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42
Chapter 2
Routing Principles
F I G U R E 2 . 3
Pinhole congestion
It is important to understand what happens when a distance-vector pro-
tocol starts up. In Figure 2.4, the four routers start off with only their directly
connected networks in the routing table. After a distance-vector protocol is
started on each router, the routing tables are updated with all route infor-
mation gathered from neighbor routers.
F I G U R E 2 . 4
The internetwork with distance-vector routing
As shown in Figure 2.4, each router has only the directly connected net-
works in each routing table. Each router sends its complete routing table out
to each active interface on the router. The routing table of each router
includes the network number, exit interface, and hop count to the network.
SO
SO
SO
SO
Network
172.16.10.0
Router B
Router A
Network
172.16.30.0
T1
56K
Network
172.16.20.0
Router D
Router C
172.16.50.0
172.16.10.0
172.16.20.0
172.16.40.0
172.16.50.0
172.16.30.0
S0
E0
2501A
F0/0
2621A
S1
S0
E0
2501B
E0
S0
2501C
Routing Table
F0/0 0
172.16.10.0
Routing Table
E0
0
172.16.10.0
S0
0
172.16.20.0
Routing Table
S0
0
172.16.20.0
Routing Table
S0
0
172.16.40.0
E0
0
172.16.30.0
S1
0
172.16.40.0
E0
0
172.16.50.0
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