Before looking at the basic ways in which
ASs can be connected to Internet service providers (ISPs), you need to review basic routing terminology and concepts.
- Static routing refers to routes, which
are manually configured in the router.
With manually configured routes, network reachability is not dependent on
the state of the network -- the router trusts the human operator to
make correct routing decisions. Whether a destination is up or down,
static routes remain in the routing table, and traffic is still
forwarded based on them.
- Default routing uses a "last
resort" route. If a router cannot find a packet's destination
network in its table, it will use a default route to forward the
traffic. Default routing is the
easiest form of routing for a domain connected to a single exit
point.
- Dynamic routing refers to routes
learned via routing protocols.
With dynamic routing, network reachability is dependent on the state
of the network. If a destination is down, the route will disappear
from the routing table, and traffic will not be sent toward it.
You will find that static, dynamic, and
default routing each have their place in a network architecture; but one
method (or combination of methods) will prove optimal depending on the situation.
This chapter considers several typical architectures and the optimal
routing method for each.
Static routes and default routes are not
your enemy! Although static routes don't provide much flexibility, they
can be the most stable configurations. Many people think they are not
technologically up-to-date just because they are not running dynamic
routing -- but trying to force dynamic routing on situation that does not
really need it is just a waste of bandwidth, effort, and money.
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