Distance Vector Routing Protocols Distance Vector Routing.
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Transcript of Distance Vector Routing Protocols Distance Vector Routing.
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Distance Vector Routing Protocols
Distance Vector Routing
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Distance Vector Routing Updates
• Routing updates occur periodically or when the topology of the network changes.
• Updates proceed from router to router.• Each router sends its entire routing table
to each of its adjacent neighbors.• Routing tables include information about:
Total path cost as defined by the metrics Logical address of the first router on the path
to each network contained in the table
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Distance Vector Update Process
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Routing Loops Introduction
• Routing loops can occur when routing tables are inconsistent due to slow convergence in a changing network.
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Converged Network
• Just before the failure of the 192.168.0.0/24 network, all routers have consistent knowledge and correct routing tables.
• The network is said to have converged.• For RTC, the preferred path to the 192.168.0.0/24
network is through RTB, and the distance from RTC to the 192.168.0.0/24 network is 3.
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Converged Network Example
RTE#show ip route<output omitted>C 192.168.0.0/24 is directly connected, FastEthernet1/0
RTA#show ip route<output omitted>R 192.168.0.0/24 [120/1] via 192.168.1.2, FastEthernet0/0
RTB#show ip route<output omitted>R 192.168.0.0/24 [120/2] via 192.168.2.2, Serial2/0
RTD#show ip route<output omitted>R 192.168.0.0/24 [120/2] via 192.168.3.2, Serial2/0
RTC#show ip route<output omitted>R 192.168.0.0/24 [120/3] via 192.168.4.2, Serial2/0
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Network Failure
• After the 192.168.0.0/24 network fails and an update is scheduled to be sent, RTE sends an update to RTA.
• RTA stops routing packets to the 192.168.0.0/24 network , but RTB, RTC, and RTD continue to do so, because they have not yet been informed of the failure.
• When RTA sends out its normal update, RTB and RTD stop routing to the 192.168.0.0/24 network .
• However, RTC has not received an update.• For RTC, the 192.168.0.0/24 network can still be reached
through RTB.
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Network Failure ExampleRTE#show ip route<output omitted>No route to 192.168.0.0/24
RTA#show ip route<output omitted>No route to 192.168.0.0/24
RTB#show ip route<output omitted>No route to 192.168.0.0/24
RTD#show ip route<output omitted>No route to 192.168.0.0/24
RTC#show ip route<output omitted>R 192.168.0.0/24 [120/3] via 192.168.4.2, Serial2/0
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The Routing Loop
• Now RTC sends a periodic update to RTD, which indicates a path to the 192.168.0.0/24 network through of RTB.
• RTD changes its routing table to reflect this incorrect information, and sends the information to RTA.
• RTA sends the information to RTB and RTE, and the process continues.
• Any packet destined for the 192.168.0.0/24 network will now loop from RTC to RTB to RTA to RTD and back again to RTC.
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The Routing Loop Example
RTE#show ip route<output omitted>C 192.168.0.0/24 is directly connected, FastEthernet1/0
RTA#show ip route<output omitted>R 192.168.0.0/24 [120/5] via 192.168.3.1, Serial3/0
RTB#show ip route<output omitted>R 192.168.0.0/24 [120/6] via 192.168.2.2, Serial2/0
RTD#show ip route<output omitted>R 192.168.0.0/24 [120/4] via 192.168.5.1, Serial3/0
RTC#show ip route<output omitted>R 192.168.0.0/24 [120/3] via 192.168.4.2, Serial2/0
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Looping Forever
• Packets will loop continuously around the network, despite the 192.168.0.0/24 network being down.
• Each time a router handles a packet it increases the packet’s hop count value. Counting to infinity
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Combating Routing Loops
• A number of dynamic routing protocol measures can prevent routing loops from occurring or limit the problems they cause. Defining a maximum hop count Split horizon Route poisoning Triggered updates Holddown timers
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Defining Infinity
• Distance-vector protocols define infinity as a specific maximum number of hops. This number usually refers to a maximum hop count. 15 for RIP, 255 for IGRP
• The routing protocol permits the routing loop to continue until the hop count reaches ‘infinity’.
• When the maximum hop count is reached, the 192.168.0.0/24 network is considered unreachable.
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Split Horizon
• Using split horizon, a router will not advertise a route update to the router that originally informed it of the route.
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Split Horizon Example (1)
• RTA, RTB, and RTD exchange updates indicating the 192.168.0.0 network is down. RTC has not received the message, yet.
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Split Horizon Example (2)
• Now, RTC sends a periodic update to RTD, which indicates a path to the 192.168.0.0/24 network through of RTB.
• RTD changes its routing table to reflect this incorrect information. RTD will not send the updated information to RTA, as RTD
originally learned the 192.168.0.0/24 network was down through RTA.
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Split Horizon ResultsRTE#show ip route<output omitted>No route to 192.168.0.0/24
RTA#show ip route<output omitted>No route to 192.168.0.0/24
RTB#show ip route<output omitted>No route to 192.168.0.0/24
RTD#show ip route<output omitted>R 192.168.0.0/24 [120/4] via 192.168.5.1, Serial3/0
RTC#show ip route<output omitted>R 192.168.0.0/24 [120/3] via 192.168.4.2, Serial2/0
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Split Horizon Completion
• Eventually, RTB will send a periodic update to RTC, indicating the 192.168.0.0/24 network is unavailable.
• Then RTC will update its routing table and send an update to RTD. Then, RTD will clear its route, again.
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Route Poisoning
• Poisoned routes are advertised with a distance higher than the maximum hop count. For RIP, poisoned routes are advertised as
being 16 hops away. Indicates an unreachable network
• By itself, route poisoning does little. When used with triggered updates, route
poisoning speeds-up convergence time, by immediately notifying other routers when a network is down.
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Triggered Updates
• By default a router running RIP advertises its routing table very 30 seconds. Every 90 seconds for routers running IGRP
• Using triggered updates, when a route fails, an update is sent immediately.
• The router that detects a topology change immediately sends an update message to neighbor routers that, in turn, send triggered updates to their neighbors.
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Holddown Timers
• When a router receives an update indicating a route is down, it marks the route as inaccessible and starts a hold-time timer.
• If the holddown timer expires, the route is completely removed from the routing table.
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Holddown Timer Changes
• If at any time before the holddown timer expires 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.
A different neighbor with a better metric than originally recorded for the network, the router marks the network as accessible and removes the holddown timer.
A different neighbor with a poorer metric:• The update is ignored.• This allows more time for the knowledge of a disruptive
change (routes frequently flapping from up to down) to propagate through the entire network.
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Holddown Timers Illustrated