Figure 3-23 Packets During Holddown
When the 10.4.0.0 network comes back up, Router B sends a trigger update to Routers A and D,
notifying them that the link is active. After the hold-down timer expires, Routers A and D add route
10.4.0.0 back to the routing table as accessible, as illustrated in Figure 3-24.
Figure 3-24 Network Up
10.4.0.0
E B C
D
A
X
Holddown
Holddown
Packet for
Network 10.4.0.0
Packet for
Network 10.4.0.0
Holddown
10.4.0.0
Link Up! E B C
D
A
Reviewing Dynamic Routing 115
Routers A and D send Router E a routing update stating that network 10.4.0.0 is up, and Router E
updates its routing table after the hold-down timer expires, as illustrated in Figure 3-25.
Figure 3-25 Network Converges
Link-State and Advanced Distance Vector Protocols
In addition to distance vector??“based routing, the second basic algorithm used for routing is the
link-state algorithm. Link-state protocols build routing tables based on a topology database. This
database is built from link-state packets that are passed between all the routers to describe the state
of a network. The shortest path ?¬?rst algorithm uses the database to build the routing table.
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