Post on 14-Jan-2016
Routing
Networks and Protocols
Prepared by: TGK First Prepared on: Last Modified on:
Quality checked by: Copyright 2009 Asia Pacific Institute of Information Technology
Networks and Protocols
Routing
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Topic & Structure of the lesson
Routing CharacteristicsRouting StrategiesRouting Algorithms
Networks and Protocols
Routing
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Learning Outcomes
At the end of this module, YOU should be able to:
• Discuss the characteristics required in performing routing functions
• Discuss the routing strategies used to determine appropriate communication path
• Discuss the operations of Link State and Distance Vector routing algorithms
Networks and Protocols
Routing
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Key Terms you must be able to use
If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:
• Routing characteristics• Routing strategies• Routing algorithms
Networks and Protocols
Routing
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Main Teaching Points
• Route selection• Minimum hop• Fixed• Flood• Random• Adaptive• Link State• Distance Vector• Dijkstra’s• Bellman-Ford• Link State Advertisements
Networks and Protocols
Routing
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Routing Characteristics
• Complex, crucial aspect of packet switched networks
• Characteristics required
• Correctness and simplicity
• Robustness and stability
• Fairness and optimality
• Efficiency
• Used for selection of route
• Minimum hop
• Least cost
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Routing Characteristics
Networks and Protocols
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Routing Strategies
• Fixed• Flooding• Random• Adaptive• Fixed:
• Single permanent route for each source to destination pair
• Determine routes using a least cost algorithm• Route fixed, at least until a change in network
topology• Flooding RoutingFlooding Routing
• No network info required• Packet sent by node to every neighbor
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• Incoming packets retransmitted on every link except incoming link
• Eventually a number of copies will arrive at destination
• Each packet is uniquely numbered so duplicates can be discarded
• Nodes can remember packets already forwarded to keep network load in bounds
• Can include a hop count in packets
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Figure 10.8 Flooding Example(hop count = 3) William Stalling –D&C Communication
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Routing Strategies
• Properties of Flooding Routing• All possible routes are tried• Very robust• At least one packet will have taken minimum hop
count route• Can be used to set up virtual circuit• All nodes are visited• Useful to distribute information (e.g. routing)
• Random RoutingRandom Routing• Node selects one outgoing path for retransmission of
incoming packet• Selection can be random or round robin• Can select outgoing path based on probability
calculation
Networks and Protocols
Routing
• No network information needed• Route is typically not least cost nor minimum hop
• Adaptive RoutingAdaptive Routing• Used by almost all packet switching networks• Routing decisions change as conditions on the
network change (failure, congestion, etc.)• Requires information about network• Decisions more complex• Tradeoff between quality of network info and
overhead• Adaptive Routing ClassificationAdaptive Routing Classification
• Based on information sources• Local (isolated)
• Route to outgoing link with shortest queue• Can include bias for each destination
Routing Strategies
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• Rarely used - do not make use of easily available info
• Adjacent nodes• All nodes
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Routing Algorithms
• Distance Vector
• Also known as Bellman-Ford algorithm
• Passes periodic copies of a routing table
• Regular updates between routers communicate
topology changes
• Perform routing decisions based on the information
by neighboring routers
• Based on the distance (number of hops) and vector
(direction)
• Before sending an update, each router adds its own
distance value to the route's metric
Networks and Protocols
Routing
• When a router receives an update, it maps the learned
network to the receiving interface
• The router then uses that interface to reach those
destinations
• Maintain a database of network topology information
• Send its entire routing table to each of its adjacent
neighbors
• Router does not know the exact topology of an
internetwork. Why?
• Update includes a list of subnets and the distances in
metric
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• Routers do not know the network topology except the
neighboring routers
• If there’s multiple routes, chooses the best route which has
the lowest metric
• Limitations
• Distance vector are classful routing protocols
• Do not send subnet information in updates
• Do not support scalability features such as Variable
Length Subnet Masking (VLSM) or super-netting
• Converge slowly compared to link-state protocols – not
suitable for complex and scalable internetworks
Routing Algorithms
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Routing Algorithms
• Examples of Distance Vector Routing Protocol
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced IGRP (EIGRP) – balanced hybrid
• Border Gateway Protocol (BGP)
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• Link State
• Uses Shortest Path First (SPF) algorithms/Dijkstra
Algorithms
• Flood routing information to all routers in the internetwork
that creates a map of the entire network
• Assemble all the information to calculate the shortest path
routes and builds routing tables showing the best path
• Once converged, send update packets, which contain only
changes rather than whole routing table
• Update packets are passed across the network in event-
triggered updates
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Routing Algorithms
• Builds a complete database of all the link states
• Router gathers enough information to create the
network map
• Each router individually runs the SPF algorithm on its
link-state database
• to identify the best paths
• These paths form a tree with the local router as its
root
• Advertise the states of their links to all other routers
in the area
Networks and Protocols
Routing
• When a network link changes state, a notification
called a link-state advertisement (LSA) is flooded
throughout the network
• All the routers note the change and adjust their routes
accordingly
• Also has periodic re-flooding of LSA but longer
interval time
• More reliable, easier to debug, and less bandwidth-
intensive
• Link-state advertisements (LSAs) – routing
information that is sent between routers
Routing Algorithms
Networks and Protocols
Routing
• Link-state advertisements (LSAs) – routing information that
is sent between routers
– Router LSA – identifies router ID, router interfaces IP
addresses, status (up or down) of each interface, cost
(metric) associated with each interface
– Link LSA – identifies each subnet and routers that are
attached to that subnet
Topological database – collection of information (data
structure) gathered from LSAs
SPF algorithm – calculation performed on the database
resulting in the SPF tree
Routing Algorithms
Networks and Protocols
Routing Routing tables – A list of the known paths and interfaces
Advantages
Faster Convergence
Improved bandwidth utilization
Support Classless Inter-Domain Routing (CIDR), VLSM,
and super-netting
Limitations
Processor overhead
Memory requirements
Bandwidth Consumption
Complex administration
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Quick Review Question
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Follow Up Assignment
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Summary of Main Teaching Points
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Q & A
Question and Answer Session
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Routing
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Topic and Structure of next session
• Networking and Internetworking Devices
Repeaters Bridges Routers Gateways
Next Session