0/0 [1/0] via 10.1.1.1 The figure shows the output of the show ip route command, which is used to display the contents of the routing table in a router. The first part of the output explains the codes, presenting the letters and the associated source of the entries in the routing table. The letter C, which is reserved for directly connected networks, labels the first and sixth entries. The letter L, which is reserved for local routes and indicating local interfaces within connected networks, labels the second and seventh entries. The letter S, which is reserved for static routes, labels the fifth and eighth entries. The asterisk (*) indicates a default route. In this example command output, the default route is a static route. The letter O, which is reserved for the OSPF routing protocol, labels the third entry. The letter D, which is reserved for EIGRP, labels the fourth entry. The letter D stands for DUAL, the update algorithm that is used by EIGRP. Click the Play button to watch a short video about routing tables. Up Next: Dynamic Routing Protocols