Static/ Dynamic, Direct/ Indirect, Shortest Path
Routing, Flooding, Distance Vector Routing, Link
State Routing, Hierarchical Routing, Broadcast
Routing, Multicast Routing
Routing Algorithm Basics
Static Vs. Dynamic routing
Routing can be classified as either
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
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, “You’re 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.
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
figure out an efficient route
their own. Why would you want to do this? In many large networks,
an accurate set of
can be a very
. Similarly, if a
router goes down
or if there is a
at some point along
the route, it may be
necessary to quickly revise the static routing tables
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 can’t I get to the Finance web site in Portland?”.
, a router communicates with other routers and when it
discovers that a route isn’t working
packets aren’t getting through
at all), it looks for and then
selects an alternate
, 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
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
is not really a routing protocol. Static routing is simply the process of manually entering
device's routing table via a configuration file that is loaded when the
device starts up. As an alternative,
can be entered by a
administrator who configures the
manually. Since these manually
don't change after they are configured (unless a human changes them) they are called 'static'
is the simplest form of
, but it is a manual process.
Use static routing when you have very few devices to configure (<5) and when you know the
also does not handle failures in external networks well because any
that is configured
manually must be updated or reconfigured manually to fix or repair any lost connectivity.
are supported by
running on the routing device (the
which dynamically learn
destinations and how to get to them and also advertise those destinations to
. This advertisement function allows all the
to learn about all the destination
exist and how to
using dynamic routing will 'learn' the
that are directly connected to the device.
that run the same
(RIP, RIP2, EIGRP, OSPF, IS
IS, BGP etc). Each
will then sort through it's list of
and select one or more 'best'
knows or has learned.
Dynamic routing protocols will then distribute this 'best route' information to other
running the same
, thereby extending the information on what
exist and can be reached. This gives
dynamic routing protocols the ability to adapt to logical
changes, equipment failures or
outages 'on the fly'.
Direct versus indirect routing
There are two types of routing:
When a machine can send an IP packet to another machine without going through a third
machine, the route the packet will travel is said to be a ``direct route'' and the selection of that
route is called ``direct routing''. In
, the machine
can trace a
direct route to any of the machines on the 10.0.118 network (that is,
When a machine wishing to send an IP packet to a second machine must send that packet
through a third machine, the route the packet will travel is said to be an ``indirect route'' and the
selection of the intermediary machine is called ``indirect routing''. The intermediary machine, a
router, has connections to more than one network and is said to provide a gateway between the
networks. Therefore, a router may also be called a ``network gateway''. In
is a network gateway between the 10.0.118 network and the 10.0.246 network.
If the machine
needs to send an IP packet to
, it must send the packet to
forwards the packet to
. Sometimes the source and destination hosts are more than one
network away from each other.