1. Switching Basics and Intermediate Routing CCNA 3 Chapter 1

2. VLSM
Variable-length subnet masks were developed to allow multiple levels of subnetted IP addresses within a single network
The routing protocol you use must support VLSM
Open Shortest Path First (OSPF)
Enhanced Interior Gateway Routing Protocol (EIGRP)
Routing Information Protocol version 2 (RIPv2)
VLSM is crucial for an effective IP addressing plan

3. VLSM Prefix Length
Prefix length is a shorthand way for expressing the subnet mask for a particular network
Number of 1s in the binary representation of the subnet mask
When bits are taken from the host part of an address and added to the network part, the number of the bits in the host part decreases
You create additional subnets at the expense of the number of host devices on each network segment

4. VLSM Prefix Length
Number of subnets can be calculated using the 2s formula, where s is the number of bits by which the default mask is extended
In IOS releases prior to 12.0, you must explicitly allow subnet 0
In IOS releases 12.0 and later, subnet 0 is enabled by default
The all-1s subnet has always been allowed

5. VLSM Prefix Length
Bits that are not part of the network or subnetwork portions of the address are the range of host address
Use the 2h – 2 formula (where h is the number of host bits) to calculate available host addresses; all 0s in host portion is the subnet identifier address, all 1s in host portion is the subnet broadcast address

6. VLSM Prefix Length
Network Mask and IP Address for the Range Through , with Host Bits Shaded
In the IP network number that accompanies the network mask, the following are true:
When the host bits are all binary 0s, that address is the beginning of the address range
When the host bits are all binary 1s, that address is at the end of the address range

7. VLSM Prefix Length
Fourth Octet for the Range Through (continued on next slide)

8. VLSM Prefix Length (continued)
Fourth Octet for the Range Through
(continued)

9. VLSM Prefix Length
In this example, PCs use the prefix length of 28 (the subnet mask ) to determine which other devices on their local network have their first 28 bits in common
A 28-bit prefix length permits 14 hosts per subnet
The PC uses ARP to find the corresponding destination MAC address if communication with any of these devices is necessary
If the destination IP address is not in the range for the subnet, the packet is forwarded to the default gateway

10. VLSM Prefix Length
A router works in a similar manner when it makes a routing decision
It compares the destination IP address of the packet to network entries in the routing table
The network entries have a prefix length associated with them
The router uses the prefix length to determine how many destination bits must match to send the packet out the corresponding outbound interface that is associated with the network number in the routing table

11. VLSM Prefix Length
The router determines from the table where to send the packet destined for
In this table, there are four entries for network
The third entry is for the subnet, which is the subnet to which belongs
Note that the next subnet, , begins with a number larger than

12. VLSM Benefits of VLSM More efficient use of IP addresses
Without use of VLSM, a single subnet mask must be implemented with an entire Class A, B, or C network
Greater capacity to use router summarization (discussed later in this chapter)
Allows more hierarchical levels within an addressing plan
Isolation of topology changes from other routers

13. VLSM Permits Flexible, Efficient Subnet Address Allocation
VLSM Benefits of VLSM
VLSM Permits Flexible, Efficient Subnet Address Allocation

14. VLSM VLSM Calculations
VLSM is used to maximize number of possible IP addresses available for a network
Point-to-point serial links require only two host addresses, so a /30 subnet does not waste scarce subnet addresses
With VLSM, you can subnet a subnet!
Next slide will show how the subnet /20 is further subnetted with a /26 prefix

15. VLSM VLSM Calculations
Further Subnetting /20 to /26 Prefixes

16. VLSM VLSM Example
VLSM Used to Define Subnets of Across the Boundary Between Octets Three and Four

17. VLSM CIDR and Route Summarization
The definition of classless inter-domain routing (CIDR):
Allocation of one or more blocks of Class C network numbers to each network service provider
Organizations using the network service provider for Internet connectivity are allocated bitmask-oriented subsets of the provider’s address space as required
CIDR (“cider”) was developed to address the problem of IP address space running out and core Internet routers running out of capacity
Route summarization is the representation by a single network of a group of contiguous networks

18. VLSM CIDR and Route Summarization
Route Summarization of Contiguous Subnets of a Class B Network

19. VLSM CIDR and Route Summarization
Route Summarization of Contiguous Subnets of a Class B Network (continued)
Router D in previous slide has these networks in its routing table
/24
/24
/24
/24
To calculate the summary route:
Find the number of highest-order bits that match in all addresses
Locate where the common pattern of digits ends
Count the number of common bits; this is the length of the summary route

20. VLSM CIDR and Route Summarization
Route Summarization of Contiguous Subnets of a Class B Network (continued)
Follow these guidelines when calculating summary routes:
Addresses that do not share the same number of bits as the prefix length of the summary route are not included in the summarization block
The IP addressing plan is hierarchical in nature to allow router to aggregate the largest number of IP addresses into a single summary route
IP networks can only be summarized in 2n networks (for some n), where the last octet of the first network in the sequence is divisible by 2n

21. VLSM Route Aggregation
By using a prefix length instead of an address class to determine the network portion of the address, CIDR allows routers to aggregate routing information
Shrinks routing table
One address and mask combination can represent the routes to multiple networks
Route aggregation is used more loosely than CIDR; describes the summarization of classful networks
Without CIDR, routers must maintain tables for individual networks

22. VLSM Route Aggregation
CIDR Permits the Aggregation of Contiguous Class B Networks

23. VLSM Route Aggregation
Summarization Employs the Furthest-to-the-Right Principle

24. VLSM Route Aggregation
In previous slide, the router can summarize routes to these networks using a 13-bit prefix which these 8 networks share = =
A single address and mask define a classless prefix that summarizes routes to the eight networks: /13

25. VLSM Route Aggregation
Using a prefix to summarize routes results in the following:
More efficient routing
A reduced number of CPU cycles when calculating a routing table or sorting through routing table entries to find a match
Reduced router memory requirements

26. VLSM Supernetting
The practice of using a summary network to group multiple classful networks into a single address is called supernetting
Subnetting breaks down a classful network
Supernetting pastes together classful networks
With Class A and B address space almost exhausted, large organizations requested multiple Class C network addresses from their service providers
A block of contiguous Class C addresses can appear as a single large network, or supernet

27. VLSM Supernetting Supernetting and route aggregation are similar
Route aggregation is used in the context of summarizing routes with BGP
Supernetting is a term used when the summarized networks are under common administrative control
Many networking professionals use the terms “route summarization” and “route aggregation” interchangeably

28. VLSM CIDR Example
CIDR Permits the Aggregation of Several Classful Networks into a Single Route Advertisement

29. Classful and Classless Routing
Behavior of classful routing is limited compared to classless routing
Classful routing protocols(RIPv1, IGRP) cannot do VLSM
Make routing decisions and send routing updates according to Class A, B, and C constructs
Classless routing protocols work independently of Class A, B, and C addresses
In the “real world,” classful routing protocols are close to becoming irrelevant

30. Classful and Classless Routing Classful Routing
RIPv1 and IGRP are the two classful routing protocols
Rare to see either of these employed on a router today
Classful routing protocols do not include subnet mask information in their updates
The router applies two options when receiving a routing update packet
If the routing update information contains the same major network number as configured on the receiving interface, the router applies the subnet mask that is configured on that interface
If the routing update information contains a different major network than the one configured on the the receiving interface, the router applies the default subnet mask

31. Classful and Classless Routing Classful Routing
The router applies two options when receiving a routing update packet (continued)
The default classful masks are:
Class A:
Class B:
Class C:
All subnets of the same major network (Classes A, B, and C) must use the same mask when using a classful routing protocol

32. Classful and Classless Routing Classful Routing
Routers running a classful routing protocol perform automatic route summarization across network boundaries
They make assumptions about networks based on their IP address class
These assumptions lead to automatic summarization of routes when routers send routing updates across major classful network boundaries
Routers send update packets to other connected routers
Routers sends entire subnet address (without mask); assume the network and the interface use the same subnet mask

33. Classful and Classless Routing Classful Routing
Router receiving the update makes the same assumption
If different masks are used, router would have wrong information in routing table
Important to use the same subnet mask on all interfaces that belong to the same classful network
When a router using a classful protocol sends an update regarding information of a subnet of a classful network across an interface belonging to a different classful network, the router assumes the remote router will use the default subnet mask for that IP address class

34. Classful and Classless Routing Classful Routing
Automatic Summarization Occurs at Classful Boundaries with RIPv1 and IGRP

35. Classful and Classless Routing Classful Routing
The process in the previous slide is automatic summarization across the network boundary
Router sends a summary of all the subnets by sending only major network information
Classful routing protocols automatically create a classful summary route at major network boundaries
Classful routing protocols do not allow summarization at other points within the major network space

36. Classful and Classless Routing Classful Routing
The router that receives the updates behaves in a similar fashion
When a routing update contains information about a different classful network than the one that is in use on its interface, the router applies the default classful mask to that update
When using classful routing protocols, assigning the same subnet mask to all subnets is called fixed-length subnet masking (FLSM) – sometimes called static-length subnet masking

37. Classful and Classless Routing Discontiguous Subnets
A classical problem with classful routing protocols:
Discontiguous subnets occur when a major network separates subnets of a major network
This can cause erroneous entries in routing tables
Traffic will not always reach its destination
Do not permit the use of discontiguous networks when using a classful routing protocol

38. Classful and Classless Routing Discontiguous Subnets
Discontiguous Subnets Present a Problem with Classful Routing

39. Classful and Classless Routing Default Routes
Routers learn paths to destinations in three ways:
The system administrator defines static routes via an attached interface or the next hop to a destination
The network engineer manually defines default routes as the path to take when no known route exists to the destination; default routes minimize the size of the routing table
Dynamic routing occurs when the router learns of paths to destinations by receiving routing updates from other routers via a routing protocol

40. Classful and Classless Routing Default Routes
You can define a static route with the ip route command:
You can define a default route with the
ip default-network command:

41. Classful and Classless Routing Default Routes
A Default Network is Configured Pointing Toward the Internet

42. Classful and Classless Routing Default Routes
You can define a default route to work with either static or dynamic routing:
The 0s represent any destination with any mask
Default routes are often referred to as quad-zero routes

43. Classful and Classless Routing Classful Routing Table
What does a router running a classful routing protocol do with packets that lie in subnets that have no entry in the routing table?
The router discards the packets!
This can be overcome by using the ip classless command
Causes the router using a classful routing protocol to evaluate all packets using the longest-match criterion
As a last resort, the router uses a configured default route

44. Classful and Classless Routing Classless Routing
All routing protocols except RIPv1 and IGRP are classless routing protocols
RIPv2, OSPF, IS-IS, EIGRP, and BGPv4 are classless routing protocols that support VLSM and CIDR
With classless routing protocols, different subnets in the same major network can have different subnet masks
Maximizes use of addresses

45. Classful and Classless Routing Classless Routing
Classful routing protocols automatically summarize to the classful network boundary; classless routing protocols allow you to control the route summarization process manually (might be needed to limit size of routing tables)
Classless routing protocols do not automatically advertise every subnet
By default, classless routing protocols perform automatic network summarization at classful boundaries, just like classful protocols

46. Classful and Classless Routing Classless Routing
Difference between classless routing protocols and their predecessors is that you can manually turn off automatic summarization
Use the no auto-summary command
Not needed with OSPF or IS-IS
Automatic summarization can cause problems in networks with discontiguous subnets
This can be fixed by turning off automatic summarization

47. Classful and Classless Routing Classless Routing
Discontiguous Subnets Presenting a Problem with Classless Routing

48. Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary
Beginning with IOS Release 12.2(8)T, EIGRP and BGP had auto-summary enabled by default
RIPv2 has always had auto-summary enabled by default
Default Behavior of RIPv2 is to Automatically Summarize at the Network Boundary

49. RIPv2 Supports VLSM with Automatic Summarization Disabled
Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary
RIPv2 Supports VLSM with Automatic Summarization Disabled

50. Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary
To disable auto-summary in RIPv2, use the
no auto-summary command as seen below

51. RIP Version 2 RIP Version 1 characteristics
Uses hop count as the metric for path selection
Maximum allowable hop count is 15, so infinite distance equals 16 hops
Uses hold-down timers to prevent routing loops with a default of 180 seconds
Employs split horizon to prevent routing loops
Failure to receive routing updates in a timely manner results in removal of routes previously learned from a neighbor

52. RIP Version 2 RIP Version 1 characteristics (continued)
The administrative distance is 120
Routing updates are broadcast every 30 seconds by default
Is capable of load-balancing over as many as six equal-cost paths; four is the default
Does not support authentication
Does not support VLSM because it is a classful routing protocol

53. RIP Version 2 RIP Version 2 characteristics
Uses hop count as the metric for path selection
Maximum allowable hop count is 15, so infinite distance equals 16 hops
Uses hold-down timers to prevent routing loops with a default of 180 seconds
Employs split horizon to prevent routing loops
Failure to receive routing updates in a timely manner results in removal of routes previously learned from a neighbor

54. RIP Version 2 RIP Version 2 characteristics (continued)
The administrative distance is 120
Routing updates are multicast every 30 seconds by default
Is capable of load-balancing over as many as six equal-cost paths; four is the default
Supports clear text and Message Digest 5 (MD5) authentication
Supports VLSM because it is a classless routing protocol
Supports manual route summarization

55. RIP Version 2 Major improvements with RIPv2: Support of authentication
Clear text is the default
MD5 used to encrypt enable secret passwords
VLSM use
Sending subnet masks in updates
Multicasting routing updates
Uses as destination
Keeps PCs and servers from having to process the broadcast

56. RIP Version 2 Multicasting routing updates (continued)
Keeps PCs and servers from having to process the broadcast (continued)
IP sends the packet to the User Datagram Protocol (UDP) and UDP checks whether RIP port 520 is available; most PCs and servers do not have a process running on this port and discard the packet
Sometimes it is running as a gateway discovery technique in TCP/IP services, such as UNIX or Windows

57. RIP Version 2 Broadcast disadvantages of RIPv1
RIPv1 can fit up to 25 networks/subnets in each update; updates are sent every 30 seconds
If the routing table has 1000 subnets, 40 packets will be sent every 30 seconds
Each of these broadcasts will have to be looked at by all devices on the network

58. RIP Version 2 Multicast advantages of RIPv2
The IP multicast address for RIPv2 has its own MAC address: 0x0100.5e
Devices such as PCs and servers read this MAC address and determine it is not for them; they discard the frame
If a device can’t distinguish this MAC address, the packet will be discarded at the IP layer (OSI network layer) as the multicast IP address is not the IP address of the device

59. RIPv2 Configuration
The router rip command starts a RIP routing process; the network command causes the implementation of these three functions:
Routing updates are multicast out an interface
Routing updates are processed if they enter that same interface
The subnet that is directly connected to that interface is advertised

60. Sample Network and Configuration of RIPv2
RIPv2 Configuration
Sample Network and Configuration of RIPv2

61. RIPv2 Configuration
In the previous slide, these commands were used to configure Router A:
Enable RIP as the routing protocol: router RIP
Identify Version 2 as the RIP being used: version 2
Specifying a directly connected network: network
Specifying a directly connected network: network

62. Verifying RIP Configuration
Sample Network for Verifying RIP Configuration

63. Verifying RIP Configuration
Most common commands for verifying RIP Configuration:
Display parameters for routing protocols: show ip protocols
Summary of IP information and status of all interfaces: show ip interface brief
Ensure that appropriate commands are configured for the RIP network: show running-config
Display contents of routing table: show ip route

64. Verifying RIP Configuration

65. Verifying RIP Configuration

66. Verifying RIP Configuration
Fields in the Routing Table Defined

67. Troubleshooting RIP Configuration
Sample Network for Troubleshooting RIP Configuration
The debug ip rip command displays real-time RIP routing updates as they are sent and received
To turn off debugging, use the no debug ip rip or the undebug all (u all) commands

68. Troubleshooting RIP Configuration
The debug ip rip command

69. Troubleshooting RIP Configuration
Sample debug ip rip output

70. Summary Classless IP addressing is implemented with:
VLSM: the ability to subnet a subnet and use different subnet masks in the same classful network
CIDR: the allocation of blocks of contiguous address space to customers by ISPs
Route summarization: a generic term that describes the use of a single network to represent a sequence of logically contiguous networks
Route aggregation: a generalized form of supernetting
Supernetting: pasting together classful networks into supernets

71. Summary Classful routing protocols: Classless routing protocols: RIPv1
IGRP
Classless routing protocols:
RIPv2
EIGRP
OSPF
IS-IS
BGPv4

72. Summary
RIPv2, EIGRP, and BGPv4 can turn automatic route summarization on and off
RIPv2 is an improvement to RIPv1
Adds authentication, VLSM support, passing of subnet masks in routing updates, and multicasting of routing updates
Configuring RIPv2 requires adding the version 2 command; adding no auto-summary is recommended
All connected networks participating in RIP are defined with the network command in the form of classful networks

73. Summary
RIP configuration can be verified with several commands: show ip protocols, show ip interface brief, show running-config, and show ip route
You can troubleshoot RIP with the debug ip rip command