Cisco CCNA mock exam questions sample test – Question 25

.Which of the following techniques is NOT used by distance vector protocols to stop routing loops in a network?

A. Split horizon
B. Spanning Tree Protocol (STP)
C. Holddowns
D. Route poisoning



Correct Answer
: B

Explanation:
Spanning Tree Protocol (STP) is not used by distance vector protocols to stop routing loops in a network. STP is used to prevent switching loops in a switched network.
Routing loops can occur due to slow convergence and inconsistent routing tables, and can cause excessive use of bandwidth or complete network failure. An example of a routing
table problem would be incorrectly configured static default routes. Suppose that Router A is connected to Router B, and the addresses of the interfaces on each end of the link
connecting the two routers are as follows:
Router A 192.168.5.1/24
Router B 192.168.5.2/24
A partial output of the routing tables of the two routers is shown below. Router B hosts the connection to the Internet.
routerA# show ip route
Gateway of last resort is 192.168.5.2 to network 0.0.0.0

routerB# show ip route
Gateway of last resort is 192.168.5.1 to network 0.0.0.0
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From the limited information shown above, you can see that Router A is pointing to Router B for the default route, and Router B is pointing to Router A for the default route. This will
cause a routing loop for any traffic that is not in their routing tables. For example, if a ping were initiated to the address 103.5.6.8 and that address was not in the routing tables of
Routers A and B, the most likely message received back would NOT be “destination unreachable” but “TTL expired in transit.” This would be caused by the packet looping between
the two routers until the TTL expired.
The following techniques are used by distance vector protocols to stop routing loops in a network:
Split horizon stops routing loops by preventing route update information from being sent back over the same interface on which it arrived.
Holddown timers prevent regular update messages from reinstating a route that is unstable. The holddown timer places the route in a suspended, or “possibly down” state in the
routing table and regular update messages regarding this route will be ignored until the timer expires.
Route poisoning “poisons” a failed route by increasing its cost to infinity (16 hops, if using RIP). Route poisoning is combined with triggered updates to ensure fast convergence in
the event of a network change.

Objective:
Routing Fundamentals

Sub-Objective:
Compare and contrast distance vector and link-state routing protocols

References:
Cisco > Articles > Network Technology > General Networking > Dynamic Routing Protocols