Monday, January 17, 2011

Routing Table


1. Every router makes its decision alone, based on the information it has in its own routing table.

2. The fact that one router has certain information in its routing table does not mean that other routers have the same information.

3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

The primary function of a router is to forward a packet toward its destination network, which is the destination IP address of the packet. To do this, a router needs to search the routing information stored in its routing table.

  • is a data file in RAM that is used to store route information about directly connected and remote networks.
  • contains network/next hop associations
  • next hop association can also be the outgoing or exit interface to the final destination.
  • network/exit-interface association can also represent the destination network address of the IP packet.
  • Directly Connected Routes - To visit a neighbor, you only have to go down the street on which you already live. This path is similar to a directly-connected route because the "destination" is available directly through your "connected interface," the street.
  • Static Routes - A train uses the same railroad tracks every time for a specified route. This path is similar to a static route because the path to the destination is always the same.
  • Dynamic Routes - When driving a car, you can "dynamically" choose a different path based on traffic, weather, or other conditions. This path is similar to a dynamic route because you can choose a new path at many different points on your way to the destination.


After the router's interface is configured and the interface is activated with the no shutdown command, the interface must receive a carrier signal from another device (router, switch, hub, etc.) before the interface state is considered "up." Once the interface is "up," the network of that interface is added to the routing table as a directly connected network.

Directly connected networks are of prime importance for routing decisions. Static and dynamic routes cannot exist in the routing table without a router's own directly connected networks. The router cannot send packets out an interface if that interface is not enabled with an IP address and subnet mask, just as a PC cannot send IP packets out its Ethernet interface if that interface is not configured with an IP address and subnet mask.

Static Routing

Remote networks are added to the routing table either by configuring static routes or enabling a dynamic routing protocol. When the IOS learns about a remote network and the interface that it will use to reach that network, it adds that route to the routing table as long as the exit interface is enabled.

Static routes should be used in the following cases:

  • A network consists of only a few routers. Using a dynamic routing protocol in such a case does not present any substantial benefit. On the contrary, dynamic routing may add more administrative overhead.
  • A network is connected to the Internet only through a single ISP. There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet.
  • A large network is configured in a hub-and-spoke topology. A hub-and-spoke topology consists of a central location (the hub) and multiple branch locations (spokes), with each spoke having only one connection to the hub. Using dynamic routing would be unnecessary because each branch has only one path to a given destination-through the central location.

Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including:

  • Network discovery
  • Updating and maintaining routing tables

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