Information Technology Reference
In-Depth Information
In the following list, fill in the missing metric or metrics for the routing protocol.
RIP:
Hop count
—Best path is chosen by the route with the lowest hop count.
■
IGRP and EIGRP:
Bandwidth
,
delay
,
reliability
, and
load
—Best path is chosen by the route
with the smallest composite metric value calculated from these multiple parameters. By default,
only
bandwidth
and
delay
are used.
■
IS-IS and OSPF:
Cost
—Best path is chosen by the route with the lowest
cost
. The Cisco
implementation of OSPF uses
bandwidth
.
■
Administrative Distances
Before the routing process can determine which route to use when forwarding a packet, it must first
determine which routes to include in the routing table. In some cases, a router can learn a route to a
remote network from more than one routing source. The exercises in the following sections reinforce
your knowledge of the concept of administrative distance.
Concept of Administrative Distance Exercise
A router might learn of a route to the same network from more than one
source
. For example, a
static
route might have been configured for the same network/subnet mask that was learned dynamically by
a dynamic routing protocol. Although less common, more than one dynamic routing protocol can be
deployed in the same network. Administrative distance (AD) defines the
preference
of a routing source.
Administrative distance is an integer value from
0
to
255
. The
lower
the value the more preferred the
route source. An administrative distance of
0
is the most preferred, which is the AD value for a
directly
connected
network.
An administrative distance of
255
means that the router will not believe the source of that route and it
will not be installed in the routing table.
On way to see the AD value for a route source is to look at the routing table. In Example 3-1, circle
the AD values for the route sources that show an AD value.
Example 3-1
Routing Table with AD Value
R2#
show ip route
<output omitted>
Gateway of last resort is not set
D 192.168.1.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0
C 192.168.2.0/24 is directly connected, Serial0/0/0
C 192.168.3.0/24 is directly connected, FastEthernet0/0
C 192.168.4.0/24 is directly connected, Serial0/0/1
R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:08, Serial0/0/1
D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0
R 192.168.7.0/24 [120/1] via 192.168.4.1, 00:00:08, Serial0/0/1
R 192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:08, Serial0/0/1
