1. Chapter 8 The Routing Table: A Closer Look

2. The Routing Table Structure

3. Understanding the Routing Table
Understanding the structure and lookup process of the routing table
Help you diagnose any routing table issue
Regardless of your level of familiarity with the routing protocol.
Is the packet is being forwarded as expected? Why or why not?
Forwarded elsewhere? Discarded?
For more details: Cisco IP Routing

4. The Routing Table Structure
Router# show ip route
/24 is subnetted, 4 subnets
S is directly connected, Serial0/0/1
R [120/1] via , 00:00:08, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
/16 is subnetted, 1 subnets
S is directly connected, Serial0/0/1
C /24 is directly connected, Serial0/0/1
S /24 is directly connected, Serial0/0/1
The structure of the routing table might seem obvious.
Help you verify and troubleshoot routing issues because you will understand the routing table lookup process.
You will know exactly what the Cisco IOS software does when it searches for a route.

5. Topology
Notice that R3 also has a /24 subnet that is disconnected, or discontiguous, from the network that R1 and R2 share.
The effects of this discontiguous subnet are examined later in this chapter when you look at the route lookup process.

6. Topology: Interface Configurations for R1 and R3
R1(config)# interface FastEthernet0/0
R1(config-if)# ip address
R1(config-if)# no shutdown
R1(config-if)# interface Serial0/0/0
R1(config-if)# ip address
R1(config-if)# clock rate 64000
R3(config)# interface FastEthernet0/0
R3(config-if)# ip address
R3(config-if)# no shutdown
R3(config-if)# interface Serial0/0/1
R3(config-if)# ip address

7. Sample Routing Table Entries
Router# show ip route
/24 is subnetted, 4 subnets
S is directly connected, Serial0/0/1
R [120/1] via , 00:00:08, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
/16 is subnetted, 1 subnets
S is directly connected, Serial0/0/1
C /24 is directly connected, Serial0/0/1
S /24 is directly connected, Serial0/0/1
Route entries from the following sources:
Directly connected networks
Static routes
Dynamic routing protocols
The source of the route does not affect the structure of the routing table

8. Level 1 Routes
R2# debug ip routing
IP routing debugging is on
R2# conf t
R2(config)# interface serial 0/0/1
R2(config-if)# ip address
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown
R2(config-if)#
00:11:06: %LINK-3-UPDOWN: Interface Serial0/0/1, changed state to up
RT: add /24 via , connected metric [0/0]
RT: interface Serial0/0/1 added to routing table
R2(config-if)# end
R2# undebug all
All possible debugging has been turned off
Serial 0/0/1 interface for R2 is configured with the /24 address
As soon as no shutdown is entered, the output from debug ip routing shows that this route has been added to the routing table.

9. Level 1 Routes
RT: add /24 via , connected metric [0/0]
RT: interface Serial0/0/1 added to routing table
R2(config-if)# end
R2# show ip route
<output omitted>
C /24 is directly connected, Serial0/0/1
The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets.
Within this structure, the hierarchy includes several levels.
For simplicity, we discuss all routes as one of two levels: level 1 or level 2.

10. Level 1 Routes
R2# show ip route
<output omitted>
C /24 is directly connected, Serial0/0/1
A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address.
/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 network.

11. Level 1 Routes We will be using this chart throughout this chapter.
A level 1 route can function as any of the following:
Default route: A default route is a static route with the address /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 (next).

12. Level 1 Routes
The level 1 route /24 can be further defined as an ultimate route.
An ultimate route is a route that includes one or both of the following:
A next-hop IP address (another path)
An exit interface

13. Level 1 Routes Directly connected network 192.168.1.0/24 is a
level 1 network route - subnet mask that is the same as its classful mask.
ultimate route - contains the exit interface Serial 0/0/1.

14. Parent and Child Routes: Classful Networks
R2(config)# interface fastethernet 0/0
R2(config-if)# ip address
R2(config-if)# no shutdown
R2(config-if)# end
R2# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,
<text omitted>
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
Level 1 Parent Route
Level 2 Child Route
Another type of level 1 network route, a parent route.
When the subnet was added to the routing table,
Another route, also added.
First entry: no next-hop IP address or exit interface information.
This route is known as a level 1 parent route.

15. Parent and Child Routes: Classful Networks
R2# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,
<text omitted>
Gateway of last resort is not set
/24 is subnetted, 1 subnets
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
Level 1 Parent Route
Level 2 Child Route
A parent route is a heading:
Indicates the presence of level 2 routes, also known as child routes.
A level 1 parent route is automatically created any time a subnet is added to the routing table.
A parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table.
The subnet is the level 2 child route of the parent route

16. Parent and Child Routes: Classful Networks
Level 1 Parent Route
Level 2 Child Route
A level 2 route is a route that is a subnet of a classful network address.
Like a level 1 route, the source of a level 2 route can be a :
directly connected network
static route
dynamic routing protocol.

17. Parent route:
: The classful network address for our subnet.
/24: The subnet mask for all the child routes. If the child routes have variable-length subnet masks (VLSM), the subnet mask will be excluded from the parent route and included with the individual child routes. (later).
is subnetted, 1 subnets: This part of the route specifies that this is a parent route and in this case has one child route (that is, one subnet).

18. Child route:
C: The route code for a directly connected network.
: The specific route entry.
is directly connected: Along with the route code of C, this specifies that this is a directly connected network with an administrative distance of 0.
FastEthernet0/0: The exit interface for forwarding packets that match this specific route entry.

19. A level 2 child route contains the route source and the network address of the route.
Notice that the subnet mask is not included with the subnet, the level 2 child route.
The subnet mask for this child route is the /24 mask included in its parent route,
Level 2 child routes are also considered ultimate routes because they contain the next-hop IP address or exit interface.

20. Adding another child route
R2(config)# interface serial 0/0/0
R2(config-if)# ip address
R2(config-if)# no shutdown
R2(config-if)# end
R2# show ip route
/24 is subnetted, 2 subnets
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
The routing table shows two child routes for the same /24 parent route.
and are members both members of the /16 classful network.
Because both child routes have the same subnet mask, the parent route still maintains the /24 mask but now shows two subnets.
Later we will see the role of the parent route.

21. Adding another child route
If child routes are deleted there is no parent route.
If there is only a single level 2 child route and that route is removed, the level 1 parent route is automatically deleted.
A level 1 parent route exists only when there is at least one level 2 child route.

22. Parent and Child Routes: Classless Networks
For this discussion, we switch briefly to the RouterX topology.

23. Parent and Child Routes: Classless Networks
RouterX# show ip route
/16 is variably subnetted, 3 subnets, 2 masks
C /30 is directly connected, Serial0/0/0
C /30 is directly connected, Serial0/0/1
C /24 is directly connected, FastEthernet0/0
All three subnets belong to the classful network /16 and are therefore level 2 child routes.
Notice that the child routes do not share the same subnet mask, as was the case in the classful example.
Implementing a network addressing scheme with VLSM.
Whenever there are two or more child routes with different subnet masks belonging to the same classful network, the routing table presents a slightly different view, which states that this parent network is variably subnetted.

24. Parent and Child Routes: Classless Networks
The parent route of now contains the classful mask /16.
In the classful example shown earlier the classful mask was not displayed.
Parent route states that the child routes are variably subnetted.
Includes the number of different masks of the child routes (2 masks).
Each child route now contains the subnet mask for that specific route.
In the non-VLSM example both child routes shared the same subnet mask, and the parent displayed their common subnet mask.

25. Parent route:
: The parent route, the classful network address associated with all child routes
/16: The classful subnet mask of the parent route
variably subnetted: States that the child routes are variably subnetted and that there are multiple masks for this classful network
3 subnets, 2 masks: Indicates the number of subnets and the number of different subnet masks for the child routes under this parent route

26. Child route:
C: The route code for a directly connected network
: The specific route entry
/30: The subnet mask for this specific route
is directly connected: Along with the route code of C, specifies that this is a directly connected network with an administrative distance of 0
Serial0/0/0: The exit interface for forwarding packets that match this specific route entry

27. Routing Table Lookup Process

28. Routing Table Lookup Process
IP Packet
Routing Table
Find “best match”
When a router receives a packet on one of its interfaces.
The routing table lookup process compares the destination IP address of the incoming packet with the entries in the routing table.
The best match between the packet’s destination IP address and the route in the routing table is used to determine to which interface to forward the packet.

29. Steps in the Route Table Lookup Process
R1(config)# router rip
R1(config-router)# network
R2(config)# router rip
R2(config-router)# network
R2(config-router)# network
R3(config)# router rip
R3(config-router)# network
R3(config-router)# network
We have specifically chosen a classful routing protocol with our discontiguous subnets.
The reason for this will become evident in a later section.

30. Steps in the Route Table Lookup Process
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:07, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
R3# show ip route
/24 is subnetted, 1 subnets
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
Neither R1 nor R2 has a route to
Also, R3 does not have routes to subnets /24, /24, or /24.

31. The Route Lookup Process
Routing Table
Step 1.
The router examines level 1 routes, including network routes and supernet routes, for the best match with the destination address of the IP packet

32. The Route Lookup Process
Routing Table
Step 1a.
If the best match is a level 1 ultimate route—a classful network, supernet, or default route—this route is used to forward the packet

33. The Route Lookup Process
Routing Table
Step 1b.
If the best match is a level 1 parent route, proceed to Step 2

34. The Route Lookup Process
Routing Table
Step 2.
The router examines child routes (the subnet routes) of the parent route for a best match.

35. The Route Lookup Process
Routing Table
Step 2a.
If there is a match with a level 2 child route, that subnet is used to forward the packet.

36. The Route Lookup Process
Routing Table
Step 2b.
If there is not a match with any of the level 2 child routes, proceed to Step 3.

37. The Route Lookup Process
Routing Table
Step 3.
Is the router implementing classful or classless routing behavior. (later)
Router(config)# no ip classless
Router(config)# ip classless

38. The Route Lookup Process
Routing Table
Step 3a.
If classful routing behavior is in effect, terminate the lookup process and drop the packet.
Router(config)# no ip classless

39. The Route Lookup Process
Routing Table
Step 3b.
If classless routing behavior is in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.

40. The Route Lookup Process
Routing Table
Step 4.
If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.

41. The Route Lookup Process
Routing Table
Step 5.
If there is not a match with any route in the routing table, the router drops the packet.

42. Longest Match (Digressing from topology)
For there to be a match between the destination IP address of a packet and a route in the routing table, a minimum number of leftmost bits must match between the IP address of the packet and the route in the routing table.
The subnet mask of the route in the routing table is used to determine the minimum number of leftmost bits that must match.
IP packet only contains the IP address and not the subnet mask.

43. Longest Match
All three routes match but Route 3 has the longest match.
The best match or longest match is the route in the routing table that has the greatest number of leftmost matching bits with the destination IP address of the packet.
The route with the greatest number of equivalent leftmost bits, or the longest match, is always the preferred route.
Subnet mask in routing table specifies the minimum number of leftmost matching bits – the only bits that are considered.

44. Example: Level 1 Ultimate Route

45. Example: Level 1 Ultimate Route
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
PC1 sends a ping to , the serial interface on R3.

46. The router first examines level 1 routes for the best match.
R1# show ip route
/24 is subnetted, 3 subnets
R /24 [120/1] via , 00:00:25, Serial0/0/0
The router first examines level 1 routes for the best match.

47. R1 uses this route and forwards the packet out interface Serial 0/0/0.
R1# show ip route
/24 is subnetted, 3 subnets
R /24 [120/1] via , 00:00:25, Serial0/0/0
There is a match between the destination IP address and the level 1 ultimate route of /24.
R1 uses this route and forwards the packet out interface Serial 0/0/0.

48. Example: Level 1 Ultimate Route
Example: Level 1 Ultimate Route
Why is there a match with the /24 level 1 route and not with one of the subnets?
Why is there not a match with any of the /24 subnets in the routing table?

49. Example: Level 1 Ultimate Route
Example: Level 1 Ultimate Route
/24 is a parent route of three subnets or child routes.
Before a child route is examined for a match, there must be at least a match between the destination IP address of the packet and the classful address of the parent route, or /16.
16 bits (/16) must match the route /16 – no match!

50. Example: Level 1 Ultimate Route
Example: Level 1 Ultimate Route
The route, , is a level 1 ultimate route and, therefore, it also contains the subnet mask, /24.
Not only does the minimum of 24 bits match, but a total of 30 bits match.
Because there is not a longer, more specific match, the packet is forwarded out the exit interface Serial 0/0/0.

51. Example: Level 1 Ultimate Route
Example: Level 1 Ultimate Route
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
No match
Match
R1 forwards the packet out Serial0/0/0.

52. Longest Match: Level 1 Parent and Level 2 Child Routes

53. Longest Match: Level 1 Parent and Level 2 Child Routes
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
Match
Examine what happens when there is a match with a level 1 parent route.
Before any level 2 child routes are examined for a match, there must be a match between the classful address of the level 1 parent route and the destination IP address of the packet.

54. Longest Match: Level 1 Parent and Level 2 Child Routes
Longest Match: Level 1 Parent and Level 2 Child Routes
PC1 sends a ping to PC2 at
R1 receives the packet and forwards it to R2.

55. Because 172.16.0.0 is a Class B address, 16 leftmost bits must match.
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
Match
The first match that occurs is with the level 1 parent route,
With non-VLSM subnets, the classful mask of the parent is not displayed.
Before any child routes (subnets) are examined for a match, there must be a match with the classful address of the parent route.
Because is a Class B address, 16 leftmost bits must match.
16 bits match the parent route,

56.
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
Examine child routes
Because there is a match with the parent route, the level 2 child routes will be examined for a match.
The actual subnet mask of /24 is used for the minimum number of leftmost bits that must match.

57. The route lookup process searches the child routes for a match.
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
Match
The route lookup process searches the child routes for a match.
In this case, there must be a minimum of 24 bits that match.
Of the three subnets there is only one route that has 24 bits that match:

58.
Match Parent
No match child
No match child
Match child
The router checks the last child route for /24 and finds a match.
The first 24 bits do match.

59. What happens if the router does not have a route?
R1# show ip route
/24 is subnetted, 3 subnets
C is directly connected, FastEthernet0/0
C is directly connected, Serial0/0/0
R [120/1] via , 00:00:25, Serial0/0/0
R /24 [120/1] via , 00:00:25, Serial0/0/0
Match Parent
No match
No match
Match child
If this child route did not have an exit interface and only included a next-hop IP address, the next-hop IP address would need to be resolved to an exit interface.
The lookup process would need to start from the beginning, this time searching the routing table for the nexthop IP address.
What happens if the router does not have a route?
In this scenario, it discards the packet.

60. Example: Route Lookup Process with VLSM

61. Example: Route Lookup Process with VLSM
RouterX# show ip route
/16 is variably subnetted, 3 subnets, 2 masks
C /30 is directly connected, Serial0/0/0
C /30 is directly connected, Serial0/0/1
C /24 is directly connected, FastEthernet0/0
Match Parent
Match child
Packet’s destination IP Address:
Using VSLM does not change the lookup process.
The only difference with VLSM is that child routes display their own specific subnet masks.
16 bits match the parent route,
For there to be a match with the child route, a minimum of 30 leftmost bits must match because the subnet mask is /30

62. Routing Behavior

63. Classful and Classless Routing Behavior
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
What happens when there is a match between the packet’s destination IP address and a level 1 parent route, but there is not a match with any of the level 2 child routes?
We might assume the routing table lookup process continues looking for a less-specific match in the routing table.
However, you will see that this might or might not be the case depending on the configuration of the router.

64. Classful and Classless Routing Behavior
Classless and classful routing behaviors are not the same as classless and classful routing protocols.
Classful and classless routing protocols affect how the routing table is populated.
Classful and classless routing behaviors determine how the routing table is searched after it is populated.

65. Topology Changes
Classful routing protocols such as RIPv1 do not support discontiguous networks.
Even though our current topology has discontiguous networks, we can configure static routes to reach those networks.

66. Topology Changes R2(config)# ip route 0.0.0.0 0.0.0.0 s0/0/1
R2(config)# router rip
R2(config-router)# default-information originate
R2(config-router)# no network
R2(config-router)# end
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1

67. Topology Changes
R3(config)# ip route s0/0/1
R3(config)# no router rip
R3(config-router)# end
R3# show ip route
/16 is variably subnetted, 2 subnets, 2 masks
C /24 is directly connected, FastEthernet0/0
S /16 is directly connected, Serial0/0/1
C /24 is directly connected, Serial0/0/1
Notice that the static route for the same network as the parent route is a child route in the routing table.
This is because there is a child route for that parent.
If there wasn’t a /24 child route, then this /16 static route would be a level 1 ultimate route.

68. Routing Behavior
Routing Table
Step 3.
Is the router implementing classful or classless routing behavior. (later)
Router(config)# no ip classless
Router(config)# ip classless

69. Classful Routing Behavior: no ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
R1(config)# no ip classless
R2(config)# no ip classless
R3(config)# no ip classless
What happens when there is a match with a parent, but none of the child routes match?
Before Cisco IOS Software Release 11.3, no ip classless was the default behavior for Cisco routers.
The command no ip classless means that the route lookup process uses classful routing table lookups by default.

70. Classful Routing Behavior: Search Process
R2 receives a packet destined for PC3 at

71. Classful Routing Behavior: no ip classless
Classful Routing Behavior: no ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
The routing process searches the routing table and finds a 16-bit match with the parent route
None of the child routes have 24 leftmost bits that match the 24 leftmost bits of the destination IP address

72. Classful Routing Behavior: no ip classless
Classful Routing Behavior: no ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
DROP PACKET
So, what happens next?
Router R2 drops the packet.
Default route is never checked or used.

73. Classful Routing Behavior: no ip classless
Classful Routing Behavior: no ip classless
Why does classful routing behavior perform like this?
The general idea of classful routing behavior comes from the time when all networks were of a classful nature.
At the beginning of the Internet’s growth, an organization received a Class A, Class B, or Class C major network address.
When an organization had a classful IP major network address, that organization would also administer all the subnets for that classful address.
All routers belonging to the organization would know about all the subnets for the major network.
If a subnet was not in the routing table, the subnet did not exist

74. Classless Routing Behavior: ip classless
Classless Routing Behavior: ip classless
Remember: R2 receives a packet destined for PC3 at

75. Classless Routing Behavior: ip classless
Classless Routing Behavior: ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
R1(config)# ip classless
R2(config)# ip classless
R3(config)# ip classless
Cisco IOS 11.3, Cisco changed the default routing behavior from classful to classless.
ip classless command is configured by default.
Classless routing behavior means that the routing process no longer assumes that all subnets for a major classful network can be reached only within the child routes of the parent.
Classless routing behavior works well for discontiguous networks and classless interdomain routing (CIDR) supernets.

76.
Step 3b.
If classless routing behavior is in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.
Step 4.
If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.

77. Classless Routing Behavior: ip classless
Classless Routing Behavior: ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
The /24 route does not have 24 leftmost bits that match the destination IP address.
/0, which means that zero or no bits need to match.
A default route will be the lowest-bit match. In classless routing behavior, if no other route matches, the default route will match.

78. Classless Routing Behavior: ip classless
Classless Routing Behavior: ip classless
R2# show ip route
/24 is subnetted, 3 subnets
R [120/1] via , 00:00:00, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0/1
S* /0 is directly connected, Serial0/0/1
Match Parent
No match
No match
No match
Now what?
Continue searching level 1 supernet routes. The default route is a match, so I is used to forward the packet.

79. Classless Routing Behavior: ip classless
Classless Routing Behavior: ip classless
Step 5.
If there is not a match with any route in the routing table, the router drops the packet.

80. Classful Routing Behavior: no ip classless
Classful Routing Behavior: no ip classless
Classless Routing Behavior: ip classless