2. Meaning of distance Vector (1/2)Meaning of distance Vector (1/2)
• A router using a distance vector routingA router using a distance vector routing
protocol does not have the knowledge ofprotocol does not have the knowledge of
the entire path to a destination network.the entire path to a destination network.
• The router only knowsThe router only knows
– The direction or interface in which packetsThe direction or interface in which packets
should be forwarded andshould be forwarded and
– The distance or how far it is to the destinationThe distance or how far it is to the destination
networknetwork
4. Operation of distance vector (1/4)Operation of distance vector (1/4)
• Some distance vector routing protocolsSome distance vector routing protocols
call for the router to periodically broadcastcall for the router to periodically broadcast
the entire routing table to each of itsthe entire routing table to each of its
neighbors.neighbors.
• This method is inefficient because theThis method is inefficient because the
updates not only consume bandwidth butupdates not only consume bandwidth but
also consume router CPU resources toalso consume router CPU resources to
process the updates.process the updates.
5. Operation of distance vector (2/4)Operation of distance vector (2/4)
• Periodic UpdatesPeriodic Updates are sent at regular intervals (30are sent at regular intervals (30
seconds for RIP and 90 seconds for IGRP).seconds for RIP and 90 seconds for IGRP).
– Even if the topology has not changed in several days,Even if the topology has not changed in several days,
periodic updates continue to be sent to all neighbors.periodic updates continue to be sent to all neighbors.
– Neighbors are routers that share a link and areNeighbors are routers that share a link and are
configured to use the same routing protocol.configured to use the same routing protocol.
– The router is only aware of the network addresses ofThe router is only aware of the network addresses of
its own interfaces and the remote network addresses itits own interfaces and the remote network addresses it
can reach through its neighborscan reach through its neighbors
6. Operation of distance vector (3/4)Operation of distance vector (3/4)
• Broadcast UpdatesBroadcast Updates are sent toare sent to
255.255.255.255.255.255.255.255.
– Neighboring routers that are configured withNeighboring routers that are configured with
the same routing protocol will process thethe same routing protocol will process the
updates.updates.
– All other devices will also process the updateAll other devices will also process the update
up to Layer 3 before discarding it.up to Layer 3 before discarding it.
– Some distance vector routing protocols useSome distance vector routing protocols use
multicast addresses instead of broadcastmulticast addresses instead of broadcast
addresses.addresses.
7. Operation of distance vector (4/4)Operation of distance vector (4/4)
• Entire Routing Table UpdatesEntire Routing Table Updates are sent,are sent,
periodically to all neighbors.periodically to all neighbors.
– Neighbors receiving these updates mustNeighbors receiving these updates must
process the entire update to find pertinentprocess the entire update to find pertinent
information and discard the rest.information and discard the rest.
– Some distance vector routing protocols likeSome distance vector routing protocols like
EIGRP do not send periodic routing tableEIGRP do not send periodic routing table
updates.updates.
8. Routing AlgorithmRouting Algorithm
• The algorithm used for the routing protocolsThe algorithm used for the routing protocols
defines the following processes:defines the following processes:
– Mechanism for sending and receiving routingMechanism for sending and receiving routing
information.information.
– Mechanism for calculating the best paths andMechanism for calculating the best paths and
installing routes in the routing table.installing routes in the routing table.
– Mechanism for detecting and reacting toMechanism for detecting and reacting to
topology changes.topology changes.
9. Routing protocol characteristicsRouting protocol characteristics
(1/3)(1/3)
• Time to ConvergenceTime to Convergence - Time to convergence- Time to convergence
defines how quickly the routers in thedefines how quickly the routers in the
network topology share routing informationnetwork topology share routing information
and reach a state of consistent knowledge.and reach a state of consistent knowledge.
– The faster the convergence, the more preferableThe faster the convergence, the more preferable
the protocol.the protocol.
– Routing loops can occur when inconsistentRouting loops can occur when inconsistent
routing tables are not updated due to slowrouting tables are not updated due to slow
convergence in a changing network.convergence in a changing network.
10. Routing protocol characteristicsRouting protocol characteristics
(2/3)(2/3)
• ScalabilityScalability - Scalability defines how large a- Scalability defines how large a
network can become based on the routingnetwork can become based on the routing
protocol that is deployed.protocol that is deployed.
– The larger the network is, the more scalable the routingThe larger the network is, the more scalable the routing
protocol needs to be.protocol needs to be.
• Classless (Use of VLSM) or ClassfulClassless (Use of VLSM) or Classful - Classless- Classless
routing protocols include the subnet mask in therouting protocols include the subnet mask in the
updates.updates.
– This feature supports the use of Variable Length SubnetThis feature supports the use of Variable Length Subnet
Masking (VLSM) and better route summarization.Masking (VLSM) and better route summarization.
– Classful routing protocols do not include the subnetClassful routing protocols do not include the subnet
mask and cannot support VLSM.mask and cannot support VLSM.
11. Routing protocol characteristicsRouting protocol characteristics
(3/3)(3/3)
• Resource UsageResource Usage - Resource usage includes the- Resource usage includes the
requirements of a routing protocol such as memoryrequirements of a routing protocol such as memory
space, CPU utilization, and link bandwidthspace, CPU utilization, and link bandwidth
utilizationutilization
– Higher resource requirements necessitate more powerfulHigher resource requirements necessitate more powerful
hardware to support the routing protocol operation inhardware to support the routing protocol operation in
addition to the packet forwarding processes.addition to the packet forwarding processes.
• Implementation and MaintenanceImplementation and Maintenance - Implementation- Implementation
and maintenance describes the level of knowledgeand maintenance describes the level of knowledge
that is required for a network administrator tothat is required for a network administrator to
implement and maintain the network based on theimplement and maintain the network based on the
routing protocol deployed.routing protocol deployed.
14. Periodic updatesPeriodic updates :RIP(1/3):RIP(1/3)
• The term periodic updates refers to the factThe term periodic updates refers to the fact
that a router sends the complete routingthat a router sends the complete routing
table to its neighbors at a predefinedtable to its neighbors at a predefined
interval.interval.
– For RIP, these updates are sent every 30For RIP, these updates are sent every 30
seconds as a broadcast (255.255.255.255)seconds as a broadcast (255.255.255.255)
whether or not there has been a topologywhether or not there has been a topology
change.change.
– This 30-second interval is a route update timerThis 30-second interval is a route update timer
that also aids in tracking the age of routingthat also aids in tracking the age of routing
information in the routing table.information in the routing table.
15. Periodic updates:RIPPeriodic updates:RIP (2/3)(2/3)
• The age of routing information in a routing table isThe age of routing information in a routing table is
refreshed each time an update is received.refreshed each time an update is received.
• This way information in the routing table can beThis way information in the routing table can be
maintained when there is a topology change.maintained when there is a topology change.
• Changes may occur for several reasons,Changes may occur for several reasons,
including:including:
– Failure of a linkFailure of a link
– Introduction of a new linkIntroduction of a new link
– Failure of a routerFailure of a router
– Change of link parametersChange of link parameters
17. RIP TimersRIP Timers (1/3)(1/3)
• In addition to the update timer, the IOS implementsIn addition to the update timer, the IOS implements
three additional timers for RIP:three additional timers for RIP:
• Invalid TimerInvalid Timer. If an update has not been received to. If an update has not been received to
refresh an existing route after 180 seconds (therefresh an existing route after 180 seconds (the
default), the route is marked as invalid by settingdefault), the route is marked as invalid by setting
the metric to 16.the metric to 16.
– The route is retained in the routing table until the flushThe route is retained in the routing table until the flush
timer expires.timer expires.
• Flush TimerFlush Timer. By default, the flush timer is set for. By default, the flush timer is set for
240 seconds, which is 60 seconds longer than the240 seconds, which is 60 seconds longer than the
invalid timer. When the flush timer expires, theinvalid timer. When the flush timer expires, the
route is removed from the routing table.route is removed from the routing table.
18. RIP TimersRIP Timers (2/3)(2/3)
• Holddown Timer. This timer stabilizesHolddown Timer. This timer stabilizes
routing information and helps preventrouting information and helps prevent
routing loops during periods when therouting loops during periods when the
topology is converging on new information.topology is converging on new information.
– Once a route is marked as unreachable, itOnce a route is marked as unreachable, it
must stay in holddown long enough for allmust stay in holddown long enough for all
routers in the topology to learn about therouters in the topology to learn about the
unreachable network.unreachable network.
– By default, the holddown timer is set for 180By default, the holddown timer is set for 180
seconds.seconds.
20. Bounded Updates :EIGRP(1/2)Bounded Updates :EIGRP(1/2)
• Unlike other distance vector routing protocols,Unlike other distance vector routing protocols,
EIGRP does not send periodic updates.EIGRP does not send periodic updates.
• Instead, EIGRP sends bounded updates about aInstead, EIGRP sends bounded updates about a
route when a path changes or the metric for thatroute when a path changes or the metric for that
route changes.route changes.
• When a new route becomes available or when aWhen a new route becomes available or when a
route needs to be removedroute needs to be removed,, EIGRP sends anEIGRP sends an
update only about that network instead of theupdate only about that network instead of the
entire table.entire table.
• This information is sent only to those routers thatThis information is sent only to those routers that
need it.need it.
21. Bounded Updates :EIGRP(2/2)Bounded Updates :EIGRP(2/2)
• EIGRP uses updates that are:EIGRP uses updates that are:
– Non-periodic because they are not sent out onNon-periodic because they are not sent out on
a regular basis.a regular basis.
– Partial updates sent only when there is aPartial updates sent only when there is a
change in topology that influences routingchange in topology that influences routing
information.information.
– Bounded, meaning the propagation of partialBounded, meaning the propagation of partial
updates are automatically bounded so thatupdates are automatically bounded so that
only those routers that need the informationonly those routers that need the information
are updated.are updated.
22. Triggered UpdateTriggered Update (1/3)(1/3)
• To speed up the convergence when there is aTo speed up the convergence when there is a
topology change,topology change, RIP uses triggered updatesRIP uses triggered updates..
• A triggered update is a routing table update thatA triggered update is a routing table update that
is sent immediately in response to a routingis sent immediately in response to a routing
changechange..
• Triggered updates do not wait for update timersTriggered updates do not wait for update timers
to expire.to expire.
– The detecting router immediately sends an updateThe detecting router immediately sends an update
message to adjacent routers.message to adjacent routers.
– The receiving routers, in turn, generate triggeredThe receiving routers, in turn, generate triggered
updates that notify their neighbors of the change.updates that notify their neighbors of the change.
23. Triggered UpdateTriggered Update (2/3)(2/3)
• Triggered updates are sent when one ofTriggered updates are sent when one of
the following occurs:the following occurs:
– An interface changes state (up or down)An interface changes state (up or down)
– A route has entered (or exited) theA route has entered (or exited) the
"unreachable" state"unreachable" state
– A route is installed in the routing tableA route is installed in the routing table
24. Triggered UpdateTriggered Update (3/3)(3/3)
• However, there are two problems with triggeredHowever, there are two problems with triggered
updates:updates:
– Packets containing the update message can bePackets containing the update message can be
dropped or corrupted by some link in the network.dropped or corrupted by some link in the network.
– The triggered updates do not happen instantaneously. ItThe triggered updates do not happen instantaneously. It
is possible that a router that has not yet received theis possible that a router that has not yet received the
triggered update will issue a regular update at just thetriggered update will issue a regular update at just the
wrong time, causing the bad route to be reinserted in awrong time, causing the bad route to be reinserted in a
neighbor that had already received the triggeredneighbor that had already received the triggered
update.update.
25. Routing Loop (1/6)Routing Loop (1/6)
• A routing loop is a condition in which aA routing loop is a condition in which a
packet is continuously transmitted within apacket is continuously transmitted within a
series of routers without ever reaching itsseries of routers without ever reaching its
intended destination network.intended destination network.
• A routing loop can occur when two orA routing loop can occur when two or
more routers have routing information thatmore routers have routing information that
incorrectly indicates that a valid path to anincorrectly indicates that a valid path to an
unreachable destination exists.unreachable destination exists.
26. Routing Loop (2/6)Routing Loop (2/6)
• The loop may be a result of:The loop may be a result of:
– Incorrectly configured static routesIncorrectly configured static routes
– Incorrectly configured route redistributionIncorrectly configured route redistribution
(redistribution is a process of handing the routing(redistribution is a process of handing the routing
information from one routing protocol to anotherinformation from one routing protocol to another
routing protocol)routing protocol)
– Inconsistent routing tables not being updatedInconsistent routing tables not being updated
due to slow convergence in a changing networkdue to slow convergence in a changing network
– Incorrectly configured or installed discard routesIncorrectly configured or installed discard routes
31. Count to infinity (1/5)Count to infinity (1/5)
• Count to infinity is a condition that existsCount to infinity is a condition that exists
when inaccurate routing updates increasewhen inaccurate routing updates increase
the metric value to "infinity" for a networkthe metric value to "infinity" for a network
that is no longer reachable.that is no longer reachable.
36. Setting a Maximum (1/2)Setting a Maximum (1/2)
• To eventually stop the incrementing of theTo eventually stop the incrementing of the
metric, "infinity" is defined by setting ametric, "infinity" is defined by setting a
maximum metric value.maximum metric value.
• For example, RIP defines infinity as 16For example, RIP defines infinity as 16
hops - an "unreachable" metric.hops - an "unreachable" metric.
• Once the routers "count to infinity," theyOnce the routers "count to infinity," they
mark the route as unreachable.mark the route as unreachable.
38. Preventing routing loop withPreventing routing loop with
holddown timerholddown timer (1/5)(1/5)
• Holddown timers are used to prevent regularHolddown timers are used to prevent regular
update messages from inappropriately reinstatingupdate messages from inappropriately reinstating
a route that may have gone bad.a route that may have gone bad.
• Holddown timers instruct routers to hold anyHolddown timers instruct routers to hold any
changes that might affect routes for a specifiedchanges that might affect routes for a specified
period of time.period of time.
• If a route is identified as down or possibly down,If a route is identified as down or possibly down,
any other information for that route containing theany other information for that route containing the
same status, or worse, is ignored for asame status, or worse, is ignored for a
predetermined amount of time (the holddownpredetermined amount of time (the holddown
period).period).
43. Split Horizon Rules (1/5)Split Horizon Rules (1/5)
• The split horizon rule says that a routerThe split horizon rule says that a router
should not advertise a network throughshould not advertise a network through
the interface from which the update came.the interface from which the update came.
48. Route Poisoning (1/4)Route Poisoning (1/4)
• Route poisoning is yet another methodRoute poisoning is yet another method
employed by distance vector routingemployed by distance vector routing
protocols to prevent routing loops.protocols to prevent routing loops.
• Route poisoning is used to mark the routeRoute poisoning is used to mark the route
as unreachable in a routing update that isas unreachable in a routing update that is
sent to other routers.sent to other routers.
• Unreachable is interpreted as a metric thatUnreachable is interpreted as a metric that
is set to the maximum.is set to the maximum.
– For RIP, a poisoned route has a metric of 16.For RIP, a poisoned route has a metric of 16.
52. Split Horizon with Poison reverseSplit Horizon with Poison reverse
(1/5)(1/5)
• The concept of split horizon with poisonThe concept of split horizon with poison
reverse is that explicitly telling a router toreverse is that explicitly telling a router to
ignore a route is better than not telling itignore a route is better than not telling it
about the route in the first place.about the route in the first place.
53. Split Horizon with Poison reverseSplit Horizon with Poison reverse
(2/5)(2/5)
• The following process occurs:The following process occurs:
• Network 10.4.0.0 becomes unavailable due to aNetwork 10.4.0.0 becomes unavailable due to a
link failure.link failure.
• R3 poisons the metric with a value of 16 andR3 poisons the metric with a value of 16 and
then sends out a triggered update stating thatthen sends out a triggered update stating that
10.4.0.0 is unavailable.10.4.0.0 is unavailable.
• R2 processes that update, invalidates theR2 processes that update, invalidates the
routing entry in its routing table, and immediatelyrouting entry in its routing table, and immediately
sends a poison reverse back to R3.sends a poison reverse back to R3.
54. Split Horizon with Poison reverseSplit Horizon with Poison reverse
(3/5)(3/5)
55. Split Horizon with Poison reverseSplit Horizon with Poison reverse
(4/5)(4/5)
56. Split Horizon with Poison reverseSplit Horizon with Poison reverse
(5/5)(5/5)
57. Time to Live (1/2)Time to Live (1/2)
• Time to Live (TTL) is an 8-bit field in the IPTime to Live (TTL) is an 8-bit field in the IP
headerheader that limits the number of hops athat limits the number of hops a
packet can traverse through the networkpacket can traverse through the network
before it is discarded.before it is discarded.
• The purpose of the TTL field is to avoid aThe purpose of the TTL field is to avoid a
situation in which an undeliverable packetsituation in which an undeliverable packet
keeps circulating on the network endlessly.keeps circulating on the network endlessly.
58. Time to Live (2/2)Time to Live (2/2)
• With TTL, the 8-bit field is set with a value by theWith TTL, the 8-bit field is set with a value by the
source device of the packet. The TTL issource device of the packet. The TTL is
decreased by one by every router on the routedecreased by one by every router on the route
to its destination.to its destination.
• If the TTL field reaches zero before the packetIf the TTL field reaches zero before the packet
arrives at its destination, the packet is discardedarrives at its destination, the packet is discarded
and the router sends an Internet Controland the router sends an Internet Control
Message Protocol (ICMP) error message backMessage Protocol (ICMP) error message back
to the source of the IP packetto the source of the IP packet
