Switching%20Basics%20and%20Intermediate%20Routing%20CCNA%203%20Chapter%201 - PowerPoint PPT Presentation

About This Presentation
Title:

Switching%20Basics%20and%20Intermediate%20Routing%20CCNA%203%20Chapter%201

Description:

Variable-length subnet masks were developed to allow multiple levels of ... can't distinguish this MAC address, the packet will be discarded at the IP layer ... – PowerPoint PPT presentation

Number of Views:1516
Avg rating:3.0/5.0
Slides: 74
Provided by: ustu6
Category:

less

Transcript and Presenter's Notes

Title: Switching%20Basics%20and%20Intermediate%20Routing%20CCNA%203%20Chapter%201


1
Switching Basics and Intermediate Routing CCNA
3Chapter 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
VLSMPrefix 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
VLSMPrefix 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
VLSMPrefix 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
VLSMPrefix Length
  • Network Mask and IP Address for the Range
    192.168.1.64 Through 192.168.1.79, 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
VLSMPrefix Length
  • Fourth Octet for the Range 192.168.1.64
    Through 192.168.1.79 (continued on next slide)

8
VLSMPrefix Length (continued)
  • Fourth Octet for the Range 192.168.1.64
    Through 192.168.1.79
  • (continued)

9
VLSMPrefix Length
  • In this example, PCs use the prefix length of 28
    (the subnet mask 255.255.255.240) 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
VLSMPrefix 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
VLSMPrefix Length
  • The router determines from the table where to
    send the packet destined for 192.168.1.67
  • In this table, there are four entries for network
    192.168.1.0
  • The third entry is for the 192.168.1.64 subnet,
    which is the subnet to which 192.168.1.67 belongs
  • Note that the next subnet, 192.168.1.80, begins
    with a number larger than 192.168.1.67

12
VLSMBenefits 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
VLSMBenefits of VLSM
  • VLSM Permits Flexible, Efficient Subnet Address
    Allocation

14
VLSMVLSM 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
    172.16.32.0/20 is further subnetted with a /26
    prefix

15
VLSMVLSM Calculations
  • Further Subnetting 172.16.32.0/20 to /26 Prefixes

16
VLSMVLSM Example
  • VLSM Used to Define Subnets of 172.16.32.0 Across
    the Boundary Between Octets Three and Four

17
VLSMCIDR 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 providers
    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
VLSMCIDR and Route Summarization
  • Route Summarization of Contiguous Subnets of a
    Class B Network

19
VLSMCIDR 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
  • 172.16.12.0/24
  • 172.16.13.0/24
  • 172.16.14.0/24
  • 172.16.15.0/24
  • 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
VLSMCIDR 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
VLSMRoute 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
VLSMRoute Aggregation
  • CIDR Permits the Aggregation of Contiguous Class
    B Networks

23
VLSMRoute Aggregation
  • Summarization Employs the Furthest-to-the-Right
    Principle

24
VLSMRoute Aggregation
  • In previous slide, the router can summarize
    routes to these networks using a 13-bit prefix
    which these 8 networks share
  • 10101100 00011000 00000000 00000000 172.24.0.0
  • 11111111 11111000 00000000 00000000 255.248.0.0
  • A single address and mask define a classless
    prefix that summarizes routes to the eight
    networks 172.24.0.0/13

25
VLSMRoute 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
VLSMSupernetting
  • 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
VLSMSupernetting
  • 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
VLSMCIDR 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 RoutingClassful 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 RoutingClassful Routing
  • The router applies two options when receiving a
    routing update packet (continued)
  • The default classful masks are
  • Class A 255.0.0.0
  • Class B 255.255.0.0
  • Class C 255.255.255.0
  • 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 RoutingClassful 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 RoutingClassful 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 RoutingClassful Routing
  • Automatic Summarization Occurs at Classful
    Boundaries with RIPv1 and IGRP

35
Classful and Classless RoutingClassful 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 RoutingClassful 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 RoutingDiscontiguous
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 RoutingDiscontiguous
Subnets
  • Discontiguous Subnets Present a Problem with
    Classful Routing

39
Classful and Classless RoutingDefault 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 RoutingDefault 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 RoutingDefault Routes
  • A Default Network is Configured Pointing Toward
    the Internet

42
Classful and Classless RoutingDefault 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 RoutingClassful 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 RoutingClassless 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 RoutingClassless 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 RoutingClassless 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 RoutingClassless Routing
  • Discontiguous Subnets Presenting a Problem with
    Classless Routing

48
Classful and Classless RoutingEffect 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
Classful and Classless RoutingEffect of
Auto-Summary and No Auto-Summary
  • RIPv2 Supports VLSM with Automatic Summarization
    Disabled

50
Classful and Classless RoutingEffect 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 224.0.0.9 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 0x0100.5e00.0009
  • Devices such as PCs and servers read this MAC
    address and determine it is not for them they
    discard the frame
  • If a device cant 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
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
    172.16.0.0
  • Specifying a directly connected network network
    10.0.0.0

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
  • 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
Write a Comment
User Comments (0)
About PowerShow.com