Mechanisms to Stop Routing Loops

Question 1

Hold-Down Timer

Answer 1

Hold-Down Timer – Router with downed network places the network in hold-down state for 180 seconds. If
the router doesn’t receive an update with a better metric to the downed network, it flushes that network
from its routing table after 60 more seconds (flush timer = 240 seconds).

Question 2

TTL (time to live)

Answer 2

TTL (time to live) – A packet has a TTL field that contains the most router hops allowed. Each time the
packet arrives at a router on its way to destination network, the TTL field is decremented by 1. When this
field equals 0, the packet is discarded. This situation occurs when destination network is no longer available

Question 3

Split Horizon

Answer 3

Split Horizon – Don’t send an update back to the router it came from. Once an update comes in an interface,
it cannot go back out. – Don’t send an update back to the router it came from. Once an update comes in an interface,
it cannot go back out.

Question 4

Route Poisoning

Answer 4

Route Poisoning – A router will insert 16 (RIP) as a metric. RIP can only traverse 15 routers. So, the router
has poisoned a route to a downed directly-connected network.

Question 5

Poison Reverse

Answer 5

Poison Reverse – A route that has been poisoned will break the split horizon rule – directly connected
routers will send an update back out the interface it came in on. This is done so the router with the downed
network does not get confused if it receives an update from a neighboring router saying that router can
reach the downed network before the neighboring router has received the update with the poisoned route