routing algorithms & protocols
TRANSCRIPT
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Routing Algorithms & Protocols
By
Utkarsh Hathi
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Routing
A process of selecting paths in a network todecide where to send the network traffic.
In most subnets, packets require multiplehops to make the journey.
Even in broadcast networks, routing is an issueif source and destination are not on the same
network.
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Routing Algorithm
Routing Algorithm is that part of the network
layer software responsible for deciding which
output line an incoming packet should be
transmitted on.
Overall performance of your network depends
on the routing algorithms
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Continued
Routing Algorithm cant be changed, only
protocols can be changed and as different
protocols use different algorithms; changing it
will change the algorithm as well.
Now, we will have a look at the different
routing protocols by the algorithms they
support.
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Routing Metric
A property of a route, consisting of a value
used by algorithms to decide which route will
perform better
Different algorithms and protocols use
different metrics and that can help us to
decide which protocol to use by the type of
services we want.
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IGP and EGP
IGP Interior Gateway Protocol, also known
as intra domain routing algorithm
Works within an autonomous system
Can be divided into three categories
Distance vector routing protocols
Link state routing protocols
Balanced Hybrid routing protocol
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Autonomous system (IGP)
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Distance Vector Routing Protocol
Requires that a router informs its neighbors
of changes in topology periodically; can be
also a triggered update if there is an instant
change
Each node maintains a table of minimum
distances to every node.
Examples: RIP V1 ,V2 and IGRP
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Routing Information Protocol (V1)
As a distance vector routing protocol, it uses
hop count as a metric.
Limitation of 15 hops for a packet to travel to
avoid the routing loops. Thus it limits the size
of network it can support.
Initially used to send update every 30 seconds
but as networks became large it became
difficult.
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Continued
RIP is not preferred choice for routing as its
convergence time and scalability are poor
compared to EIGRP, OSPF or IS-IS.
RIP V1 doesnt support subnet while RIP V2
does thus supporting Classless Inter-domain
routing.
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IGRP
Interior gateway routing protocol (IGRP),
another distance vector routing protocol
invented by CISCO and increases the hop limit
to 255 from 15 of RIP.
Supports multiple metrics such as bandwidth,
delay, load, MTU and reliability.
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IGRP metric count
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Link state routing protocol
Every node constructs the map of the
connectivity to the network, in the form of a
graph showing which nodes are connected to
other nodes
Then each node calculates the best logical
path from it to every possible destination.
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Each node independently runs
an algorithm over the map to determine
the shortest path from itself to every other
node in the network
Uses link as the metric and will choose the
path from where the packet will reach the
fastest
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Example:
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Open Shortest Path First (OSPF)
The most widely-used interior gatewayprotocol (IGP) in large enterprise networks
Support variable length Subnet maskingsupporting Classless Inter domain routing.
Detects changes in the topology, such as linkfailures, very quickly and converges on a newloop-free routing structure within seconds
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Continued
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OSPF Areas
Allows an AS to be split into different areas for
ease of management and the routing
information transmitted between areas is
summarized to reduce network bandwidth
consumption.
Special area called the backbone
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OSPF Computation
OSPF route computation
OSPF route computation is described as follows:
1. Based on the network topology around itself, each routergenerates Link State Advertisements (LSA) and sends them to otherrouters in update packets.
2. Each OSPF router collects LSAs from other routers to compose aLSDB (Link State Database). An LSA describes the network topologyaround a router, so the LSDB describes the entire network topologyof the AS.
3. Each router transforms the LSDB to a weighted directed graph,which actually reflects the topology architecture of the entirenetwork. All the routers have the same graph.
4. Each router uses the SPF algorithm to compute a Shortest PathTree that shows the routes to the nodes in the autonomous system.The router itself is the root of the tree.
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Border Gateway Protocol
Was created to replace Exterior gateway
protocol, that is used for interconnecting
different autonomous systems.
Most Internet service providers must use BGP
to establish routing between one another
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BGP between different AS
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The exchange of the routing information
between two routers using BGP takes place in
a session.
For that BGP uses the services of TCP; when a
TCP connection is created for BGP it lasts for
long.
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Types of BGP messages