Distance Vector Routing Protocols

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Distance Vector Routing Protocols

• W.lilakiatsakun

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Meaning of distance Vector (1/2)

• A router using a distance vector routing protocol
  does not have the knowledge of the entire path to
  a destination network.
• The router only knows
• The direction or interface in which packets
  should be forwarded and
• The distance or how far it is to the destination
  network

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Meaning of distance Vector (2/2)
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Operation of distance vector (1/4)

• Some distance vector routing protocols call for
  the router to periodically broadcast the entire
  routing table to each of its neighbors.
• This method is inefficient because the updates
  not only consume bandwidth but also consume
  router CPU resources to process the updates.

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Operation of distance vector (2/4)

• Periodic Updates are sent at regular intervals
  (30 seconds for RIP and 90 seconds for IGRP).
• Even if the topology has not changed in several
  days, periodic updates continue to be sent to all
  neighbors.
• Neighbors are routers that share a link and are
  configured to use the same routing protocol.
• The router is only aware of the network addresses
  of its own interfaces and the remote network
  addresses it can reach through its neighbors

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Operation of distance vector (3/4)

• Broadcast Updates are sent to 255.255.255.255.
• Neighboring routers that are configured with the
  same routing protocol will process the updates.
• All other devices will also process the update up
  to Layer 3 before discarding it.
• Some distance vector routing protocols use
  multicast addresses instead of broadcast
  addresses.

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Operation of distance vector (4/4)

• Entire Routing Table Updates are sent,
  periodically to all neighbors.
• Neighbors receiving these updates must process
  the entire update to find pertinent information
  and discard the rest.
• Some distance vector routing protocols like EIGRP
  do not send periodic routing table updates.

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Routing Algorithm

• The algorithm used for the routing protocols
  defines the following processes
• Mechanism for sending and receiving routing
  information.
• Mechanism for calculating the best paths and
  installing routes in the routing table.
• Mechanism for detecting and reacting to topology
  changes.

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Routing protocol characteristics (1/3)

• Time to Convergence - Time to convergence defines
  how quickly the routers in the network topology
  share routing information and reach a state of
  consistent knowledge.
• The faster the convergence, the more preferable
  the protocol.
• Routing loops can occur when inconsistent routing
  tables are not updated due to slow convergence in
  a changing network.

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Routing protocol characteristics (2/3)

• Scalability - Scalability defines how large a
  network can become based on the routing protocol
  that is deployed.
• The larger the network is, the more scalable the
  routing protocol needs to be.
• Classless (Use of VLSM) or Classful - Classless
  routing protocols include the subnet mask in the
  updates.
• This feature supports the use of Variable Length
  Subnet Masking (VLSM) and better route
  summarization.
• Classful routing protocols do not include the
  subnet mask and cannot support VLSM.

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Routing protocol characteristics (3/3)

• Resource Usage - Resource usage includes the
  requirements of a routing protocol such as memory
  space, CPU utilization, and link bandwidth
  utilization
• Higher resource requirements necessitate more
  powerful hardware to support the routing protocol
  operation in addition to the packet forwarding
  processes.
• Implementation and Maintenance - Implementation
  and maintenance describes the level of knowledge
  that is required for a network administrator to
  implement and maintain the network based on the
  routing protocol deployed.

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Distance Vector Routing Protocols
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Comparison of Routing Protocol
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Periodic updates RIP(1/3)

• The term periodic updates refers to the fact that
  a router sends the complete routing table to its
  neighbors at a predefined interval.
• For RIP, these updates are sent every 30 seconds
  as a broadcast (255.255.255.255) whether or not
  there has been a topology change.
• This 30-second interval is a route update timer
  that also aids in tracking the age of routing
  information in the routing table.

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Periodic updatesRIP (2/3)

• The age of routing information in a routing table
  is refreshed each time an update is received.
• This way information in the routing table can be
  maintained when there is a topology change.
• Changes may occur for several reasons, including
• Failure of a link
• Introduction of a new link
• Failure of a router
• Change of link parameters

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Periodic updatesRIP (3/3)
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RIP Timers (1/3)

• In addition to the update timer, the IOS
  implements three additional timers for RIP
• Invalid Timer. If an update has not been received
  to refresh an existing route after 180 seconds
  (the default), the route is marked as invalid by
  setting the metric to 16.
• The route is retained in the routing table until
  the flush timer expires.
• Flush Timer. By default, the flush timer is set
  for 240 seconds, which is 60 seconds longer than
  the invalid timer. When the flush timer expires,
  the route is removed from the routing table.

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RIP Timers (2/3)

• Holddown Timer. This timer stabilizes routing
  information and helps prevent routing loops
  during periods when the topology is converging on
  new information.
• Once a route is marked as unreachable, it must
  stay in holddown long enough for all routers in
  the topology to learn about the unreachable
  network.
• By default, the holddown timer is set for 180
  seconds.

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RIP Timers (3/3)
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Bounded Updates EIGRP(1/2)

• Unlike other distance vector routing protocols,
  EIGRP does not send periodic updates.
• Instead, EIGRP sends bounded updates about a
  route when a path changes or the metric for that
  route changes.
• When a new route becomes available or when a
  route needs to be removed, EIGRP sends an update
  only about that network instead of the entire
  table.
• This information is sent only to those routers
  that need it.

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Bounded Updates EIGRP(2/2)

• EIGRP uses updates that are
• Non-periodic because they are not sent out on a
  regular basis.
• Partial updates sent only when there is a change
  in topology that influences routing information.
• Bounded, meaning the propagation of partial
  updates are automatically bounded so that only
  those routers that need the information are
  updated.

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Triggered Update (1/3)

• To speed up the convergence when there is a
  topology change, RIP uses triggered updates.
• A triggered update is a routing table update that
  is sent immediately in response to a routing
  change.
• Triggered updates do not wait for update timers
  to expire.
• The detecting router immediately sends an update
  message to adjacent routers.
• The receiving routers, in turn, generate
  triggered updates that notify their neighbors of
  the change.

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Triggered Update (2/3)

• Triggered updates are sent when one of the
  following occurs
• An interface changes state (up or down)
• A route has entered (or exited) the "unreachable"
  state
• A route is installed in the routing table

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Triggered Update (3/3)

• However, there are two problems with triggered
  updates
• Packets containing the update message can be
  dropped or corrupted by some link in the network.
• The triggered updates do not happen
  instantaneously. It is possible that a router
  that has not yet received the triggered update
  will issue a regular update at just the wrong
  time, causing the bad route to be reinserted in a
  neighbor that had already received the triggered
  update.

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Routing Loop (1/6)

• A routing loop is a condition in which a packet
  is continuously transmitted within a series of
  routers without ever reaching its intended
  destination network.
• A routing loop can occur when two or more
  routers have routing information that incorrectly
  indicates that a valid path to an unreachable
  destination exists.

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Routing Loop (2/6)

• The loop may be a result of
• Incorrectly configured static routes
• Incorrectly configured route redistribution
  (redistribution is a process of handing the
  routing information from one routing protocol to
  another routing protocol)
• Inconsistent routing tables not being updated due
  to slow convergence in a changing network
• Incorrectly configured or installed discard
  routes

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Routing Loop (3/6)
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Routing Loop (4/6)
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Routing Loop (5/6)
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Routing Loop (6/6)
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Count to infinity (1/5)

• Count to infinity is a condition that exists when
  inaccurate routing updates increase the metric
  value to "infinity" for a network that is no
  longer reachable.

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Count to infinity (2/5)
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Count to infinity (3/5)
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Count to infinity (4/5)
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Count to infinity (5/5)
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Setting a Maximum (1/2)

• To eventually stop the incrementing of the
  metric, "infinity" is defined by setting a
  maximum metric value.
• For example, RIP defines infinity as 16 hops - an
  "unreachable" metric.
• Once the routers "count to infinity," they mark
  the route as unreachable.

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Setting a Maximum (2/2)
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Preventing routing loop with holddown timer (1/5)

• Holddown timers are used to prevent regular
  update messages from inappropriately reinstating
  a route that may have gone bad.
• Holddown timers instruct routers to hold any
  changes that might affect routes for a specified
  period of time.
• If a route is identified as down or possibly
  down, any other information for that route
  containing the same status, or worse, is ignored
  for a predetermined amount of time (the holddown
  period).

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Preventing routing loop with holddown timer (2/5)
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Preventing routing loop with holddown timer (3/5)
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Preventing routing loop with holddown timer (4/5)
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Preventing routing loop with holddown timer (5/5)
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Split Horizon Rules (1/5)

• The split horizon rule says that a router should
  not advertise a network through the interface
  from which the update came.

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Split Horizon Rules (2/5)
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Split Horizon Rules (3/5)
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Split Horizon Rules (4/5)
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Split Horizon Rules (5/5)
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Route Poisoning (1/4)

• Route poisoning is yet another method employed by
  distance vector routing protocols to prevent
  routing loops.
• Route poisoning is used to mark the route as
  unreachable in a routing update that is sent to
  other routers.
• Unreachable is interpreted as a metric that is
  set to the maximum.
• For RIP, a poisoned route has a metric of 16.

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Route Poisoning (2/4)
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Route Poisoning (3/4)
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Route Poisoning (4/4)
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Split Horizon with Poison reverse (1/5)

• The concept of split horizon with poison reverse
  is that explicitly telling a router to ignore a
  route is better than not telling it about the
  route in the first place.

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Split Horizon with Poison reverse (2/5)

• The following process occurs
• Network 10.4.0.0 becomes unavailable due to a
  link failure.
• R3 poisons the metric with a value of 16 and then
  sends out a triggered update stating that
  10.4.0.0 is unavailable.
• R2 processes that update, invalidates the routing
  entry in its routing table, and immediately sends
  a poison reverse back to R3.

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Split Horizon with Poison reverse (3/5)
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Split Horizon with Poison reverse (4/5)
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Split Horizon with Poison reverse (5/5)
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Time to Live (1/2)

• Time to Live (TTL) is an 8-bit field in the IP
  header that limits the number of hops a packet
  can traverse through the network before it is
  discarded.
• The purpose of the TTL field is to avoid a
  situation in which an undeliverable packet keeps
  circulating on the network endlessly.

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Time to Live (2/2)

• With TTL, the 8-bit field is set with a value by
  the source device of the packet. The TTL is
  decreased by one by every router on the route to
  its destination.
• If the TTL field reaches zero before the packet
  arrives at its destination, the packet is
  discarded and the router sends an Internet
  Control Message Protocol (ICMP) error message
  back to the source of the IP packet