Discussion

Chapter
CHAPTER
8
8
IP Routing
8
8.0 Introduction
Routing is the act of forwarding packets toward a given destination from one network
segment or interface to the next. Routing tables, also sometimes called routing
information bases (RIBs), are the databases that routers use to route traffic toward
their destination. These tables contain the network addresses and prefixes that have
been learned from dynamic routing protocols such as RIP, IS-IS, OSPF, and BGP;
that have been learned statically from static (configured) routing-table entries; and
that have been learned from the router’s network interfaces. Each address and prefix
in a routing table has a next hop associated with it that takes the packet one hop
closer to its destination.
Each IP packet that a router receives contains two types of information: the packet
data itself (the packet’s payload) and information that identifies the packet. In IP
packets, the identifying information is at the beginning of the packet, in the header.
One of these header fields is the source address, which states the packet’s origin;
another, which is key to the routing tables, is the destination address, which tells
where the packet is going when the router uses standard destination-based forwarding.
(Routing policy can alter the path toward a destination.) When the router is
determining the path toward the destination, it checks the routing table for a route
that matches the packet’s destination and then sends the packet to the next hop
associated with that route. If there is no exact match, the router locates a more general
route, selecting the longest match, which is the route that matches the most bits
in the network portion of the address. For example, if the packet’s destination is
10.0.16.2 and the routing table contains a route to 10.0.16.2/32, which is the
address of the specific host, the packet is sent using the next hop associated with
that route. If the only matching routes in the table are 10.0.0.0/8 and 10.0.16.0/24,
the latter route is used because it is the longest match.
If no match is found in the routing table, the default route of 0.0.0.0/0 is used if it
exists. If no default route is configured or learned, the traffic is dropped.
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