CCNA – Semester 2




           Chapter 8:
The Routing Table: A Closer Look

       CCNA Exploration version 4.0
Objectives

  •   Describe the various route types found in the routing
      table structure
  •   Describe the routing t...
Routing Table Structure




                          3
Lab Topologies




                 4
Routing Table Structure




•   Routing table entries come from the following sources
     – Directly connected networks
 ...
Routing Table Structure




 Level 1 Routes
 • As soon as the no shutdown command is issued the route
   is added to routi...
Routing Table Structure


• Cisco IP routing table
  is a hierarchical
  structure
   – The reason for this
      is to sp...
Routing Table Structure


  •   A level 1 route is a route with a subnet mask equal to or less than
      the classful mas...
Routing Table Structure




 Ultimate Route
 • The level 1 route 192.168.1.0/24 can be further defined as an
    ultimate ...
Routing Table Structure

                          • Parent and Child
                              Routes
               ...
Classful Networks


• Automatic creation of parent
    routes
     – Occurs any time a subnet
       is added to the routi...
Routing Table Structure




                          12
Routing Table Structure




 •   Both child routes have the same subnet mask
      – This means the parent route maintains...
Routing Table Structure




  •   In classless networks, child routes do not have to
      share the same subnet mask
    ...
Routing Table Structure
 •   Parent & Child Routes: Classless Networks



                              Term variably     ...
Routing Table Structure
• Parent & Child Routes: Classless Networks




                                              16
Routing Table Lookup Process




                               17
Discontiguous
• Classful routing
  with discontiguous




                       18
Result routing




                 19
Routing Table Lookup Process
• Let's examine in more depth how the routers determine
   the best routes to use when sendin...
Routing Table Lookup Process
 Examine level 1 routes
    – If best match a level 1 ultimate route and is not a parent
   ...
Routing Table Lookup Process
• Longest Match: Level 1 Network Routes
   – Best match is also known as the longest match
  ...
Routing Table Lookup Process
 • Finding the subnet mask
   used to determine the
   longest match
     Scenario:
    – PC1...
Routing Table Lookup Process




  •   The process of matching
       – 1st there must be a match made between the parent ...
Routing Table Lookup Process




• Finding a match between packet’s destination IP address and the
  next route in the rou...
Routing Table Lookup Process




 Level 1 Parent & Level 2 Child Routes
 • Before level 2 child routes are examined, there...
Routing Table Lookup Process
• After the match with parent route has been made Level 2 child
  routes will be examined for...
Routing Table Lookup Process




                               28
Routing Table Lookup Process
•   How a router finds a match with one of the level 2 child routes
     – First router exami...
Routing Table Lookup Process

•   Example: Route Lookup
    Process with VLSM
    – The use of VLSM does not
      change ...
Routing Behavior




                   31
Routing Behavior




                   •   Classful & classless routing
                       protocols
                ...
Routing Behavior




                   33
Routing Behavior
• Classful Routing
    Behavior: no ip
    classless
•   What happens if there
    is not a match with an...
Routing Behavior
 •   Classful Routing Behavior – Search Process
 •   An example of when classful routing behavior is in e...
Routing Behavior




                   First 24 bits must match




                                              36
Routing Behavior
  Classful Routing Behavior – Search Process
  • The routing table process will not use the default route...
Routing Behavior
Classless Routing Behavior: ip classless
• Commands: Router(config)#ip classless
• Beginning with IOS 11....
Routing Behavior
• Classless Routing Behavior: ip classless
• Route lookup process when ip classless is in use
   – If cla...
Routing Behavior




                   40
Routing Behavior




                   41
Routing Behavior




                   42
Routing Behavior




                   43
Routing Behavior




•   Classful vs. Classless Routing Behavior
     – It is recommended to use classless routing behavio...
Summary




          45
46
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Ca Ex S2 M8 The Routing Table

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Ca Ex S2 M8 The Routing Table

  1. 1. CCNA – Semester 2 Chapter 8: The Routing Table: A Closer Look CCNA Exploration version 4.0
  2. 2. Objectives • Describe the various route types found in the routing table structure • Describe the routing table lookup process. • Describe routing behavior in routed networks. 2
  3. 3. Routing Table Structure 3
  4. 4. Lab Topologies 4
  5. 5. Routing Table Structure • Routing table entries come from the following sources – Directly connected networks – Static routes – Dynamic routing protocols 5
  6. 6. Routing Table Structure Level 1 Routes • As soon as the no shutdown command is issued the route is added to routing table 6
  7. 7. Routing Table Structure • Cisco IP routing table is a hierarchical structure – The reason for this is to speed up lookup process 7
  8. 8. Routing Table Structure • A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address. 192.168.1.0/24 is a level 1 network route, because the subnet mask is equal to the network's classful mask. /24 is the classful mask for class C networks, such as the 192.168.1.0 network. • A level 1 route can function as a: – Default route - A default route is a static route with the address 0.0.0.0/0. – Supernet route - A supernet route is a network address with a mask less than the classful mask. – Network route - A network route is a route that has a subnet mask equal to that of the classful mask. A network route can also be a parent route. Parent routes will be discussed in the next section. 8
  9. 9. Routing Table Structure Ultimate Route • The level 1 route 192.168.1.0/24 can be further defined as an ultimate route. An ultimate route is a route that includes: – either a next-hop IP address (another path) – and/or an exit interface 9
  10. 10. Routing Table Structure • Parent and Child Routes – A parent route is a level 1 route – A parent route does not contain any next-hop IP address or exit interface information • The subnet is the level 2 child route of the parent route. 10
  11. 11. Classful Networks • Automatic creation of parent routes – Occurs any time a subnet is added to the routing table • Child routes – Child routes are level 2 routes – Child routes are a subnet of a classful network address 11
  12. 12. Routing Table Structure 12
  13. 13. Routing Table Structure • Both child routes have the same subnet mask – This means the parent route maintains the /24 mask 13
  14. 14. Routing Table Structure • In classless networks, child routes do not have to share the same subnet mask 14
  15. 15. Routing Table Structure • Parent & Child Routes: Classless Networks Term variably Subnet mask Includes the # Parent route’s subnetted is included Network of different Classful mask seen in parent with each type masks of child is displayed route in routing child route routes table entry Classful No No No No Classless Yes Yes Yes Yes 15
  16. 16. Routing Table Structure • Parent & Child Routes: Classless Networks 16
  17. 17. Routing Table Lookup Process 17
  18. 18. Discontiguous • Classful routing with discontiguous 18
  19. 19. Result routing 19
  20. 20. Routing Table Lookup Process • Let's examine in more depth how the routers determine the best routes to use when sending packets and why classful routing protocols do not work with discontiguous designs. We will consider: 1. What happens when a router receives an IP packet, examines the IP destination address, and looks that address up in the routing table? 2. How does the router decide which route in the routing table is the best match? 3. What effect does the subnet mask have on the routing table lookup process? 4. How does the router decide whether or not to use a supernet or default route if a better match is not found? 20
  21. 21. Routing Table Lookup Process  Examine level 1 routes – If best match a level 1 ultimate route and is not a parent route this route is used to forward packet – If the best match is a level 1 parent route examines level 2 routes  Router examines level 2 (child) routes – If there is a match with level 2 child route then that subnet is used to forward packet – If no match then – Determine routing behavior type  Router determines classful or classless routing behavior – If classful then packet is dropped – If classless then router searches level one supernet and default routes – If there exists a level 1 supernet or default route match then packet is forwarded. If not packet is dropped. 21
  22. 22. Routing Table Lookup Process • Longest Match: Level 1 Network Routes – Best match is also known as the longest match – The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table. 22
  23. 23. Routing Table Lookup Process • Finding the subnet mask used to determine the longest match Scenario: – PC1 pings 192.168.1.2 – Router examines level 1 route for best match – There exist a match between192.168.1.2 & 192.168.1.0 / 24 – Router forwards packets out s0/0/0 23
  24. 24. Routing Table Lookup Process • The process of matching – 1st there must be a match made between the parent route & destination IP • If a match is made then an attempt at finding a match between the destination IP and the child route is made. 24
  25. 25. Routing Table Lookup Process • Finding a match between packet’s destination IP address and the next route in the routing table The figure shows a match between the destination IP of 192.168.1.0 and the level one IP of 192.168.1.0 / 24 then packet forwarded out s0/0/0 25
  26. 26. Routing Table Lookup Process Level 1 Parent & Level 2 Child Routes • Before level 2 child routes are examined, there must be a match between classful level one parent route and destination IP address. 26
  27. 27. Routing Table Lookup Process • After the match with parent route has been made Level 2 child routes will be examined for a match – Route lookup process searches for child routes with a match with destination IP 27
  28. 28. Routing Table Lookup Process 28
  29. 29. Routing Table Lookup Process • How a router finds a match with one of the level 2 child routes – First router examines parent routes for a match – If a match exists then: • Child routes are examined • Child route chosen is the one with the longest match Destination of 172.16.3.10 10101100 00010000 00000011 00001010 IP Packet Level 1 Parent 172.16.0.0/16 10101100 00010000 00000000 00000000 Route Level 2 Child 172.16.1.0/24 10101100 00010000 00000001 00000000 Route Level 2 Child 172.16.2.0/24 10101100 00010000 00000010 00000000 Route Level 2 Child 172.16.3.0/24 10101100 00010000 00000011 00000000 Route 23rdrd Matches Parent Route 24 bit 24 bit does not match. First 24 bit bit does not match. First First 24 bits match much match  next route entry much match  next route entry 29
  30. 30. Routing Table Lookup Process • Example: Route Lookup Process with VLSM – The use of VLSM does not change the lookup process – If there is a match between destination IP address and the level 1 parent route then – Level 2 child routes will be searched 30
  31. 31. Routing Behavior 31
  32. 32. Routing Behavior • Classful & classless routing protocols – Influence how routing table is populated • Classful & classless routing behaviors – Determines how routing table is searched after it is filled 32
  33. 33. Routing Behavior 33
  34. 34. Routing Behavior • Classful Routing Behavior: no ip classless • What happens if there is not a match with any level 2 child routes of the parent? – Router must determine if the routing behavior is classless or classful • The commands no ip classless and ip – If router is utilizing classless are global configuration classful routing commands and can be viewed by behavior then lookup typing show running-config. process is terminated • In IOS versions 11.3 and later, the and packet is dropped command ip classless is the default, implementing a classless route lookup process. 34
  35. 35. Routing Behavior • Classful Routing Behavior – Search Process • An example of when classful routing behavior is in effect and why the router drops the Packet – The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits. This means packet is dropped 35
  36. 36. Routing Behavior First 24 bits must match 36
  37. 37. Routing Behavior Classful Routing Behavior – Search Process • The routing table process will not use the default route, 0.0.0.0/0, or any other route. • A common error is to assume that a default route will always be used if the router does not have a better route. In our example, R2's default route is not examined nor used, although it is a match. This is often a very surprising result when a network administrator does not understand the difference between classful and classless routing behavior. • The reason why the router will not search beyond the child routes  Originally networks were all classful  This meant an organization could subnet a major network address and “enlighten” all the organization’s routers about the subnetting  Therefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped 37
  38. 38. Routing Behavior Classless Routing Behavior: ip classless • Commands: Router(config)#ip classless • Beginning with IOS 11.3, ip classless was configured by default • Classless routing behavior works for: – Discontiguous networks – CIDR supernets 38
  39. 39. Routing Behavior • Classless Routing Behavior: ip classless • Route lookup process when ip classless is in use – If classless routing behavior in effect then  Search level 1 routes  Supernet routes Checked first – If a match exists then forward packet – Default routes Checked second – If there is no match or no default route then the Packet is dropped 39
  40. 40. Routing Behavior 40
  41. 41. Routing Behavior 41
  42. 42. Routing Behavior 42
  43. 43. Routing Behavior 43
  44. 44. Routing Behavior • Classful vs. Classless Routing Behavior – It is recommended to use classless routing behavior so supernet and default routes can be used whenever needed – Remember that classful and classless routing behaviors are independent from classful and classless routing protocols. A router could be configured with classful routing behavior (no ip classless) and a classless routing protocol, such as RIPv2. A router could also be configured with classless routing behavior (ip classless) and a classful routing protocol, such as RIPv1. 44
  45. 45. Summary 45
  46. 46. 46

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