Static vs. Dynamic Routing
Routing can be classified as either static or dynamic. In static routing, the network administrator sets up turn by turn directions for how traffic gets from one subnet to another, and from those subnets to other networks such as the Internet. For each destination network or subnet, the administrator adds a static route that specifies the next router on that network that should receive the packet, in order for it to get to its final destination.
When that next router receives the packet, it looks at its own static routes and determines the next turn the data must take on the way to its final destination. Eventually, a router along the path will look at the data packet and know, Youre here! No more turns! and deliver the packet to a host on a network to which it is directly connected. This provides a network admin with explicit control over the path of packets through the network.
Dynamic routing works similarly, except instead of the network administrator having to manually specify how to get packets from one router to the next optimally, she can rely on the routers to figure out an efficient route on their own. Why would you want to do this? In many large networks, maintaining an accurate set of static routing tables can be a very difficult task. Similarly, if a router goes down or if there is a cable break at some point along the route, it may be necessary to quickly revise the static routing tables to reflect new temporary routes between some subnets or networks. Again, this could be a time-consuming and error-prone task, made all the more difficult by impatient users calling to ask, Why cant I get to the Finance web site in Portland?.
With dynamic routing, a router communicates with other routers and when it discovers that a route isnt working well (because its too slow, or packets arent getting through at all), it looks for and then selects an alternate route, much like you would when encountering congestion on the Interstate. This alternate route might not be as efficient or as reliable as the first one under ideal circumstances, but it is usually better than no route at all with no extra work on the part of the network administrator.
Entire texts are devoted for efficient, optimal ways to perform this dynamic routing task. As far as Network+ goes, be aware that the most common dynamic routing protocols are OSPF, IGRP, BGP and RIP.
To get the best of both worlds, it can be appropriate to combine static and dynamic routing under certain circumstances (which can include political as well as technical considerations, so consider yourself forewarned).
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