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Switching Basics and Intermediate Routing CCNA 3 Chapter 2

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Title: Switching Basics and Intermediate Routing CCNA 3 Chapter 2


1
Switching Basics and Intermediate Routing CCNA
3 Chapter 2
2
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • Link-state routing algorithms, also known as
    shortest path first (SPF) algorithms, build a
    complex database of topology information
  • The algorithms compute the shortest path between
    nodes
  • Maintains full knowledge of distant routers and
    how they interconnect

3
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • Link-state routing uses link-state advertisements
    (LSAs)
  • A basic building block that describes a routers
    local topology and is distributed to all other
    routers in the area
  • Link-state routing uses a topological database
    (or link-state database)
  • The set of all links learned from the flooding of
    LSAs
  • Synchronized with all other routers in the area

4
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • OSPF and Intermediate System-to-Intermediate
    System (IS-IS) are link-state routing protocols
  • Collect routing information from all other
    routers in the area
  • Each router calculates all the best paths to all
    destinations in the network
  • Because each router calculates best paths, they
    are less likely to propagate incorrect
    information learned from a neighboring router

5
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • Link-state routing protocols were designed to
    overcome the limitations of distance vector
    routing protocols
  • Respond quickly to network changes
  • Send only triggered updates
  • Send periodic updates at long intervals, such as
    every 30 minutes
  • A hello mechanism determines reachability of
    neighbors

6
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • Link-State Routing Relies on Complex Mechanisms
    to Permit Stable, Synchronous and High-Speed
    Routing

7
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • When a failure occurs in a network
  • Link-state protocols flood LSAs use a special
    multicast address
  • Each link-state router takes a copy of the LSA,
    updates its topological database, and forwards
    the LSA to neighboring routers
  • All link-state routers in the area recalculate
    their routing tables using the Dijkstra SPF
    algorithm
  • A link is similar to an interface on a router
  • The state of the link is a description of the
    interface and its relation to its neighboring
    routers

8
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • OSPF Uses a Two-Layer Hierarchy

9
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • Two primary elements exist in the two-layer
    hierarchy
  • Area A grouping of contiguous networks
  • Areas are logical subdivisions of the autonomous
    system
  • Each area must be connected directly to the
    backbone area (known as area 0)
  • Autonomous System (AS) A collection of networks
    under a common administration
  • Share a common routing strategy
  • Can be logically subdivided into multiple areas

10
Link-State Routing Overview Maintaining Routing
Information Via Link States
  • The backbone area is the transition area
  • All other areas communicate through it
  • All non-backbone areas are connected to it
  • These can be configured as a stub area, a totally
    stubby area, or a not-so-stubby area (NSSA) (not
    covered in this curriculum) to reduce the sizes
    of the link-state database and the routing table

11
Link-State Routing Overview Link-State Routing
Protocol Algorithms
  • Link-State Routing Protocol Algorithms
  • Rely on SPF protocols to maintain a complex
    database of the network topology
  • Develop and maintain a full knowledge of the
    network routers and how they interconnect
  • Use LSAs to exchange information with other
    routers
  • Each router that has exchanged LSAs constructs a
    topological database
  • The SPF algorithm is used to compute reachability
    to destination networks
  • A routing table is built from this information,
    containing only lowest-cost routes

12
Link-State Routing Overview Link-State Routing
Protocol Algorithms
  • (continued)
  • LSA exchanges are triggered events
  • Greatly speed up convergence process
  • No need to wait for a series of timers to expire
    before the networked routers can begin to converge

13
Link-State Routing Overview Link-State Routing
Protocol Algorithms
  • Cost Metric Determines Shortest Path for
    Link-State Routing Protocols

14
Link-State Routing Overview Link-State Routing
Protocol Algorithms
  • Next Hops and Costs for Destination Routes
    (Previous Slide)

15
Link-State Routing Benefits of Link-State
Routing
  • Link-state protocols use cost metrics to choose
    paths
  • Cost metric reflects the capacity of the links
  • Routing updates are less frequent
  • Network can be segmented into area hierarchies
  • Limits the scope of route changes
  • Link-state protocols send only updates of a
    topology change
  • Use triggered, flooded updates which lead to
    faster convergence times

16
Link-State Routing Benefits of Link-State
Routing
  • Each router has a complete and synchronized
    picture of the network
  • Difficult for routing loops to occur
  • LSAs are sequenced and aged
  • Routers always base their routing information on
    the most recent set of information
  • With careful design work, size of link-state
    databases can be minimized
  • Smaller Dijkstra calculations and faster
    convergence

17
Link-State Routing Limitations of Link-State
Routing
  • In addition to a routing table, link-state
    protocols require
  • A topological database
  • An adjacency database
  • Lists all the relationships formed between
    neighboring routers for the purpose of exchanging
    routing information
  • A forwarding table
  • A data structure of a stripped down association
    between network prefixes and next hops

18
Link-State Routing Limitations of Link-State
Routing
  • Dijkstras algorithm requires CPU cycles to
    calculate best paths through the network
  • If the network is large or unstable, this can
    require a significant amount of CPU time
  • Not a problem for most modern routers
  • A strict hierarchical network design is required
    to divide the network into smaller areas
  • Reduces the excessive use of memory and CPU
    cycles
  • Reduces size of topology tables and Dijkstra
    calculations
  • Areas must be contiguous at all times

19
Link-State Routing Limitations of Link-State
Routing
  • Although configuration of link-state networks is
    usually simple, configuring a large network can
    be challenging
  • Trouble-shooting is usually easier, as every
    router has a copy of the topology
  • However, interpreting the information requires a
    good understanding of link-state routing concepts
  • Link-state protocols usually scale to bigger
    networks than distance vector protocols

20
Link-State Routing Limitations of Link-State
Routing
  • Link-state routing raises two concerns
  • During the initial discovery process, link-state
    routing protocols flood the network with LSAs
  • Significantly decreases the networks capability
    to transport data
  • This is temporary, but noticeable
  • Link-state routing is both memory- and
    processor-intensive
  • Greater demand requires higher-end routers that
    cost more

21
Single-Area OSPF Concepts
  • OSPF was developed by the Interior Gateway
    Protocol (IGP) group of the Internet Engineering
    Task Force (IETF)
  • Created in mid 1990s because RIP was unable to
    serve large, heterogeneous networks
  • OSPF has two primary characteristics
  • Protocol is an open standard, not proprietary
  • Based on the SPF algorithm

22
Single-Area OSPF Concepts Comparing OSPF with
Distance Vector Routing Protocols
  • OSPF is a link-state protocol, RIP and IGRP are
    distance vector protocols
  • Distance vector protocols send all, or a portion
    of, their routing table in updates to their
    neighbors
  • A link is an interface on a router
  • The state of the link describes the interface and
    its relationship to neighboring routers
  • Can include IP address, subnet mask, type of
    network
  • The collection of link states forms a link-state
    database

23
Single-Area OSPF Concepts Comparing OSPF with
Distance Vector Routing Protocols
  • An OSPF router sends LSA packets to periodically
    advertise its link states instead of sending
    routing table updates
  • Information about attached interfaces and metrics
    are included
  • LSAs are flooded to all routers in the area
  • As OSPF routers accumulate link-state
    information, they use the SPF algorithm to
    calculate the shortest path to each destination

24
Single-Area OSPF Concepts Comparing OSPF with
Distance Vector Routing Protocols
  • A topological (link-state) database is an overall
    picture of networks in relationship to routers
  • Contains the collection of LSAs received from all
    routers in the same area
  • Database is pieced together from the LSAs
  • Routers in the same area have identical
    topological databases

25
Single-Area OSPF Concepts Comparing OSPF with
Distance Vector Routing Protocols
  • OSPF can operate within a hierarchy
  • The largest entity is the Autonomous System (AS)
  • A collection of networks under a common
    administration that share a common routing
    strategy
  • An AS can be divided into several areas, which
    are groups of contiguous networks and attached
    hosts

26
Single-Area OSPF Concepts OSPF Hierarchical
Routing
  • OSPFs capability to separate a large network
    into multiple areas is known as hierarchical
    routing
  • Hierarchical routing enables you to separate a
    large internetwork (AS) into smaller
    internetworks called areas
  • Routing still occurs between areas
  • Many of the minute internal routing operations,
    such as recalculating the database, are kept
    within an area

27
Single-Area OSPF Concepts OSPF Hierarchical
Routing
  • OSPF Uses Areas to Provide Hierarchy

28
Single-Area OSPF Concepts OSPF Hierarchical
Routing
  • OSPFs hierarchical topology possibilities have
    the following advantages
  • Reduced frequency of SPF calculations
  • Smaller routing tables
  • Reduced link-state update overhead

29
Single-Area OSPF Concepts Dijkstras Algorithm
  • In Dijkstras algorithm, the best path is the
    lowest cost path
  • Named for Edsger Wybe Dijkstra, a Dutch computer
    scientist
  • Each link has a cost
  • Each node has a name
  • Each node has a complete topological database

30
Single-Area OSPF Concepts Dijkstras Algorithm
  • Dijkstras Algorithm Uses Cost Metric

31
Single-Area OSPF Concepts Dijkstras Algorithm
  • Dijkstras algorithm places each router at the
    root of a tree
  • Calculates the shortest path to each node based
    on the cumulative cost to reach the destination
  • Each router has its own view of the topology
  • Each router uses the information in its
    topological database to calculate a shortest-path
    tree, with itself as the root
  • The router uses this tree to route network traffic

32
Single-Area OSPF Concepts Dijkstras Algorithm
  • The cost, or metric, of an interface indicates
    the overhead that is required to send packets
    across that interface
  • The OSPF cost of an interface is inversely
    proportional to that interfaces bandwidth
  • Higher bandwidth equals lower cost
  • Cost 100,000,000 / bandwidth in bps

33
Single-Area OSPF Concepts Dijkstras Algorithm
  • Shortest Path is Measured from Each Root Node to
    Build a Shortest Path Tree

34
Single-Area OSPF Configuration Basic OSPF
Configuration
  • The router ospf command takes a process
    identifier as an argument
  • Router (config) router ospf process-id
  • The process ID is a locally significant number
    between 1 and 65,535 that you select to identify
    the routing process
  • It does not need to match the OSPF process ID on
    other OSPF routers

35
Single-Area OSPF Configuration Basic OSPF
Configuration
  • The network command identifies which IP networks
    on the router are part of the OSPF network
  • Router(config-router)network address
    wildcard-mask area area-id (all on one command
    line)
  • Parameters of a network Command

36
Single-Area OSPF Configuration Basic OSPF
Configuration
  • The wildcard mask is sometimes called an inverse
    mask because it is the inverse of the subnet mask
    for the network
  • This is not required many network administrators
    use the 0.0.0.0 option to match the interface
  • Basis OSPF Network with Each Router in Area 0

37
Single-Area OSPF Configuration Basic OSPF
Configuration
  • Using the network statement in OSPF

38
Single-Area OSPF Configuration Basic OSPF
Configuration
  • A router uses the OSPF hello protocol to
    establish neighbor relationships
  • Hello packets let other routers know they are
    still functional
  • On networks supporting more than two routers
    (multiaccess networks), such as Ethernet
    networks, the hello protocol elects
  • A designated router (DR)
  • Generates LSAs
  • Manages link-state synchronization
  • A backup designated router (BDR)
  • Becomes the DR if the existing DR fails

39
Single-Area OSPF Configuration Loopback
Interfaces
  • The OSPF router ID is the number by which the
    router is known to OSPF
  • To modify the OSPF router ID to a loopback
    address use this command
  • Router(config)interface loopback number
  • The highest IP address on an active interface of
    a router at startup can be overridden by using a
    loopback address
  • OSPF is more reliable if a loopback interface is
    configured because a loopback interface is always
    active

40
Single-Area OSPF Configuration Modifying the
OSPF Cost Metric
  • OSPF uses cost as the metric to determine the
    best route
  • Cost is associated with the output side of an
    interface
  • It is calculated with the formula
  • cost 100,000,000/bandwidth in bps
  • The lower the cost, the more likely the route is
    to be used

41
Single-Area OSPF Configuration Modifying the
OSPF Cost Metric
  • OSPF Cost Values

42
Single-Area OSPF Configuration Modifying the
OSPF Cost Metric
  • It is essential for proper OSPF operation that
    the correct interface bandwidth is set
  • Router(config)interface serial 0
  • Router(config-if)bandwidth 56
  • Cost can be changed to influence the outcome of
    OSPF cost calculation
  • When costs are from different vendors are
    unequal, might want to make change to match costs
  • Might need to change cost to account for Gigabit
    Ethernet
  • Use this command to change cost
  • Router(config-if)ip ospf cost number

43
Single-Area OSPF Configuration OSPF
Authentication
  • A router trusts the information that is coming
    from a router that should be sending it the
    information
  • To guarantee this trust, routers in a specific
    area can be configured to authenticate each other
    with OSPF authentication
  • Each interface can present an authentication key
    that the router uses to send OSPF information to
    other routers on the segment
  • The key, known as a password, is a shared secret
    between the routers
  • The key can be up to eight characters long
  • The key generates the authentication data in the
    OSPF header

44
Single-Area OSPF Configuration OSPF
Authentication
  • Use the following syntax to configure OSPF
    authentication
  • Router(config-if)ip ospf authentication-key
    password
  • After the password is configured, authentication
    must be enabled
  • Router(config-router)area area-number
    authentication
  • With simple authentication, the password is sent
    as plain text (security risk)
  • Configure encryption of the password

45
Single-Area OSPF Configuration OSPF
Authentication
  • Authentication password encryption syntax
  • Router(config-if)ip ospf message-digest-key
    key-id encryption-type md5 key (all on one
    line!)
  • The key-id is an identifier with a value of
    between 1 and 255
  • The encryption-type refers to the type of
    encryption, where 0 means none and 7 means
    proprietary
  • The following is configured in router
    configuration mode on a router with an interface
    in the area area-id
  • Router(config-router)area area-id authentication
    message-digest
  • MD5 creates a message digest, which is scrambled
    data based on the password and the message
    contents
  • If the digests match, the receiving router trusts
    the data

46
Single-Area OSPF Configuration OSPF Network
Types and OSPF Timers
  • OSPF interfaces automatically recognize three
    OSPF network types
  • Broadcast multiaccess, such as Ethernet
  • Point-to-point networks
  • Nonbroadcast multiaccess networks (NBMA), such as
    Frame Relay
  • An administrator can manually configure a fourth
    OSPF network type point-to-multipoint
  • In a multiaccess network, it is not known in
    advance how many routers will be connected
  • In point-to-point networks, only two routers will
    be connected

47
Single-Area OSPF Configuration OSPF Network
Types and OSPF Timers
  • In a broadcast multiaccess network segment, many
    routers can be connected
  • If every router has to establish adjacency with
    every other router, n (n-1) / 2 adjacencies
    need to be formed
  • For 5 routers the formula would be 5(5-1) / 2
    54 / 2 20 / 2 10 adjacencies
  • Routers hold an election for a DR router
  • This router becomes adjacent to all other routers
    in the broadcast segment
  • All other routers send their link-state
    information to the DR
  • The DR sends link-state information to all other
    routers on the segment by using the 224.0.0.5
    multicast address

48
Single-Area OSPF Configuration OSPF Network
Types and OSPF Timers
  • Despite the gain in efficiency that electing a DR
    provides, a disadvantage exists
  • The DR is a single point of failure
  • A second router is elected the BDR to take over
    in case the DR fails
  • To make sure that both the DR and BDR see the
    link states that all routers send on the segment,
    the 224.0.0.6 multicast address is used
  • On point-to-point networks, no DR or BDR is
    elected both routers become fully adjacent

49
Single-Area OSPF Configuration OSPF Network
Types and OSPF Timers
  • OSPF Network Type, Characteristics, and DR
    Election

50
Single-Area OSPF Configuration OSPF Network
Types and OSPF Timers
  • OSPF uses
  • Hello intervals
  • Default of 10 seconds on broadcast networks
  • Default of 30 seconds on nonbroadcast networks
  • Dead intervals (4 times the hellow interval by
    default)
  • Default of 40 seconds on broadcast networks
  • Default of 120 seconds on nonbroadcast networks
  • To change the default times
  • Router(config-if)ip ospf hello-interval seconds
  • Router(config-if)ip ospf dead-interval seconds

51
Single-Area OSPF Configuration Propagating a
Default Route
  • OSPF routing ensures loop-free paths to every
    network in the routing domain
  • To reach networks outside the domain, either OSPF
    must know about the network or OSPF must have a
    default route
  • To have an entry for every network in the world
    would require enormous resources for each router
  • A practical alternative is to add a default route
    to the OSPF router connected to the outside
    network
  • This default route can be redistributed to each
    router in the AS through normal OSPF updates

52
Single-Area OSPF Configuration Propagating a
Default Route
  • To configure a static default route
  • Router(config)ip route 0.0.0.0 0.0.0.0
    interface next hop address
  • This is referred to as the quad-zero route
  • Any destination network address is matched
  • To propagate this route to all the routers in a
    normal OSPF area
  • Router(config-router)default-information
    originate
  • All routers in the OSPF area learn a default
    route provided that the interface of the border
    router to the gateway router is active

53
Single-Area OSPF Configuration Verifying OSPF
Configuration
  • Several show commands display information about
    OSPF configuration
  • Display parameters about timers, filters, metrics
    and networks show ip protocols
  • Display the routes that are known to the router
    show ip route
  • Verify that interfaces have been configured in
    the intended areas show ip ospf interface
  • Display OSPF neighbor information on a
    per-interface basis show ip ospf neighbor

54
Single-Area OSPF Configuration Troubleshooting
OSPF
  • Output from the debug ip ospf events Command

55
Single-Area OSPF Configuration Troubleshooting
OSPF
  • The debug ip ospf events output might appear if
  • The IP subnet masks for routers on the same
    network do not match
  • The OSPF hello interval does not match that
    configured for a neighbor
  • The OSPF dead interval does not match that
    configured for a neighbor
  • If a router configured for OSPF does not see a
    router on an attached network
  • Make sure both routers are configured with the
    same subnet mask, OSPF hello and dead intervals
  • Make sure both neighbors are part of the same
    area type

56
Single-Area OSPF Configuration Troubleshooting
OSPF
  • Sample Output from the debug ip ospf packet
    Command

57
Single-Area OSPF Configuration Troubleshooting
OSPF
  • Fields in debug ip ospf packet Output

58
Single-Area OSPF Configuration Troubleshooting
OSPF
  • Fields in debug ip ospf packet Output (continued)

59
Summary
  • Link-state routing protocols such as OSPF and
    IS-IS quickly and reliably propagate routing
    information within an AS
  • Link-state routing protocols build link-state
    databases, which are synchronized with link-state
    advertisements (LSAs)
  • The link-state protocol then applies Dijkstras
    algorithm (SPF) to determine the best path(s) to
    each destination, which are then installed in the
    routing table
  • OSPF is the most commonly deployed link-state
    protocol
  • Employs DRs and BDRs on broadcast segments to
    optimize propagation of link-state information
  • Each link uses hello and dead interval timers
    depending on OSPF network type broadcast
    multiaccess, NBMA, point-to-point,
    point-to-multipoint

60
Summary
  • OSPF is configured by
  • Defining which interfaces will participate in a
    given OSPF process for a specific area
  • Use the network statements coupled with inverse
    masks
  • Inverse masks are often created to exactly match
    the subnet mask of the network associated with
    the given link, or they can be defined simply
    with a 0.0.0.0 mask to exactly match their
    interface ID
  • Verifying OSPF configurations is done with these
    commands show ip protocol, show ip route, show
    ip ospf interface, show ip ospf neighbor
  • Troubleshooting OSPF is done with these commands
    debug ip ospf events, debug ip ospf packets
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