Nonroutable, Routed, and Routing Protocols

1
Chapter 4

• Nonroutable, Routed, and Routing Protocols

2
Learning Objectives

• Differentiate nonroutable, routed, and routing
  protocols
• Define Interior Gateway Protocols, Exterior
  Gateway Protocols, distance-vector routing
  protocols and link-state routing protocols
• Explain the concepts of count-to-infinity, split
  horizon, split horizon with poison reverse, and
  hold-down timers
• Describe, configure, and monitor the interior
  routing protocols RIP and IGRP

3
Chapter Overview

• In Chapter 4 of CCNA Guide to Networking
  Fundamentals, you were introduced to the concepts
  of routed versus routing protocols.
• This chapter is a continuation and expansion of
  that discussion.
• In particular, this chapter adds the concept of
  nonroutable protocols while at the same time,
  expanding and defining several classes of routed
  protocols.
• In addition, you will learn the proper way to
  configure and monitor Routing Information
  Protocol (RIP) and Interior Gateway Routing
  Protocol (IGRP) on Cisco routers.

4
In The Beginning

• In the beginning, networks were small collections
  of computers tied together for the purpose of
  sharing expensive peripherals such as high-end
  laser printers.
• Few companies could afford to link all their
  computers together on a Local Area Network (LAN).
  
• Instead, using coaxial cable, computers were
  hooked together in work groups as shown on the
  next slide.

5
Workgroup Configuration
6
Nonroutable Protocols

• Early networks were sometimes configured as
  peer-to-peer networks because they communicated
  with and provided services to their peers.
• Peer-to-peer networks do not pass packets between
  multiple networks.
• All communication occurs on the one small segment
  where the peer-to-peer network exists.
• Protocols that contain network source and
  destination information, and are large and
  inefficient, are not needed.
• Instead, peer-to-peer networks can use the small
  and efficient nonroutable protocols.

7
NetBeui

• Several nonroutable protocols exist in todays
  networking world, but NetBEUI, is the most
  common.
• NetBEUI ships with all Microsoft Windows
  operating systems.
• In small, peer-to-peer networks, NetBEUI is easy
  to configure and use. Since it is very small, it
  is fast and efficient.
• Unfortunately, NetBEUI cannot scale into large
  internetworks because it cannot hold Network
  layer information in its frame header. Without
  this information, packets cannot be routed
  between multiple network segments.
• If you try to use any nonroutable protocol in a
  network with multiple networks, communication
  between the networks will fail.

8
Routed Protocols

• Routed protocols have packet headers that can
  contain Network layer addresses.
• Routed protocols use network addresses to build
  route tables that routers use to determine to
  which network a particular packet is destined.
• Routed protocols were developed to support
  networks consisting of multiple network segments
  or multiple networks.
• The next slide shows a sample internetwork.

9
Internetwork Example
10
Routed Protocols Continued

• In the sample internetwork, Host Alpha can
  communicate with Host Bravo only if Host Alpha
  uses a protocol that can add Network layer
  addressing to each packet header.
• Without the Network layer information, all
  packets are only able to communicate within
  Network 1.
• Transmission Control Protocol/Internet Protocol
  (TCP/IP) and Internetwork Packet
  eXchange/Sequence Packet eXchange (IPX/SPX) are
  two protocol stacks that support Network layer
  information.

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IP Addresses

• For routed protocols to work on a TCP/IP network,
  every device (computer, printer, and router
  interface port) must be configured with a unique
  IP address.
• These Network layer, logical addresses allow
  TCP/IP packets to be routed throughout the
  internetwork.
• Figure 4-3 on the next slide shows a common
  internetwork with IP addresses.

12
Internetwork With IP
13
Quick Quiz

• What was the older definition of a computer
  workgroup?
• Why cant NetBEUI be used on internetworks?
• What are two common protocol stacks that support
  network layer information?
• In terms of a packet or frame, where does the
  network layer address live?
• How does a router determine which network a
  packet should be forwarded to?

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Routing Protocols

• For proper network connectivity, you need more
  than just routed protocols on large
  internetworks.
• Because routers must be able to find the correct
  paths to route routed protocols, they use routing
  protocols to build route tables that specify
  exactly where every network in the internetwork
  is located.
• Routing protocols are protocols used by routers
  to make path determination choices and to share
  those choices with other routers.
• The next slide shows a conceptual routing table
  for RouterB.

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Routing Protocols Example
16
Routing Protocols Continued

• The previous table shows that RouterB can reach
  any of the networks in our internetwork.
• The Distance column refers to hop count as the
  single metric used in this route table.
• Hop count is the number of routers a packet must
  pass through to reach a particular network.
• A metric is a value used to define the
  suitability of a particular route.
• Routers use metrics to determine which routes are
  better than other routes.

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Routing Protocols Continued

• In the internetwork shown, routing metrics are
  simple because of the single path nature of the
  internetwork.
• The route to Network 4 from RouterB will be via
  RouterA and Router C there is no better path
  available.

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Interior Gateway Routing Protocols

• An Autonomous System (AS), which uses Interior
  Gateway Protocols as routing protocols, is a
  group of routers under the control of a single
  administration.
• The next slide shows Big Tin Inc.'s autonomous
  system.
• Big Tin Inc. has an autonomous system consisting
  of the four routers under the control of local
  network engineers.
• In general, AS systems run a single routing
  protocol.

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Autonomous System Example
20
Interior Gateway Routing Protocols Continued

• Routing protocols come in two major categories
• Interior Gateway Protocols (IGPs) are the routing
  protocols used within an AS
• Exterior Gateway Protocols (EGP) are routing
  protocols used to route between multiple
  autonomous systems.
• Routing Information Protocol (RIP), Interior
  Gateway Routing Protocol (IGRP), Enhanced
  Interior Gateway Routing Protocol (EIGRP), and
  Open Shortest Path First (OSPF) are examples of
  IGP.
• Border Gateway Protocol (BGP) and Exterior
  Gateway Protocol (EGP) are examples of EGP.

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Two Types of IGP Routing Protocols

• Interior Gateway Routing protocols are subdivided
  into two major types of routing protocols
• distance-vector
• link-state
• These protocol types accomplish the same
  jobdetermining routes within an autonomous
  systembut they do so via different mechanisms.

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Distance Vector Routing Protocols

• Distance vector routing protocols broadcast their
  entire routing table to each neighbor router at
  predetermined intervals. The next slide shows how
  this process occurs.
• As the updates propagate throughout the network,
  RouterC will only receive information about
  RouterBs route table via RouterA.
• This is sometimes referred to as routing by
  rumor. It also is one of the main problems with
  distance-vector routing protocols.
• If RouterB and RouterA have an update interval of
  30 seconds, RouterC will not learn about network
  topology changes on RouterB for up to a minute.

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Distance-Vector Routing Example
24
Distance Vector Routing Continued

• The example shows that the time it will take the
  network to converge depends on the amount of time
  between update intervals on RouterB and RouterA.
  
• Due to its small size, the amount of time between
  convergence would be fairly minimal.
• Still, for a short amount of time, RouterC is
  working on the assumption that the network is
  configured differently than it actually is.

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Routing Loops

• Problems, such as routing loops, can occur with
  distance-vector protocols if control measures are
  not put in place.
• Routing loops are often referred to as
  count-to-infinity problems because loops, without
  preventive measures cause packets to bounce
  around the internetwork for an infinite amount of
  time.
• The next slide illustrates the types of problems
  that can occur with routing loops.

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Routing Loops Example
27
Routing Loops Continued

• In this internetwork, true loops are not possible
  because of the linear nature of the network
  design.
• Still, the scenario presented in the example
  shows that the internetwork could, without proper
  precautions, readvertise a route that was not
  accessible.
• To prevent these problems, techniques such as
  defining a maximum, split horizon, split horizon
  with poison reverse, and hold-down timers are
  used.
• These measures reduce the chances that incorrect
  route table information will be propagated.

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Hop Count

• Defining a maximum is one of the easiest ways to
  limit count-to-infinity problems.
• If you assign a packet a maximum hop count, it
  cannot bounce infinitely around the internetwork.
  
• RIP, one of the most common distance-vector
  protocols, defines a maximum hop count of 15.
• If a routing loop did occur on a RIP
  internetwork, the packet only travels through 15
  hops before the packet exceeds its time to live
  (TTL) and is dropped.

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Split Horizon

• Split horizon and split horizon with poison
  reverse are two other common ways to prevent
  routing loops.
• Split horizon controls what information a router
  will send out about particular routes.
• Routers will not send information back through an
  interface about an advertised route from that
  interface.
• For example, if RouterA shown in the previous
  example uses split horizon, it will not accept
  the update from RouterC at Time 4.
• It wont accept it because that update could not
  be sent out of the interface from which RouterA
  learned that E0 on RouterB was down in the first
  place.

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Split Horizon With Poison Reverse

• If RouterA uses split horizon with poison
  reverse, it not only refuses to send RouterCs
  update to RouterB, but it also responds to
  RouterCs attempted update.
• RouterA will tell RouterC that the route to E0 is
  no longer available by indicating that the hop
  count has been exceeded.
• In other words, it poisons the erroneous route
  advertised by RouterC so that no other router
  will see this as a viable route.

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Hold-Down Timers

• Another common technique used to stop routing
  loops is the hold-down timer.
• Hold-down timers allow a router to place a route
  in a state where it will not accept any changes
  to that route.
• If RouterA uses hold-down timers in the previous
  example, the update from RouterC is ignored
  because the route would be in hold down for a
  period of time after it was marked down.

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Link-State Routing Protocols

• Routers configured with a link-state routing
  protocol use Link-State Advertisements (LSAs) to
  inform all routers on the internetwork of their
  route tables.
• Link State Packets (LSPs) send out LSAs, to
  allow every router in the internetwork to share a
  common view of the topology of the internetwork.
  
• Link-state routing protocols use the Shortest
  Path First (SPF) algorithm to determine the best
  paths in the internetwork.
• The next slide shows how a router configured with
  a link-state routing protocol floods or
  broadcasts LSPs to the network.

33
Link-State Routing Example
34
Link-State Routing Example Continued

• In the example, the network quickly reaches a
  state of convergence due to the flooding of
  link-state packets.
• This is a big advantage link-state routing
  protocols have over distance-vector routing
  protocols.
• Later updates by the routers in the internetwork
  will be triggered updates.
• These updates occur due to network topology
  changes, not periodic route table advertisements.
  
• This announcement contains only the changes in
  the route table, not the entire route table. This
  conserves bandwidth on the internetwork links.

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Link-State Problems

• The Shortest Path First algorithm is very complex
  and so has it has high overhead. This complexity
  requires demands more memory and processing power
  than distance- vector routing protocols.
• The table below summarizes distance-vector and
  link-state.

36
Quick Quiz

• What are routing protocols used for?
• What is an autonomous system?
• What are the two major categories of routing
  protocols?
• What is hop count?
• What is a metric?

37
RIP Details

• RIP is a distance-vector routing protocol that
  broadcasts entire routing tables to neighbors
  every 30 seconds out of every interface that on
  which it is configured.
• RIP uses hop count as its sole metric.
• RIP has a maximum hop count of 15. As a result,
  RIP does not work in large internetworks.
• RIP is susceptible to all the problems normally
  associated with distance-vector routing
  protocols.
• slow to converge
• 2nd hand information
• entire table is passed
• periodic updates

38
Configuring RIP

• RIP is an easy to install and configure protocol.
  
• To configure RIP on a Cisco router using TCP/IP,
  you must perform the following two tasks
• Enable RIP routing globally
• Configure each major network
• You need only to configure major network numbers
  with RIP because RIP does not maintain subnet
  mask information within the route tables it
  produces.
• In other words, RIP only knows about networks,
  not hosts.
• Remember that there are actually two versions of
  RIP. This section focuses on RIP version 1.
  RIPv1 does not pass subnet mask information with
  its route table updates.

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Enabling RIP Routing

• The next slide illustrates an example network for
  this discussion.
• We will focus on enabling RIP on RouterB.
• Assume that RIP has already been enabled on
  RouterA and RouterC.
• To start configuring RIP, you must first enter
  privileged EXEC mode and then global
  configuration mode on your router.
• Once in global configuration mode, you must
  enable RIP with the router rip command.

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Enabling RIP Example
41
RIP for Each Major Network

• The commands necessary to configure RIP on
  RouterB are shown on the next slide.
• The network network command turns RIP routing
  on for the major class B network 172.22.0.0.
• This major network is subnetted but only the
  major network is configured.
• If you have multiple major networks configured on
  a router, an individual network network
  command must be issued for each separate network.

42
Configuring RIP
43
Updates

• After you have enabled RIP routing globally and
  configured each major network that the router
  will advertise with RIP updates, RIP is fully
  configured on the router.
• After the update interval of 30 seconds passes on
  each router, RouterB will learn of all networks.
  
• The show IP route command displays all ip routes
  on the router and how those routes were learned.
• The show IP route command for the previous
  example is shown on the next slide.

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Show IP Route Output
45
Administrative Distance

• The show IP route output shown on the next slide
  illustrates an extremely important concept called
  administrative distance.
• Administrative distance is a value used to
  determine the reliability of a particular route.
  
• The table below shows common routing protocols
  and their administrative distances.

46
Administrative Distance Continued

• If a route is being discovered using both RIP and
  a directly connected interface, the route
  available via the directly connected interface
  will be the preferred route.
• The router will choose the directly connected
  interface because it has a lower administrative
  distance.
• Likewise, if both IGRP and RIP advertise a route
  for a particular network, the IGRP route will be
  used because it is considered more reliable due
  to its lower administrative distance.

47
Show IP Prot Debug IP Commands

• Use the show ip protocol and debug ip rip
  commands to monitor RIP.
• Type the show ip protocol command in either user
  mode or privileged mode.
• Show ip protocol command output is shown on the
  next slide.

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Show IP Protocol Output
49
Show IP Protocol Command

• RIP updates on TCP/IP networks, as stated
  previously, occur every 30 seconds.
• A route is considered invalid if six consecutive
  update intervals pass without an update from that
  route.
• The hold down time of 180 seconds allows the
  router to stabilize its route table to help
  prevent routing loops when a network path does go
  down.
• The flush interval is the time at which a route
  will be totally removed from the route table if
  no updates are received.

50
Debug IP Rip Command

• The debug ip rip command, like all debug
  commands, should only be used when
  troubleshooting RIP.
• This command places very high processing demands
  on your router and could affect network
  performance.
• The next slide shows the output of the debug ip
  rip command.
• Notice that you must be in privileged EXEC mode
  to use the debug commands.

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Debug IP Rip Output
52
Convergence

• RIP, like most distance-vector routing protocols
  is slow to converge.
• Use the show ip route command to display the
  change in status of a network when it becomes
  available.
• The results of the show ip route command after
  Ethernet0 on RouterC becomes inaccessible is
  shown on the next slide.

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Show IP Route Output
54
Convergence Continued

• Eventually, a downed route will be flushed from
  the route table.
• If you issue the show ip route command after the
  route has been flushed from the route table, you
  will get the router output displayed below.

55
Hop Count Dilemma

• Note that RIP relies on hop count as its single
  metric.
• In the network shown below, a router configured
  to use RIP would always route packets to the
  subnet 172.22.5.0 via the 56kbps link between
  RouterB and RouterC, because of the hop count of
  one.

56
Quick Quiz

• What is a distance-vector protocol?
• What is a link-state protocol?
• What are the disadvantages of RIP?
• What are the disadvantages of a link-state
  protocol?
• What are the two commands used to configure RIP?

57
IGRP

• IGRP is a distance-vector routing protocol
  created by Cisco to solve some of the problems
  associated with RIP.
• A larger hop count metric of 255 allows IGRP to
  be used on larger networks.
• Also, IGRP uses the following additional metrics
• Load The measure of the greatest load on a link
  in the route
• Bandwidth The slowest link in the route
• Reliability Measures reliability with a scale of
  0 to 255
• Delay Delay of all links on a route
• Maximum Transmission Unit (MTU) The smallest MTU
  along the path is used to find the route

58
IGRP Details

• By default, IGRP computes the best available
  route using only bandwidth and delay, but it can
  be configured to use all of the metrics mentioned
  previously.
• The ability to use bandwidth as a factor in the
  route selection process, along with reliability
  and delay, allows IGRP to make more intelligent
  route choices than RIP.
• IGRP also has the ability to use multiple,
  different cost paths to allow for redundancy and
  load balancing.
• IGRP can support up to four different cost paths.
  

59
Configuring IGRP

• Configuring IGRP on a Cisco router using TCP/IP
  is almost as simple as configuring RIP.
• Use the router igrp autonomous system and
  network network commands.
• The commands to configure IGRP are shown below.

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Monitoring IGRP

• Use the show ip route command to monitor all
  available IGRP routes.
• Command output is shown below.

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Monitoring IGRP Continued

• To see the IGRP timers, use the show ip protocol
  command.
• It will present output similar to that shown
  below.

Figure 4-20
62
More IGRP Details

• Notice from the previous slide that IGRP sends
  out routing table updates every 90 seconds.
• This reduces the amount of broadcast traffic used
  to maintain routing tables.
• It is possible to run multiple routing protocols,
  for example, RIP and IGRP on one router.
• In the real world, you would normally not
  configure your routers to run both RIP and IGRP
  on the same interfaces.
• Doing so would waste network bandwidth on RIP
  updates that will always be considered inferior
  to IGRP updates.

63
Debug IP IGRP Command

• A final command available to monitor IGRP is the
  debug ip igrp command.
• This command has two options
• events
• transactions
• The events subcommand provides details on what
  updates are being sent and received and on which
  interfaces.
• The transactions subcommand provides details on
  the contents of IGRP updates being received and
  sent includes network numbers and metrics.
• Example output from the debug ip igrp events
  command is shown on the next slide.

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Debug IP IGRP Events Output
65
Quick Quiz

• What are the advantages of IGRP?
• What are the commands used to configure IGRP?
• What information will the show IP route command
  display?
• What information will the show IP protocol
  command display?
• What information will the debug IP RIP command
  display?

66
Chapter Summary

• Protocols vary in their functions.
• Some protocols are designed to be used in small
  networks without the need for Network layer
  addressing.
• These protocols are described as nonroutable
  protocols.
• The most common nonroutable protocol is NetBEUI.
• Other protocols were designed with the ability to
  move between multiple networks via Network layer
  addressing.
• These protocols are routed protocols.

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Chapter Summary Continued

• Protocols must be available to find the best path
  throughout an internetwork and relay that
  information to routers.
• Routing protocols serve this function on modern
  networks.
• Routing protocols are classed in the two major
  groups Interior Gateway Protocols and Exterior
  Gateway Protocols.
• Interior Gateway Protocols are routing protocols
  that function within a single autonomous system.
  
• Exterior Gateway Protocols function as routing
  protocols between autonomous systems.

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Chapter Summary Continued

• Routing protocols are further divided into
  distance-vector routing protocols and link-state
  routing protocols.
• These two types of Interior Gateway Protocols use
  very different methods to determine the best path
  in an internetwork.
• Distance-vector protocols periodically broadcast
  entire routing tables to neighbor routers.
• Link-state protocols broadcast updates to all
  other routers on the internetwork upon startup
  and when network topology changes.

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Chapter Summary Continued

• Two common distance-vector IGPs are RIP and IGRP.
  
• RIP is an easy to install routing protocol that
  uses hop count as its sole metric.
• RIP has a hop count limit of 15.
• RIP uses split horizon, split horizon with poison
  reverse, and hold-down timers to help limit
  routing loops.
• IGRP is a distance-vector routing protocol also.
  It has a maximum hop count of 255.
• IGRP is not limited to hop count as its sole
  metric.
• IGRP can also use load, bandwidth, reliability,
  delay and maximum transmission unit, when
  determining best path.

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End of Chapter 4