Ch.16/Mod.7

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Ch.16/Mod.7 Distance Vector Routing
ProtocolsPart 2 of 2 Distance Vector Routing
and IGRP
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IGRP Features

• IGRP is a distance vector routing protocol
  developed by Cisco.
• IGRP sends routing updates at 90 second
  intervals, advertising networks for a particular
  autonomous system.
• Key design characteristics of IGRP are a follows
• The versatility to automatically handle
  indefinite, complex topologies
• The flexibility needed to segment with different
  bandwidth and delay characteristics
• Scalability for functioning in very large
  networks

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IGRP Features

• By default, the IGRP routing protocol uses
  bandwidth and delay as metrics.
• Additionally, IGRP can be configured to use a
  combination of variables to determine a composite
  metric.
• Those variables include
• Bandwidth
• Delay
• Load
• Reliability

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IGRP Metrics
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IGRP Metrics

• The metrics that IGRP uses are
• Bandwidth The lowest bandwidth value in the
  path
• Delay The cumulative interface delay along the
  path
• Reliability The reliability on the link towards
  the destination as determined by the exchange of
  keepalives
• Load The load on a link towards the destination
  based on bits per second
• NO MTU The Maximum Transmission Unit value of
  the path. MTU has never been used by IGRP or
  EIGRP as a routing metric.
• IGRP has an administrative distance of 100, more
  trustworthy than RIP at 120.
• This means a Cisco router will prefer an IGRP
  learned route over a RIP learned route to the
  same network.

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Administrative Distances
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IGRP Metrics
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IGRP Routes

• InteriorInterior routes are routes between
  subnets of a network attached to a router
  interface. If the network attached to a router is
  not subnetted, IGRP does not advertise interior
  routes.
• Clarification
• IGRP also advertises three types of routes
• interior, system, and exterior.
• Interior routes are routes between subnets in the
  network attached to a router interface.
• If the network attached to a router is not
  subnetted, IGRP does not advertise interior
  routes.

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IGRP Routes

• SystemSystem routes are routes to networks
  within an autonomous system. The Cisco IOS
  software derives system routes from directly
  connected network interfaces and system route
  information provided by other IGRP-speaking
  routers or access servers. System routes do not
  include subnet information.

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IGRP Routes

• ExteriorExterior routes are routes to networks
  outside the autonomous system that are considered
  when identifying a gateway of last resort. The
  Cisco IOS software chooses a gateway of last
  resort from the list of exterior routes that IGRP
  provides. The software uses the gateway (router)
  of last resort if a better route is not found and
  the destination is not a connected network. If
  the autonomous system 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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IGRP Timers

• IGRP has a number of features that are designed
  to enhance its stability, such as
• Holddowns
• Split horizons
• Poison reverse updates

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IGRP Timers
Update timer

• The update timer specifies how frequently routing
  update messages should be sent.
• The IGRP default for this variable is 90 seconds.

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IGRP Timers
Invalid timer

• The invalid timer specifies how long a router
  should wait in the absence of routing-update
  messages about a specific route before declaring
  that route invalid (unreachable), but still in
  the routing table.
• The IGRP default for this variable is three times
  the update period or 270 seconds.
• Then placed in the holddown state.
• If I havent heard from you in 270 seconds, I am
  considering this route as unreachable, I will
  start the holddown timer, but I will keep it in
  the routing table until the flush timer expires.

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IGRP Timers
Holddown timer

• The holddown timer specifies the amount of time
  for which information about poorer routes are
  ignored.
• Zinin Holddown specifies the number of seconds
  that a route must spend in holddown state after
  expiration of the Invalid Timer.
• The IGRP default for this variable is three times
  the update timer period plus 10 seconds 280
  seconds.
• The original route is still in the routing table
  but marked as unreachable, until the flush timer
  expires.

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IGRP Timers
Flush timer

• Finally, the flush timer indicates how much time
  should pass before a route is flushed from the
  routing table.
• The IGRP default is seven times the routing
  update timer or 630 seconds.
• Zinin Flush specifies the number of seconds
  that a route must remain in the routing table in
  the garbage collection state after it exits the
  holddown state.
• Each time an update is received the invalid and
  flush timers are reset.
• If the invalid timer expires before another
  update is heard, the route is marked as
  unreachable, but remains in the routing table.
• If the flush timer then expires before another
  update is heard, the route will be deleted from
  the routing table.

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IGRP Timers

• All timers begin at the same time.
• Update timer 90 seconds
• Invalid timer 270 seconds
• Holddown timer 280 seconds
• Flush timer 630 seconds
• Today, IGRP is showing its age, it lacks support
  for variable length subnet masks (VLSM).
• Enhanced IGRP (EIGRP) supports VLSM.

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Configuring IGRP

• Same network commands as RIP.
• IGRP AS number must be the same on all routers.

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Configuring IGRP
Router(config-router)router igrp
100 Router(config-router)timers basic update
invalid holddown flush sleeptime Router(config-r
outer) no timers basic

• timers basic (IGRP)
• To adjust Interior Gateway Routing Protocol
  (IGRP) network timers, use the timers basic
  router configuration command. To restore the
  default timers, use the no form of this command.

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Migrating from RIP to IGRP

• Router(config)router rip
• Router(config-router)network 172.16.0.0
• Router(config-router)network 192.168.1.0
• Router(config-router)exit
• Router(config)router igrp 10
• Router(config-router)network 172.16.0.0
• Router(config-router)network 192.168.1.0
• Router(config-router)exit
• Router(config)no router rip

• Enable IGRP
• Suggestion Remove RIP configuration from routers
  even though the administrative distance will
  prefer RIP

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Verifying IGRP
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Verifying IGRP
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Verifying IGRP
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Verifying IGRP
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Verifying IGRP
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Troubleshooting IGRP
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Troubleshooting IGRP
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Troubleshooting IGRP
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(No Transcript)
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Summary
But there is still more!
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IGRP Metric Information
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Metric Calculation
Routergt show interfaces s1/0 Serial1/0 is up,
line protocol is up Hardware is QUICC Serial
Description Out to VERIO Internet address is
207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit,
DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set Keepalive
set (10 sec) ltoutput omittedgt
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Displaying Interface Values
Routergt show interface s0/0 Serial0/0 is up, line
protocol is up Hardware is QUICC Serial
Description Out to VERIO Internet address is
207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit,
DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set Keepalive
set (10 sec) ltoutput omittedgt
Bandwidth
Delay
Reliability
Load

• Routing Table Metric
• Default Slowest of bandwidth plus the sum of the
  delays of all outgoing interfaces from this
  router to the destination network.

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Metric Calculation

• Bandwidth
• Expressed in kilobits (show interface)
• This is a static number and used for metric
  calculations only.
• Does not necessarily reflect the actual bandwidth
  of the link.
• It is an information parameter only.
• You cannot adjust the actual bandwidth on an
  interface with this command.
• Use the show interface command to display the raw
  value
• The default values
• Default bandwidth of a Cisco interface depends on
  the type of interface.
• Default bandwidth of a Cisco serial interface is
  1544 kilobits or 1,544,000 bps (T1), whether that
  interface is attached to a T1 line (1.544 Mbps)
  or a 56K line.
• IGRP metric uses the slowest bandwidth of all of
  the outbound interfaces to the destination
  network.

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Metric Calculation

• Changing the bandwidth informational parameter
• The bandwidth can be changed using
• Router(config-if) bandwidth kilobits
• To restore the default value
• Router(config-if) no bandwidth

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Metric Calculation

• Delay
• Like bandwidth, delay it is a static number.
• Expressed in microseconds, millionths of a second
  
• (Uses the Greek letter mu with an S, ?S, NOT
  ms which is millisecond or thousandths of a
  second)
• Use the show interface command to display the raw
  value
• It is an information parameter only.
• The default values
• The default delay value of a Cisco interface
  depends upon the type of interface.
• Default delay of a Cisco serial interface is
  20,000 microseconds, that of a T1 line.
• IGRP metric uses the sum of all of the delays of
  all of the outbound interfaces to the destination
  network.

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Metric Calculation

• Changing the delay informational parameter
• The delay can be changed using
• Router(config-if) delay tens-of- ?S
• (microseconds)
• Example of changing the delay on a serial
  interface to 30,000 microseconds
• Router(config-if) delay 3000
• To restore the 20,000 microsecond default value
  
• Router(config-if) no delay

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Metric Calculation

• IGRP
• bandwidth (10,000,000/bandwidth)
• delay delay/10

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IGRP Metrics
Values displayed in show interface commands and
sent in routing updates.
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Metric Calculation
Routergt show interfaces s1/0 Serial1/0 is up,
line protocol is up Hardware is QUICC Serial
Description Out to VERIO Internet address is
207.21.113.186/30 MTU 1500 bytes, BW 1544 Kbit,
DLY 20000 usec, rely 255/255, load 246/255
Encapsulation PPP, loopback not set Keepalive
set (10 sec) ltoutput omittedgt
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From Casablanca to 172.20.40.0/24
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From Casablanca to 172.20.40.0/24

• Using BWIGRP and DLYIGRP to calculate the IGRP
  metric
• The slowest bandwidth has the highest BWIGRP
  value.
• IGRP metric
• highest BWIGRP total of the DLYIGRP
• 19,531 (100 2,000 2,000 100)
• 19,531 4,200
• 23,731

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• Calculating the IGRP MetricUsing the Raw
  Values Bandwidth and Delay

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From Casablanca to 172.20.40.0/24
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Calculating Bandwidth

• So how is Bandwidth, BWIGRP, calculated?
• The bandwidth metric is calculated by taking 107
  (10,000,000) and dividing it by the slowest
  bandwidth metric along the route to the
  destination.
• This is known as taking the inverse of the
  bandwidth scaled by a factor of 107 (10,000,000)
• The lowest bandwidth on the route is 512K or 512
  (measured in kilobits), the outgoing interface of
  the Quebec router.
• Divide 10,000,000 by 512 and you get the
  bandwidth!
• Bandwidth 10,000,000/512
• 19,531
• Which is the lowest BWIGRP along the route

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Calculating Delay

• So how is Delay, DLYIGRP, calculated?
• Delay is the total sum of delays on the outgoing
  interfaces, in 10-microsecond units
• The sum of the delays on each of the outgoing
  interfaces between Casablanca and Yalta, from
  172.20.1.0/24 through 172.20.40.0/24 is
• 1,000 (Casablanca) 20,000 (Teheran) 20,000
  (Quebec) 1,000 (Yalta) 42,000
• We need this in 10-microsecond units
• (1,000/10)(20,000/10) (20,000/10)
  (1,000/10)
• 100 2,000 2,000 100
• or
• (1,000 20,000 20,000 1,000) / 10
• In either case the total sum is
• Delay 4,200
• Which is the total of the DLYIGRP, the total
  Delays along the route!

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Slowest Bandwidth Sum of Delays

• IGRP metric Bandwidth Delay
• IGRP metric 19,531 4,200
• 23,731
• IF we were using RIP, the RIP metric would be 3
  hops.

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show ip route 172.20.40.0

• Casablanca show ip route 172.20.40.0
• Known via igrp 1, distance 100, metric 23,731
• 172.20.1.2, from 172.20.1.2 on Ethernet 0
• Route metric is 23,731
• Total delay is 42,000 microseconds,
• minimum bandwidth is 512 Kbit
• ...
• Not to be redundant, but if we were using RIP,
  the RIP metric would be 3 hops.

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Ch.16/Mod. 7 Distance Vector Routing
ProtocolsPart 2 of 2 Distance Vector Routing
and IGRP