Routing Protocol Basics
277
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.
In Figure 5.5, the routing tables are complete because they include
information about all the networks in the internetwork. They are consid-
ered converged. When the routers are converging, no data is passed.
That's why fast convergence time is a plus. One of the problems with RIP,
in fact, is its slow convergence time.
F I G U R E 5 . 5
Converged routing tables
The routing table in each router keeps information regarding the remote
network number, the interface to which the router will send packets to reach
that network, and the hop count or metric to the network.
Routing Loops
Distance-vector routing protocols keep track of any changes to the internet-
work by broadcasting periodic routing updates to all active interfaces. This
broadcast includes the complete routing table. This works fine, although it
takes up CPU process and link bandwidth. However, if a network outage
happens, problems can occur. The slow convergence of distance-vector rout-
ing protocols can cause inconsistent routing tables and routing loops.
Routing loops can occur because every router is not updated close to the
same time. Let's say that the interface to Network 5 in Figure 5.6 fails. All
routers know about Network 5 from Router E. Router A, in its tables, has
a path to Network 5 through Router B.
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
Routing Table
Routing Table
S0
0
172.16.20.0
Routing Table
E0
0
172.16.30.0
S1
0
172.16.40.0
S0
1
172.16.10.0
S1
1
172.16.50.0
F0/0 0
172.16.10.0
F0/0 1
172.16.20.0
F0/0 2
172.16.30.0
F0/0 2
172.16.40.0
F0/0 3
172.16.50.0
E0
0
172.16.10.0
S0
0
172.16.20.0
S0
1
172.16.30.0
S0
1
172.16.40.0
S0
2
172.16.50.0
S0
0
172.16.40.0
E0
0
172.16.50.0
S0
3
172.16.10.0
S0
1
172.16.20.0
S0
1
172.16.30.0
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