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