4.0.0 is
inaccessible. Because Router E received a triggered update from Routers A and D, it sends a
poison reverse to Routers A and D. Figure 3-22 illustrates the sending of poison reverse updates.
Figure 3-22 Route Holddown
Routers A, D, and E will remain in holddown until one of the following events occurs:
?– The hold-down timer expires.
?– Another update is received, indicating a new route with a better metric.
?– A ?¬‚ush timer, which is the time a route will be held before being removed, removes the route
from the routing table.
10.4.0.0
E B C
D
A
X
Holddown
Holddown
Holddown
B C E
A
D
10.4.0.0
X
Holddown
Holddown
Holddown
Poison Reverse
Poison Reverse
Poison Reverse
Poison Reverse
114 Chapter 3: Medium-Sized Routed Network Construction
During the hold-down period, Routers A, D, and E assume that the network status is unchanged
from its original state and attempt to route packets to network 10.4.0.0. Figure 3-23 illustrates
Router E attempting to forward a packet to network 10.4.0.0. This packet will reach Router B.
However, because Router B has no route to network 10.4.0.0, Router B will drop the packet and
return an Internet Control Message Protocol (ICMP) network unreachable message.
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