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Module 4
Distance Vector Routing Protocols
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Identify the characteristics of distance vector routingprotocols.
Describe the network discovery process of distancevector routing protocols using Routing InformationProtocol (RIP).
Describe the processes to maintain accurate routingtables used by distance vector routing protocols. Identify the conditions leading to a routing loop and
explain the implications for router performance. Recognize that distance vector routing protocols are in
use today
Objectives
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Introduction to Distance VectorRouting Protocols
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Distance Vector Technology
A router using distance vector routing protocols knows 2 things: Distance to final destination Vector or direction traffic should be directed
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Distance Vector Technology
Characteristics of Distance Vector routing protocols: Periodic updates Neighbors - Routing by rumor Broadcast updates 255.255.255.255 Entire routing table is included with routing update
Routers using distance vector routing are not aware of the networktopology. 5
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DV Routing Protocol Algorithms
At the core of the distance vector protocol is the algorithm . The algorithmused 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 . 6
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DV Routing Protocols Characteristics
Advantages Disadvantages
Simple implementation andmaintenance
Slow convergence
Low resource requirement Limited scalability
Routing loops
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Routing Protocols Characteristics
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Network Discovery
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Cold Start
After a cold start and before the exchange of routing information, the routersinitially discover their own directly connected networks and subnet masks.
Network Discovery Cold Start
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RTA RTB RTCNetwork W Network X Network Y Network Z
Routing Table(Distance) (Vector)
Net. Hops Exit-int.
W 0
Routing Table(Distance) (Vector)
Net. Hops Exit-int.
X 0
Routing Table(Distance) (Vector)
Net. Hops Exit-int.
Y 0
RTA RTB RTCNetwork W Network X Network Y Network Z
Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTA
Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTB
Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTC
Exchange of Routing Information
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RTA RTB RTCNetwork W Network X Network Y Network Z
Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTA
Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTB
Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.W 0 Y 1 RTB
Routing Table(Distance) (Vector)
Net. Hops Exit-int.X 0 W 1 RTAZ 1 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.Y 0 X 1 RTB
RTA RTB RTCNetwork W Network X Network Y Network Z
Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTA
Y 2 RTA
Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTB
W 2 RTBZ 2 RTB
Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTC
X 2 RTC
Exchange of Routing Information
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RTA RTB RTC
Network WNetwork X Network Y
Network Z
Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTAY 2 RTA
Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTBW 2 RTBZ 2 RTB
Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTCX 2 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.W 0 Y 1 RTBZ 2 RTB
Routing Table(Distance) (Vector)
Net. Hops Exit-int.X 0 W 1 RTAZ 1 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.Y 0 X 1 RTBW 2 RTB
Routing Table(Distance) (Vector)
Net. Hops Exit-int.W 0 Y 1 RTB
Routing Table(Distance) (Vector)
Net. Hops Exit-int.X 0 W 1 RTAZ 1 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.Y 0 X 1 RTB
ExistingRoutingTables
NewRoutingTables
Exchange of Routing Information
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RTA RTB RTCNetwork W Network X Network Y Network Z
Routing Table(Distance) (Vector)
Net. Hops Exit-int.W 0 Y 1 RTBZ 2 RTB
Routing Table(Distance) (Vector)
Net. Hops Exit-int.X 0 W 1 RTAZ 1 RTC
Routing Table(Distance) (Vector)
Net. Hops Exit-int.Y 0 X 1 RTBW 2 RTB
RoutingTables
Convergence !
Exchange of Routing Information
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Routing Table Maintenance
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Periodic Updates : RIPv1 & IGRP
Periodic Updates refers to the fact that a router sends the complete routingtable to its neighbors at a predefined interval .
Changes may occur for several reasons, including: Failure of a link ;Introduction of a new link ; Failure of a router ; Change of link parameters .
Periodic Updates
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Periodic Updates: RIPv1 & IGRP
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Bounded Updates: EIGRP
EIGRP uses updates that are: Non-periodic because they are not sent out on a regular basis ->Triggered by topology changes 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 areupdated.
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Triggered Updates
To speed up the convergence when there is a topology change , RIP usestriggered updates .
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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Routing Loops
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Definitions & Implications
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1. Network 10.4.0.0 goes down
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2. Before R3 can send an update, R2 sends an update
Definitions & Implications
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3. R3 install a bad route to 10.4.0.0
Definitions & Implications
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Definitions & Implications
What is a Routing Loop ? A routing loop is a condition in which a packet is continuously transmitted
within a series of routers without ever reaching its intended destinationnetwork.
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4. The Network now has a loop
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Problem: Count to InfinityR3 install a bad route to 10.4.0.0 with a hop count of 2
R2 increases the hop count to 3 for 10.4.0.0
R1 increases the hop count to 4 for 10.4.0.0
25Each round of updates continues to increase hop count
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Solution: Setting a Maximum Metric
Once the routers " count to infinity " they mark the route as unreachable . For example:
RIP defines infinity as 16 hops - an "unreachable" metric IGRP : default maximum hop count is 100 , but can configure to 255 EIGRP : maximum hop count is 224
10.4.0.0 is unreachable. Hop count is 16
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i i i h
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Preventing Routing Loops withHolddown Timers
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P i R i L i h
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Preventing Routing Loops withHolddown Timers
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X
P i R i L i h
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Preventing Routing Loops withHolddown Timers
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P i R i L i h
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Preventing Routing Loops withHolddown Timers
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To
P i R i L i h
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Preventing Routing Loops withHolddown Timers
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P ti R ti L ith
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Preventing Routing Loops withHolddown Timers
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P ti R ti L ith
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Preventing Routing Loops withHolddown Timers
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Split Horizon Rule
The split horizon rule says that a router should not advertise a networkthrough the interface from which the update came .
Note: Split horizon can be disabled by an administrator. Under certainconditions, this has to be done to achieve the proper routing
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R2 only advertises10.2.0.0 and 10.1.0.0 to R3
R1 only advertises10.1.0.0 to R2 R2 only advertises
10.3.0.0 and 10.4.0.0 to R1
R3 only advertises10.4.0.0 to R2
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The Network is converged on poisoned route
Route Poisoning
Route poisoning is used to mark the route as unreachable in a routing updatethat is sent to other routers
Route poisoning speeds up the convergence process as the information about10.4.0.0 spreads through the network more quickly than waiting for the hopcount to reach "infinity". 35
Route Poisoning1. Network 10.4.0.0 goes down2. R3 poisons route with an infinite metric3. R3 sends triggered Poison Update to R24. 23 sends triggered Poison Update to R1
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IP & TTL
Time to Live (TTL) is an 8-bit field in the IP header that limits the number ofhops a packet can traverse through the network before it is discarded.
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Distance Vector Routing
Protocols Today
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RIP and EIGRP
The decision about which routing protocol to use in a given situationis influenced by a number of factors including:
Size of the network Compatibility between models of routers
Administrative knowledge required
Distance Vector Routing Protocol Compared
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RIP and EIGRP Features of RIP :
Supports split horizon and split horizon with poison reverse to prevents loops. Is capable of load balancing up to six equal cost paths. The default is four equal cost
paths. Easy to configure Works in a multi vendor router environment
RIPv2 introduced the following improvements to RIPv1 : Includes the subnet mask in the routing updates, making it a classless routing
protocol. Has authentication mechanism to secure routing table updates. Supports variable length subnet mask (VLSM). Uses multicast addresses instead of broadcast. Supports manual route summarization.
EIGRP features include : Triggered updates (EIGRP has no periodic updates). Use of a topology table to maintain all the routes received from neighbors (not only the
best paths). Establishment of adjacencies with neighboring routers using the EIGRP hello protocol. Support for VLSM and manual route summarization. These allow EIGRP to create
hierarchically structured large networks. Cisco proprietary protocol
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Good luck with this module!
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