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Distance Vector Routing Protocols

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Title: Distance Vector Routing Protocols


1
Distance VectorRouting Protocols
  • Module 7
  • CCNA 2 Version 3.0

2
Dynamic Routing
  • Dynamic routing - allows routers to react to
    changes in the network and adjust their routing
    tables automatically
  • It is important that a routing protocol be
    efficient in updating the routing tables
  • 2 broad categories of routing protocols are
  • interior routing within an AS
  • exterior routing between ASs
  • Routing protocols can also be classified as
  • Distance vector (updates neighbors, slow
    convergence)
  • Link state (LSAs to all network routers, fast
    convergence, topological database, memory
    intensive)

3
Distance Vector Routing Updates
  • Routing table updates occur
  • Periodically
  • each router sends its entire routing table to
    each of its adjacent neighbors
  • After a topology change
  • updates proceed systematically from router to
    router
  • The routing tables include
  • information about the total path cost (defined by
    the metric)
  • the logical address of the first router on the
    path to each network contained in the table

4
Routing Variables Metrics
  • Below are variables that can contribute to the
    routing metric, depending on the specific routing
    protocol used

5
Distance Vector Routing Loops
  • What causes routing loops?
  • inconsistent routing tables due to slow
    convergence
  • How do routing tables become inconsistent?
  • A network fails
  • The attached router is aware of the failure but
    other routers must wait for an update
  • In the meantime, the other routers continue to
    route packets to the failed network

6
How a Routing Loop Occurs
  • Network is converged (all routers have same
    knowledge)

3. Router A sends an update to Routers B and D
who stop routing to Network 1
2. Router A stops routing packets to Network 1
but all other routers continue to do so
4. Router C is still not aware of the failed
network
1. Network 1 fails so Router E sends an update
to Router A that does not include the failed
network
5. Router C sends an update to Routers B D
that includes the failed network
7. Routers B D send an update to Router A that
includes the failed network
6. Routers B D include the failed network in
their routing table
7
Count to Infinity Condition
  • The invalid updates of Network 1 continue to loop
    until some other process stops the looping (this
    condition is called count to infinity)
  • During a count to infinity, routers continually
    increment the hop count (metric) to other
    networks
  • To avoid this prolonged problem, distance vector
    protocols define infinity as a specific maximum
    number
  • This number refers to a routing metric (perhaps a
    hop count)
  • The routing protocol permits the routing loop to
    continue until the metric exceeds its maximum
    allowed value
  • Then the network is considered unreachable and
    the packet is discarded

8
Routing Loops Resolved Through Split Horizon
  • Split Horizon prevents routers from sending info
    about a particular route out an interface on
    which they originally received info about that
    route

Router 1
Router 2
9
Route Poisoning to Avoid Routing Loops
  • Failed routes are advertised with infinite
    metrics (this makes them unreachable)

10
More on Route Poisoning
  • When route poisoning is used with triggered
    updates it will speed up convergence time
  • Why?
  • Neighboring routers do not have to wait 30
    seconds before advertising the poisoned route
  • Essentially
  • Route Poisoning Split Horizon with Poison
    Reverse

11
Preventing Routing Loops with Hold down Timers
  • How holddown timers work
  • A router receives an update from a neighbor
    indicating that a previously accessible network
    is now inaccessible
  • The router marks the route as inaccessible and
    starts a holddown timer
  • If an update is received from the same neighbor
    indicating that the network is again accessible,
    the router marks the network as accessible and
    removes the holddown timer
  • If an update arrives from a different neighboring
    router with a better metric than originally
    recorded for the network, the router marks the
    network as accessible and removes the holddown
    timer
  • If at any time before the holddown timer expires
    an update is received from a different
    neighboring router with a poorer metric, the
    update is ignored
  • allows more time for the knowledge of a
    disruptive change to propagate through the entire
    network

12
RIP (Routing Information Protocol)
  • RIP is a distance vector routing protocol
  • Hop count is used as a metric for best path
    selection
  • Maximum hop count is 15
  • Routing updates are broadcast every 30 seconds

13
Rip Configuration
  • Include network statements for all networks
    directly connected to the router
  • The network command is then used to tell the
    router on which interfaces to run RIP
  • The router begins sending and receiving RIP
    updates on these interfaces
  • If you omit a network, the router will not
    advertise this network to its neighboring
    routers
  • RIP routers maintain only the best route to a
    destination but can maintain multiple equal-cost
    paths to the destination

14
RIP Triggered Updates
  • A router running RIP can be configured to send a
    triggered update when the network topology
    changes
  • The command is
  • router(config-if) ip rip triggered
  • This command is issued only on serial interfaces
  • After a configuration change
  • the router immediately updates its routing table
  • begins transmitting routing updates to other
    network routers
  • Triggered updates are sent independently of the
    regularly scheduled updates that RIP routers
    forward

15
The IP Classless Command
  • A supernet route is a route that covers a greater
    range of subnets with a single entry
  • For example, an enterprise might use the network
    10.0.0.0 /8 with a subnet 10.10.0.0/16
  • A supernet route for 10.10.10.0 /24 would be
    10.10.0.0 /16 (this includes the 10.10.10.0/24
    subnet)
  • Lets say a router receives packets destined for
    an unknown subnet of a network that has directly
    connected subnets
  • Ideally these packets should be routed to the
    best supernet route possible
  • The command router(config)ip classless
  • The ip classless command is enabled by default in
    Cisco IOS Software Release 11.3 and later

16
More on the IP Classless Command
  • SCENERIO A packet is received on a router that
    is attached to a subnetted network...
  • The destination address is for an unknown subnet
  • The router assumes that the subnet does not exist
  • The router will drop the packet, even if there is
    a default route
  • Configuring ip classless on the router allows the
    router to ignore the classful boundaries of the
    networks in its routing table and simply route to
    the default route

17
RIP Routing
  • To reduce routing loops and counting to infinity,
    RIP uses the following techniques
  • Count-to-infinity
  • Split horizon
  • Poison reverse
  • Holddown counters
  • Triggered updates
  • Some of these methods may require configuration
    and others dont

18
RIP Configuration
  • To change the holddown timer from the default 180
    (speeds convergence)
  • Router(config-router)timers basic update invalid
    holddown flush sleeptime
  • To change the update interval from the default 30
    seconds (conserves bandwidth)
  • GAD(config-router)update-timer seconds
  • To disable the sending of routing updates on
    specified interfaces
  • GAD(config-if) passive-interface

19
Verifying RIP Operations
  • Two commands to verify RIP operations are
  • show ip route and show ip protocols

20
Verifying RIP Operations
  • Additional commands to check RIP configuration
    are as follows
  • show interface interface
  • show ip interface interface
  • show running-config

21
Debug IP RIP Command
  • To find RIP update issues use the
  • debug ip rip command
  • Displays RIP routing updates as they are sent and
    received

22
Load Balancing with RIP
  • Load balancing is a concept that allows a router
    to take advantage of multiple best paths to a
    given destination
  • RIP is capable of load balancing over as many as
    six equal-cost paths, with four paths being
    default
  • RIP performs what is referred to as round robin
    load balancing (RIP takes turns forwarding
    packets over the parallel paths)

23
Static Routes
  • Static routes are user-defined routes that force
    packets moving between a source and a destination
    to take a specific path
  • waycross(config)ip route 172.16.3.0
    255.255.255.0 172.16.4.1 130
  • The 130 is a number that refers to the
    administrative distance (AD) or reliability of
    the route (the lower the better)
  • Static routes can specifying a gateway of last
    resort, commonly referred to as a default route
  • If a packet is destined for a subnet that is not
    explicitly listed in the routing table, the
    packet is forwarded to the default route
  • waycross(config)ip route 0.0.0.0 0.0.0.0
    next-hop-address outgoing interface
  • A static route can be configured as a backup by
    giving it a higher AD than a dynamic route

24
IGRP (Interior Gateway Routing Protocol)
  • IGRP
  • distance vector routing protocol
  • developed by Cisco
  • sends routing updates at 90 second intervals
  • Maximum hop count is 255
  • Key design characteristics of IGRP are
  • The versatility to automatically handle
    indefinite, complex topologies
  • The flexibility needed to segment with different
    bandwidth and delay characteristics
  • Scalability for functioning in very large
    networks

25
IGRP (Interior Gateway Routing Protocol)
  • IGRP uses a composite metric that includes
  • Bandwidth The lowest bandwidth value in the
    path
  • Delay The cumulative interface delay along the
    path
  • Reliability The reliability on the link towards
    the destination as determined by the exchange of
    keepalives
  • Load The load on a link towards the destination
    based on bits per second
  • MTU The Maximum Transmission Unit value of the
    path
  • By default, only bandwidth and delay are
    considered

26
IGRP Routes
  • IGRP advertises three types of routes
  • Interior (routes between subnets of a network
    attached to a router interface)
  • System (routes to networks within an autonomous
    system does not include subnet information)
  • Exterior (Exterior routes are routes to networks
    outside the autonomous system that are considered
    when identifying a gateway of last resort)

27
IGRP Stability Features
  • IGRP has a number of features that are designed
    to enhance its stability, such as
  • Holddowns
  • Split horizons
  • Poison reverse updates

28
IGRP Timers
  • IGRP maintains a number of timers and variables
    containing time intervals. These include
  • update timer
  • specifies how frequently routing update messages
    should be sent
  • invalid timer
  • specifies how long a router should wait in the
    absence of routing-update messages about a
    specific route before declaring that route
    invalid
  • holddown timer
  • specifies the amount of time for which
    information about poorer routes is ignored
  • flush timer
  • indicates how much time should pass before a
    route is flushed from the routing table

29
IGRP Configuration
30
Verifying IGRP Operation
  • To verify that IGRP has been configured properly,
    enter the show ip route command and look for IGRP
    routes signified by an "I

Next Hop Address
Metric
Administrative Distance (Default for IGRP is 100)
31
Verifying IGRP Operations
  • Additional commands for checking IGRP
    configuration are
  • show interface interface
  • show running-config
  • show running-config interface interface
  • show running-config begin interface interface
  • show running-config begin igrp
  • show ip protocols
  • show ip route
  • ping
  • traceroute

32
Troubleshooting IGRP
33
Distance VectorRouting Protocols
THE END
  • Module 7
  • CCNA 2 Version 3.0
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