The troubleshooting solutions presented in the previous sections work together to prevent routing
loops in a more complex network design. As depicted in Figure 3-20, the routers have multiple
routes to each other. As soon as Router B detects the failure of network 10.4.0.0, Router B removes
its route to that network. Router B sends a trigger update to Routers A and D, poisoning the route
to network 10.4.0.0 by indicating an in?¬?nite metric to that network.
Figure 3-20 Implementing Multiple Solutions
Routers D and A receive the triggered update and set their own hold-down timers, noting that the
10.4.0.0 network is ???possibly down.??? Routers D and A, in turn, send a triggered update to Router
E, indicating the possible inaccessibility of network 10.4.0.0. Router E also sets the route to
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 X
Network 10.4.0.0
is unreachable.
Network 10.4.0.0
is unreachable.
Network 10.4.0.0
is unreachable.
B C E
A
D
10.4.0.0
X
Reviewing Dynamic Routing 113
10.4.0.0 in holddown. Figure 3-21 depicts the way Routers A, D, and E implement hold-down
timers.
Figure 3-21 Route Fails
Router A and Router D send a poison reverse to Router B, stating that network 10.
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