IP Routing Information Tables Overview

A router makes forwarding decisions based on the information that is contained in its routing table. Routers announce and receive route information to and from other routers. They build tables of routes based on the collected information about all the best paths to all the destinations they know how to reach.

Each configured protocol has one or more local routing tables, sometimes referred to as a routing information base (RIB). This table is a database local to the protocol that contains all the routes known by that protocol to the prefixes in the table. For example, OSPF might have four different routes to 10.23.40.5.32. Only one of these routes is the best route to that prefix known to OSPF, but all four routes are in the OSPF local routing table.

The global routing table is a database maintained by IP on the switch route processor (SRP) module. It contains at most one route per protocol to each prefix in the table. Each of these routes is the best route known by a given protocol to get to that prefix. The IP routing table does not, for example, have two OSPF routes to 10.5.11.0/24; it will have only one (if any) OSPF route to that prefix. It might also have a BGP route to the prefix, and a RIP route to the prefix, but no more than one route to a prefix per protocol.

IP compares the administrative distances for the routes to each prefix and selects the overall best route regardless of protocol. The best route to 10.5.11.0/24 might be via IS-IS. The best route to 192.168.0.0/16 might be via EBGP, and so on. These selected overall best routes to each prefix are used to create the forwarding table. The forwarding table is pushed to each line module. The line modules use their local instance of the forwarding table to forward the packets that they receive. When the global IP routing table is updated, so are the forwarding tables on the line modules.

Figure 1 illustrates a very simple network composed of three networks and two routers. The hosts that are attached to each network are not shown, because each router makes its forwarding decisions based on the network number and not on the address of each individual host. The router uses Address Resolution Protocol (ARP) to find the physical address that corresponds to the Internet address for any host or router on networks directly connected to it.

Figure 1: Routers in a Small Network

Table 1 and Table 2 represent information from the routing tables for routers NY and LA. Each routing table contains one entry for each route for each protocol or route type. Each routing table entry includes the following information:

• The destination IP network address.
• The IP address of the next-hop router.
• The type of network, such as static, directly connected, or the particular protocol.
• An administrative distance that is used to select the least-cost route among multiple routes to the same destination network. The least-cost (best) route is placed in the forwarding table. The administrative distance is not included in the forwarding table.
• A metric that is used by protocols to which the route is redistributed to select the least-cost route among multiple routes to the same destination network. The metric is not used to determine the best route to be placed in the forwarding table. The metric is also not listed in the forwarding table.

  Table 1: Routing Table for Router NY

  Destination Network	Next-Hop Router	Route Type	Administrative Distance	Metric
  10.1.0.0/16	10.1.0.1	connected	0	0
  10.2.0.0/16	10.5.0.3	OSPF	110	10
  10.2.0.0/16	10.5.0.3	IS-IS	115	10
  10.2.0.0/16	10.5.0.3	EBGP	20	15
  10.2.0.0/16	10.5.0.3	RIP	120	5
  10.5.0.0/30	10.5.0.2	connected	0	0

  Table 2: Routing Table for Router LA

  Destination Network	Next-Hop Router	Route Type	Administrative Distance	Metric
  10.1.0.0/16	10.5.0.2	static	1	0
  10.1.0.0/16	10.5.0.2	OSPF	110	10
  10.1.0.0/16	10.5.0.2	RIP	120	4
  10.2.0.0/16	10.2.0.1	connected	0	0
  10.5.0.0/30	10.5.0.3	connected	0	0