Direct versus indirect routing - IMED MCA 10+

reekydizzyNetworking and Communications

Oct 28, 2013 (3 years and 7 months ago)

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Routing Algorithms

Lecture
-
29
-

30

Static/ Dynamic, Direct/ Indirect, Shortest Path
Routing, Flooding, Distance Vector Routing, Link
State Routing, Hierarchical Routing, Broadcast
Routing, Multicast Routing


Routing Algorithm Basics
-

IIT
Kharagpur

Notes

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, “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.


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 can’t 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 isn’t working
well
(because it’s
too slow
, or
packets aren’t 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
.



Static Routing


Static
routing

is not really a routing protocol. Static routing is simply the process of manually entering
routes

into a
device's routing table via a configuration file that is loaded when the
routing

device starts up. As an alternative,
these
routes

can be entered by a
network

administrator who configures the
routes

manually. Since these manually
configured
routes

don't change after they are configured (unless a human changes them) they are called 'static'
routes
.


Static
routing

is the simplest form of
routing
, but it is a manual process.


Use static routing when you have very few devices to configure (<5) and when you know the
routes

will probably
never change.


Static
routing

also does not handle failures in external networks well because any
route

that is configured
manually must be updated or reconfigured manually to fix or repair any lost connectivity.


Dynamic Routing


Dynamic
routing protocols

are supported by
software

applications

running on the routing device (the
router
)
which dynamically learn
network

destinations and how to get to them and also advertise those destinations to
other
routers
. This advertisement function allows all the
routers

to learn about all the destination
networks

that
exist and how to
to

those
networks
.


A
router

using dynamic routing will 'learn' the
routes

to all
networks

that are directly connected to the device.
Next, the
router

will learn
routes

from other
routers

that run the same
routing protocol

(RIP, RIP2, EIGRP, OSPF, IS
-
IS, BGP etc). Each
router

will then sort through it's list of
routes

and select one or more 'best'
routes

for each
network

destination the
router

knows or has learned.


Dynamic routing protocols will then distribute this 'best route' information to other
routers

running the same
routing protocol
, thereby extending the information on what
networks

exist and can be reached. This gives
dynamic routing protocols the ability to adapt to logical
network topology

changes, equipment failures or
network

outages 'on the fly'.



Direct versus indirect routing


There are two types of routing:



direct

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
``Example internetwork''
, the machine
columbia

can trace a
direct route to any of the machines on the 10.0.118 network (that is,
seine
,
thames
, and
volga
). The
machine
columbia

cannot reach
london

or
paris

directly.



indirect

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
``Example internetwork''
,
the machine
volga

is a network gateway between the 10.0.118 network and the 10.0.246 network.
If the machine
seine

needs to send an IP packet to
paris
, it must send the packet to
volga
, which
forwards the packet to
paris
. Sometimes the source and destination hosts are more than one
network away from each other.