CISCO 4.txt

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CISCO 4.txt
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2010-08-09 15:29:16
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cisco 4
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  1. What are the 4 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.
    • 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. The holddown timer is discussed in more detail later in this chapter.
    • Update Timer-updates neighboring routers every 30 seconds.
  2. How can you verify the timers?
  3. The timer values can be verified with two commands: show ip route and show ip protocols. Notice in the output from show ip route that each route learned through RIP shows the elapsed time since the last update, expressed in seconds.
  4. Does EIGRP send periodic updates?
  5. 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.
  6. When are RIP triggered updates sent?
  7. An interface changes state (up or down)
    • A route has entered (or exited) the "unreachable" state
    • A route is installed in the routing table
  8. What are 2 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.
  9. What is Synchronization?
  10. Sending updates at the same time is known as the synchronization of updates. Synchronization can become a problem with distance vector routing protocols due to their usage of periodic updates. As more routers' timers become synchronized, more collisions of updates and more delays occur in the network. Initially, the updates of routers will not be synchronized. But over time, the timers across a network will become globally synchronized.
  11. How can you prevent synchronization?
  12. To prevent the synchronization of updates between routers, the Cisco IOS uses a random variable, called RIP_JITTER, which subtracts a variable amount of time to the update interval for each router in the network. This random jitter, or variable amount of time, ranges from 0% to 15% of the specified update interval. In this way, the update interval varies randomly in a range from 25 to 30 seconds for the default 30-second interval.
  13. 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 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.
  14. What may be a result of a routing loop?
    • 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 and is discussed in CCNP-level courses)
    • Inconsistent routing tables not being updated due to slow convergence in a changing network
    • Incorrectly configured or installed discard routes
  15. What is TTL?
    : The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.
  16. What conditions can a routing loop can create?
  17. Link bandwidth will be used for traffic looping back and forth between the routers in a loop.
    • A router's CPU will be strained due to looping packets.
    • A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.
    • Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse.
    • Packets may get lost in "black holes."
  18. What can prevent routing loops?
  19. Holddown timers
    • Split horizon
    • Route poisoning or poison reverse
    • Triggered updates
  20. For distance vector routing protocols, there really are only two choices, what are they? The decision about which routing protocol to use in a given situation is influenced by a number of factors including:
    RIP and EIGRP

    • Size of the network
    • Compatibility between models of routers

    Administrative knowledge required
  21. What are some features of RIP?
    Supports split horizon and split horizon with poison reverse to prevent loops.

    Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.
  22. What improvements does RIPv2 have over 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.
  23. What are the advantages of EIGRP?
  24. Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count.
    • Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.
    • Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.
    • EIGRP supports multiple Network layer protocols through Protocol Dependent Modules, which include support for IP, IPX, and AppleTalk.
  25. Which event will cause a triggered update?
    1. an update routing timer expires
    2. a corrupt update message is received
    3. a route is installed in the routing table
    4. the network is converged
    3.
  26. Three routers running a distance-vector routing protocol lost all power, including the battery backups. When the routers reload, what will happen?
    1.They will share all routes saved in NVRAM prior to the power loss with their directly connected neighbors.
    2.They will multicast hello packets to all other routers in the network to establish neighbor adjacencies. 3.They will send updates that include only directly connected routes to their directly connected neighbors.
    4.They will broadcast their full routing table to all routers in the network.
    3.
  27. What does the RIP holddown timer do?
    1.ensures an invalid route has a metric of 15
    2.prevents a router from sending any updates after it has introduced a routing loop into the network
    3.ensures every new route is valid before sending an update
    4.instructs routers to ignore updates, for a specified time or event, about possible inaccessible routes
    4.
  28. Which two statements are true regarding the function of the RIPv1 routing updates? (Choose two).
    1.updates are broadcast only when there are changes to the topology
    2.updates are broadcast at regular intervals
    3.broadcast are sent to 0.0.0.0
    4.broadcasts are sent to 255.255.255.255


    updates contain the entire network topology


    only changes are included in the updates
    2. and 4.
  29. Which of the following statements are correct about RIP?
    1.uses a broadcast to update all other routers in the network every 60 seconds
    2.uses a multicast address to update other routers every 90 seconds
    3.will send out an update if there is a failure of a link
    4.updates only contain information about routes that have changed since last update
    3.
  30. Which two statements describe EIGRP? (Choose two.)
    1.EIGRP can be used with Cisco and non-Cisco routers.
    2.EIGRP sends triggered updates whenever there is a change in topology that influences the routing information.
    3.EIGRP has an infinite metric of 16.
    4.EIGRP sends a partial routing table update, which includes just routes that have been changed.
    5.EIGRP broadcasts its updates to all routers in the network.
    2. and 4.
  31. Which statement is true regarding cisco’s RIP_JITTER variable?
    1.It prevents the synchronization of routing updates by buffering the updates as they leave the router interfaces.
    2.It prevents the synchronization of routing updates by subtracting a random length of time ranging from 0% to 15% of the specified interval time from the next routing update interval.
    3.It prevents the synchronization of routing updates by causing the router to skip every other scheduled update time.
    4.It prevents the synchronization of routing updates by forcing the router to listen when its time for other updates on the lines before sending its update.
    2.
  32. Which of the following methods does split horizon use to reduce incorrect routing information?
    1.Routing updates are split in half to reduce the update time.
    2.Information learned from one source is not distributed back to that source.
    3.New route information must be learned from multiple sources to be accepted.
    4.The time between updates is split in half to speed convergence.
    5.New route information is suppressed until the system has converged.
    2.
  33. What is the purpose of the TTL field in the IP header?
    1.used to mark routes as unreachable in updates sent to other routers
    2.prevents regular update messages from reinstating a route that may have gone bad
    3.prevents a router from advertising a network through the interface from which the update came
    4.limits the time or hops that a packet can traverse through the network before it should be discarded
    5.defines a maximum metric value for each distance vector routing protocol by setting a maximum hop count
    4.
  34. Which of the following can exist in a distance vector network that has not converged? (Choose three.)
    1.routing loops
    2.inconsistent traffic forwarding
    3.no traffic forwarding until system converges
    4.inconsistent routing table entries
    5.routing table updates sent to wrong destinations
    1.,2., and 4.
  35. Which three routing protocols are distance vector routing protocols? (Choose three).
    1.RIPv1
    2.EIGRP
    3.OSPF
    4.IS-IS
    5.RIPv2
    1.,2., and 5.
  36. What is a routing loop?
    1.a packet bouncing back and forth between two loopback interfaces on a router
    2.a condition where a return path from a destination is different from the outbound path forming a "loop"
    3.a condition where a packet is constantly transmitted within a series of routers without ever reaching its intended destination
    4.the distribution of routes from one routing protocol into another
    3.
  37. Which two conditions are most likely to cause a routing loop? (Choose two.)
    1.random jitter
    2.implementation of classful addressing
    3.inconsistent routing tables
    4.incorrectly configured static routes
    5.a network converging too quickly
    3. and 4.

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