2.0.0
10.3.0.0
10.4.0.0
10.1.0.0
S0
S1
S1
S0
0
0
1
1
Routing Table
10.3.0.0
10.4.0.0
10.2.0.0
10.1.0.0
S0
E0
S0
S0
0
Down
1
2
X
Metric = Infinity
A C B E0
S0
S0
10.1.0.0 10.4.0.0 10.2.0.0
S0
S1 E0
10.3.0.0
Routing Table
10.1.0.0
10.2.0.0
10.3.0.0
10.4.0.0
E0
S0
S0
S0
0
0
1
2
Routing Table
10.2.0.0
10.3.0.0
10.4.0.0
10.1.0.0
S0
S1
S1
S0
0
0
1
Routing Table
10.3.0.0
10.4.0.0
10.2.0.0
10.1.0.0
S0
S0
S0
S0
0
Infinity
1
2
X
Poison
Reverse
Possibly
Down
Reviewing Dynamic Routing 111
some period of time. The hold-down period is usually calculated to be just greater than the time
necessary to update the entire network with a routing change.
Hold-down timers perform route maintenance as follows:
1. When a router receives an update from a neighbor indicating that a previously accessible
network is now inaccessible, the router marks the route as inaccessible and starts a hold-down
timer.
2. If an update arrives from a neighboring router with a better metric than originally recorded
for the network, the router marks the network as accessible and removes the hold-down timer.
3. If at any time before the hold-down timer expires, an update is received from a different
neighboring router with a poorer metric, the update is ignored.
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