Routing Hierarchy - D-Link DFL-1600 User Manual

70
fields, such as Interface, Network, Gateway, Destination, etc.
When a packet arrives at a router, it refers to the routing table to
make path determination. The router compares the destination
address of the packet with the entries it has in the routing table, and
finds out the associated interface and next hop from the matching
route to forward the packet. The paths stored in the table are
computed by certain routing algorithm defined for the router, which
always tries to make the "best" choice. The "best" means a path
selection having the "least cost" for transporting. In practice, the
concern of "cost" are normally the bandwidth, path length(hops),
average delay, and etc., which are introduced in
metrics.
Routing algorithm is also responsible for keeping the routing table up
to date, so that the router can obtain correct path information for
every decision. The two most prevalent classes of routing algorithms
are covered in the next section.
Packet switching
After a path is chosen, the packet switching function takes control of
how the packet is actually moved. According to the information of
the selected route, the firewall/router rewrites the physical address of
the packet to the address of the next hop, and forwards the packet to
the next hop with the destination IP address unchanged. In a real-life
scenario, many firewalls/routers may come into play during the
packet forwarding process, each of them delivers the packet to its
nearby neighbor until the packet finally arrives at the receiving host.
10.2

Routing Hierarchy

In a complex network environment, as the number of routers becomes large,
the domain of routing is often divided into different areas to provide better
scalability. Routers reside under the same administrative control are
aggregated into one region called "autonomous system (AS)".
An AS can be, for example, all computer networks owned by a university
or a company's private network. The organization is able to run and
administer its network with its own policies and preferable routing
algorithm independently, while still being able to connect to the "outside"
D-Link Firewalls User's Guide
Chapter 10. Routing
10.3.2 Routing