Transcript [RIP] - School of Computing

Routing Information Protocol
RIP
Cisco CCNA student
presentation @ Napier
University.
Why is a Routing Protocol needed?
Purpose of Routing information
Which route?
Metrics
Routing tables
RIP datagram and RIP port
Questions
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Why is a routing protocol needed?
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Early requirements to
exchanges data
between computers
over interconnected
networks.
Routing entities had
to make a judgement
on which path to route
traffic to destination.
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Background to RIP
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RIP dates back to 1969, the early networking days and
ARPNET when Xerox and Berkley’s Unix implemented it
broadly similar protocols.
RIP distributed through ‘route d’ application, included in
early Unix O.S.
RIP uses a single class of routing algorithm known as
distance vector - based on a simple hop count algorithm
(derived from Bellman’s equation).
Although superseded by more complex algorithms, its
simplicity means is still found widely in smaller
autonomous systems.
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Purpose of Routing Protocol
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The purpose of RoutING protocols is to supply information
needed to do routing of datagrams from router to router.
RIP intended for use in IP based network environment.
Operating at layer 3 of OSI (Network)
RIP makes no formal distinction between networks and
hosts.
Routers typically provide a gateway for datagrames to leave
one network or AS and be forwarded onward to another
network.
Routers therefore, have to make decisions if there is a
choice of forwarding path on offer.
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Which way?
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A path must be found
between the source and
destination.
Without a direction to
follow packet would have
to randomly circulate the
whole internetwork
looking for its destination.
If the networks are not
adjacent then the path will
be through several
routers.
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Routing metrics
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Routing entities keep a database (look up table) of basic
information based on numeric result s (metric) of an
algorithm to forward a datagram onward to its next
destination.
Each entity participating in routing decisions sends update
messages to its neighbour.
In order to provide complete network routing information
every router within the AS must participate in the protocol.
Each router has a lookup table which contains one entry for
every destination that is reachable.
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How does a metric work?
Metrics are the result of a formula based on a choice of
measurement criteria.
Example, travel cost by taxi:
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£10 to go by taxi from Edinburgh to Livingston. (P1)
£25 to go from Livingston to Glasgow (P2)
£15 to go from Edinburgh to Falkirk (P3)
£30 to go from Falkirk to Glasgow (P4)
Cost (Edinburgh, Glasgow)
= [P1+P2] = £35
also/or [P3+P4] = £45
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What is in a RIP routing table?
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Address - IP address (IPv4) of host or network destination.
Router - First router along the route to destination.
Interface - The physical network which must be used to
reach the next router.
Metric - A number indicating the distance to the destination.
This number is the sum of the ‘costs’ that have to be
transversed to get to the destination.
Timers - Time since entry was last updated and others.
Flags - Various flags to indicate status of various adjacent
routers (for example).
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Other entries in the routing table
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The entries for directly connected networks typically have a
value of 1 (a simple hop count).
Initially subnet masks were not included in RIP protocol
implementations, but were included later to support feature
extensions and to identify different subnets within local and
distant networks.
Administrators may also add static routes for example,
which are outside the scope of the routing system.
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The RIP datagram
Header
1 byte
1 byte
2 bytes
Command
Version
Must be zero
Request/
Response
1 (or 2)
Every RIP message contains a RIP
header that contains a command and a
version number. The command field
specifies the purpose of the message
(requested or a response). Response
can be an unsolicited RIP message.
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Routing Entry
Addr family ID
Must be zero
IPv4 ADDRESS
Must be zero
Must be zero
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RIP is a UDP-based
protocol.
Small regular messages,
no need for windowing,
handshaking or retransmission.
Frames received and
transmitted on UDP port
number 520 (Rip 1&2)
1 - 25 RIP routing entries
RTEs.
METRIC (value 1 to 15) 16 =infinity
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RIP
– Stephen Webster
– Gino Rigitano
– Telewest Communications
– 0131 539 0002
7/21/2015
Telewest Communications Group
user@domain