Chapter 5 – Routing Protocols: IGRP

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Chapter 5 Routing Protocols IGRP
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Building a Network

• To Be Reliable provide error detection and
  ability to correct errors
• To Provide Connectivity incorporate a variety
  of hardware software products so they can
  function together
• To Be Easy To Use users need to have no concern
  for the networks structure and implementation
• To Be Easy To Modify allow itself to evolve and
  adapt as needs change
• Be Easy To Implement must follow industry
  standards and allow a variety of configurations

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Path Determination

• Usually the responsibility of the router

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Path Determination

• Enables a router to evaluate the available paths
  to a destination and establish best path for the
  packet
• Most routing protocols use simply use the
  shortest and best path
• Analogy Packet routing is like driving a car
  Routers through the use of protocols make path
  decisions based on routing tables, and people
  driving cars determine their paths by reading
  road signs

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Metrics

• Routing tables must be updated and accurate
• Each routing protocol interprets the best path
  in its own way
• Protocol generates a value, METRIC, for each path
  through the network
• Smaller the metric the better the path
• Metrics differ depending on the protocol being
  used

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Metrics
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Metrics
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Metrics
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Router Forwarding Decisions

• Router examines the packets destination protocol
  address it either knows or does not know how to
  forward to the next hop
• Does not know and no default route assigned - the
  packet is typically dropped
• Network portion of IP address used by router
  within the network cloud
• The source and destination IP addresses never
  changes as a packet traverses the network

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Router Forwarding Decisions

• Network portion of the address is used to make
  path selection
• Router responsible for passing the packet to the
  next network along the path
• Switching function allows the router to accept a
  packet on one interface and forward it another
  interface

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Routing Protocols

• ROUTED protocols moved over a network
• TCP/IP
• IPX
• AppleTalk

• ROUTING protocols route routed protocols
  through a network
• IGRP
• EIGRP
• OSPF
• BGP
• OSI routing
• Advanced Peer-to-Peer Networking(APPN)
• Intermediate System-to-Intermediate System(IS-IS)
• RIP

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Routing Protocols Continued

• End systems (computers) use routed protocols to
  talk to each other
• IP
• Routers (intermediate systems) use routing
  protocols to talk to each other about networks
  and paths

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Multi-Protocol Routing

• Routers are capable of supporting multiple
  independent routing protocols
• IGRP
• RIP
• Allows routers to deliver packets from several
  routed protocols over the same data links
• TCP/IP
• IPX
• Apple Talk

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Differentiating Routing Protocols from One Another

• Can be differentiated from one another by
• Particular goals of the designer
• Various types - each has a different effect on
  the network
• Routing protocols use a variety of metrics to
  identify best paths

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Routing Protocols

• Interior Protocols
• Used for routing information within networks that
  are under a common administration
• All IP interior protocols must be specified with
  a list of associated networks before routing can
  occur
• CISCO supports RIP and IGRP

• Exterior Protocols
• Used to exchange information between networks
• Require the following before routing can begin
• List of neighbor routers
• List of networks to advertise as directly
  reachable
• EGP and BGP

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Goals of Routing Protocols

• Optimal Route
• Select best route
• Depends on metric and metric weighting
• Simplicity and Efficiency
• Efficiency is important when software
  implementing the routing protocol must run on a
  computer with limited resources
• Robustness
• Should perform correctly at all times
• Hardware failures, high load conditions and
  incorrect implementation

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Goals of Routing Protocols Continued

• Rapid Convergence
• Must converge rapidly
• Speed and ability of a group of devices to agree
  on the topology after a change has occurred
• Flexibility
• Quickly and accurately adapt to a variety of
  network circumstances

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Routing Loops

• Packet continues to bounce back and forth between
  to devices until
• Device is updated
• Packet is switched the maximum number of times
  allowed
• Different routing protocols have different
  maximums
• IGRP has a maximum hop count of 255 it defaults
  to 100
• Usually set to 50 or less

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Static and Dynamic Routing
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Classifications of Routing Protocols

• Most can be classified into three basic
  approaches
• Distance vector routing
• Determines the direction and distance to any link
  in the network
• IGRP and RIP
• Link-state routing (shortest path first)
• Re-creates exact topology of the entire network
• OSPF, IS-IS, NLSP
• Hybrid approach
• Combines both aspect of link-state and distance
  vectoring
• EIGRP

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Classes of Routing Protocols
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IP Routing Configuration

• Any routing protocol must follow two steps
• Create routing process with one of the router
  commands
• Configure the protocol specifics
• Interior routing protocols also must have a list
  of networks specified before routing begins
• IGRP requires an AS (Autonomous System) number

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IP Routing Configuration

• Choosing a routing protocol consider the
  following
• Network size and complexity
• Network traffic levels
• Security needs
• Reliability needs
• Network delay characteristics
• Organizational policies
• Organizational acceptance of change

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IGRP Overview
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IGRP Metrics

• CISCO proprietary developed to supercede RIP
• Distance vector interior routing protocol
• Uses a combination of metrics
• Network delay, bandwidth, reliability and load
  all factored into the routing decision

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IGRP Continued

• Provides a wide range for its metrics
• For example
• Reliability value between 1 and 255
• Bandwidth values reflecting speeds from 1200
  bps to 10 Gbps
• Delay value from 1 to 224
• Default values related to weightings for IGRP
  give the most importance to bandwidth makes
  IGRP superior to RIP
• Contrast with RIP(does not weigh metrics) it
  only uses one

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Distance-Vector Routing
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IGRP Operation

• Goal of IGRP was create a robust protocol for
  routing within an AS
• IGRP uses a combination of user-configurable
  metrics
• IGRP advertises three types of routes
• Interior
• System
• Exterior

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IGRP OperationInterior Routes

• Interior routes between subnets in the network
  attached to a router interface
• If the network attached to router is not
  subnetted IGRP does NOT advertise interior
  routes
• Subnet information is not included in IGRP
  updates

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IGRP Operation-System Routes

• Routes to other major networks within AS
• Router derives system routes from directly
  connected interfaces and system route information
  from other routers that use IGRP
• System routes do NOT include subnetting
  information

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IGRP Operation-Exterior Routes

• Routes to networks outside the AS that are
  considered when identifying the gateway of last
  resort
• Router uses the gateway of last resort if it does
  not have a better route
• If AS has more than one connection to an external
  network - different routers can choose different
  exterior routers as the gateway of last resort

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Autonomous Systems
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Command Sequence for Enabling IGRP on a Router
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Features that Enhance Stability of IGRP -
Holddowns

• Used to avoid routing loops in the network
• Effect of increasing topology convergence time
• Used to prevent regular update messages from
  reinstating a route that may have gone bad
• Tell routers to hold down any changes that might
  effect routes for some period of time
• Period of time is calculated to be just greater
  than time to update entire network

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Features that Enhance Stability of IGRP Split
Horizons

• Occur when router tries to send information about
  a route back in the direction that it came
• Helps prevent routing loops
• Implemented in IGRP

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Features that Enhance Stability of IGRP Poison
Reverse Updates

• Intended to defeat larger routing loops
• Poison reverse updates are sent to remove the
  route and place it in holddown
• Poisoning the route can speed convergence

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IGRP Metrics and routing Updates

• IGRP uses several types of metric information
• For each path through an AS IGRP records
• Segment with lowest bandwidth
• Accumulated delay
• Smallest maximum transmission unit (MTU)
• Reliability and load

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IGRP Metrics and routing Updates

• Bandwidth by default is given the most importance
• Router running IGRP sends updates every 90
  seconds
• Declares route inaccessible if an update from
  first router is not sent within 3 update periods
  (270 seconds)
• after 5 update periods (450 seconds) router
  removes the route from the routing table
• Uses flash update and poison reverse to speed up
  convergence

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IGRP Metrics and routing Updates

• Flash Update
• Sending of update sooner than standard periodic
  update interval
• Poison Reverse Updates
• Intended to defeat larger routing loops
• Sent to remove a route and place it in holddown

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IGRP Maximum Hop Count

• Maximum hop count 255
• Normally set lower than the default of 100
• Should be number at least greater than the
  maximum number of routers a route might have to
  go through

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The End