IP Routing
Routing is the
technique by which data finds its way from one host computer to another.
In the Internet context there are
three major aspects of routing
The routing algorithm is as
follows:
Within each host there is a routing
table that the host uses to determine which physical interface address to use for outgoing
IP datagrams.
There are four basic items of
information in such a table
A host dedicated to providing a
gateway service between several networks is known as a Router and may have a very
large routing table (64 MB is not unknown) and will run special protocols to interchange
routing information with other hosts and routers.
Routers are basically Network
Layer Relays.
Two methods are used for
information dissemination:
DISTANCE VECTOR |
LINK STATE |
Loops are sometimes formed. | Looping is not formed, since each router has the complete network topology. |
Slow convergence problems. | Faster convergence. |
Bandwidth intensive process because updates are sent every 60 secs. | Routing updates are sent every 15-30 minutes or when there is a change in network topology. |
Low processing overhead. | Increased route calculation processing. |
Routers use dynamic routing
protocols to communicate with each other.
An Autonomous System (AS)
is an individually administered entity (network).
All autonomous systems have a
distinctive and unique number.
Details of autonomous systems are
available from the whois servers in the same way as details of IP networks.
Unfortunately the syntax of such
queries differs between the various servers, the ripe server requires ASnnnn
whereas the arin server requires just the number.
Routing protocols can be broadly
classified into two types:
Routing
Information Protocol (RIP)
Uses a datagram
(UDP) for proliferating routing tables.
The maximum number of hops allowed
is 15 (A limitation!)
RIP recognizes two types of user
devices:
The active users send updates every
30 seconds.
Routes are considered disconnected
unless updates are received. (i.e. they timeout after 180 seconds).
RIP information is processed by
daemon processes (either
routed
or gated
on Unix hosts) listening on the well known port number 520.
Exterior
Gateway Protocol (EGP)
A distance vector
routing protocol.
It does not employ routing tables.
It uses point-point communication
rather than broadcast.
Border Gateway
Protocol (BGP)
It
enhances EGP.
A distance
vector external routing protocol.
The
enhancements are:
Read the BGP Frequently
Asked Questions.
For detailed information, look into
the BGP Page.
Open Shortest
Path First (OSPF)
The O means open,
i.e. a non-proprietary protocol.
It is a link state internal gateway
protocol.
OSPF uses IP directly.
A router using OSPF calculates the
shortest path to the other routers in the AS by considering itself as a ROOT.
The advantages of using OSPF are as
follows:
References:
Maintained by Mohan Atreya , Last update April 8, 1999 First created on November 10, 1998 |