Information Technology Reference
In-Depth Information
Best Path and Metrics: Completion and Short Answer Exercise
Complete the paragraphs that follow by filling in appropriate words and phrases.
Best Path
The best path is selected by a routing protocol based on the value or
metric
it uses to determine the
distance to reach a network. Some routing protocols, such as RIP, use simple
hop count
, which is sim-
ply the number of routers between a router and the destination network. Other routing protocols, such
as OSPF, determine the shortest path by examining the
bandwidth
of the links.
A
metric
is the quantitative value used to measure the distance to a given route. The best path to a net-
work is the path with the
lowest metric
.
The primary objective of the routing protocol is to determine the
best paths
for each route to include
in the routing table. The routing
algorithm
generates a value, a
metric
for each path through the net-
work. The smaller the value of the
metric
, the better the path.
Provide a short answer for the following questions.
Comparing Hop Count and Bandwidth
When hop count is used as the metric, the resulting path can sometimes be suboptimal. Explain why
this might happen.
Answers can vary but should include the following concept: Hop count does not consider the band-
width or speed on the links between source and destination, nor does it consider the current load on a
link. Hop count will choose a path that only has two hops as opposed to a path that has three hops—
even though the three-hop path might be faster.
Equal-Cost Load Balancing
What happens if a routing table has two or more paths with the same metric to the same destination
network?
The router will perform equal-cost load balancing using both paths (or multiple paths) to route to the
same destination. The routing table will contain the single destination network but will have multiple
exit interfaces, one for each equal-cost path.
Path Determination and Switching Function Exercise
Can you describe the exact details of what happens to a packet at Layer 2 and Layer 3 as it travels
from source to destination? Use the topology and Figure 1-11, the addressing table in Table 1-4, and
the routing tables in Example 1-1 to fill in the blanks in the steps that follow.
Figure 1-11
Path Determination and Switching Function Topology
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
S0/0/0
S0/0/1
S0/0/0
DCE
Switch1
S0/0/1
DCE
Switch3
Fa0/0
R1
R2
R3
Fa0/0
PC1
PC3
