Routing Components

Routing involves two functions: determining the best routing path, and transporting data traffic in the form of packets across the best path through an internetwork. The routing process also is referred to as packet switching. Even though packet switching is straightforward, determining the path for the traffic to take to its intended destination can be complex.

Path Determination

Routing protocols use metrics to measure and determine which path is the best for a packet to travel through an internetwork. Metrics are standards of measurement, such as path bandwidth, used by routing algorithms to determine the best path to a destination network or host. To determine the best path to a destination host/network, routing algorithms build and maintain routing tables which contain route information for each available path. The information in these routing tables varies depending on which routing algorithm is used.

Destination/next hop associations tell a router that a particular destination can be reached by sending the packet to a particular router, representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address against its routing tables and associates this address with a next hop if available.

If there is no next hop available in the routing table, the router sends the packets to the default gateway. The default gateway is considered the path of last resort in that if the router doesn't know what to do with a packet it forwards it out the default gateway interface to the associated attached network. Not all default gateways are pointed to an attached network; there are instances where the default gateway represents what is called a "bit bucket," or null interface. The null interface is a logical interface where the packet is sent and subsequently dropped from the router.

Routing tables can contain other information, such as data about path desirability. Routers compare the metrics of each path in its routing table to determine optimal routes. These metrics differ depending on the design of the routing algorithm used. Metrics are discussed in the "Routing Metrics" section of this chapter.

Routers communicate with one another and maintain their routing tables through the exchange of routing update messages. The routing update message consists of all, or a portion, of a routing table. By analyzing routing updates from other routers in the network, a router can build a detailed picture of the network topology. Link-state advertisements (LSAs) are another example of routing update messages exchanged between routers. LSAs inform other routers in the network of the state of connected network links. This link information is used to build a complete picture of network topology, enabling routers to determine the best path to a network destination.

In most cases, a router determines that it must send a packet to another router in the network so the packet can reach its intended destination. The next router in the packet's path is known as the downstream neighbor, or the next hop router. The router knows the physical (MAC) address of this next hop router, (learned through other means), and forwards the packet to this next hop router with both the physical (MAC) and logical (network) addresses.

As the router examines the packet's destination network address, the router determines that it either does, or does not, know how to forward the packet to the next hop. If the router knows how to forward the packet, it changes the packet's destination physical (MAC) address to that of the next hop and sends the packet. If the router does not know how to forward the packet, it is often configured to drop the packet.

The next hop might be the ultimate destination; if not, the next hop is often another router in the network, which in turn executes the same routing decision process. As the packet moves through the internetwork, its physical (MAC) address changes, but its logical (network) address remains constant.

The International Organization for Standardization (ISO) developed a hierarchical terminology useful in describing the process of forwarded packets between source and destination. The ISO hierarchical terminology is as follows:

• End systems (ES) Network devices without the ability to forward packets between sub-networks.

• Intermediate systems (IS) Network devices with the ability to forward packets between subnetworks are called intermediate systems (ISs). ISs are further divided into those that can communicate within routing domains (intradomain ISs) and those that communicate both within and between routing domains (interdomain ISs).

• Autonomous systems (AS) Routing domains under common administrative authority regulated by a particular set of administrative guidelines. Routing domains can be divided into areas, but intradomain routing protocols are used for routing within and between these areas.