D-Link DFL-2500 User Guide - Page 89
Routing Hierarchy
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70 Chapter 10. Routing 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 10.3.2 Routing 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
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