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Administrative distance and metrics are two important
factors when it comes to which routing protocols and which routes a router
uses. Being knowledgeable about these two aspects can make all the difference
in network performance, reliability, and circuit selection.
If you’re not familiar with administrative distance and
metrics, you’ve probably seen them before and just not paid any attention to
them. If you enter a show ip route
command, you’ll notice two numbers in brackets, listed directly after the route
in the table. Here’s an example:
O 10.1.103.0/24 [110/791] via 10.1.100.2, 00:39:44, Serial1/0:0.21
In this case, 110 represents the administrative distance,
and 791 signifies the metric. You can get more details by using the same show ip route command and specifying a
single route. Here’s an example:
Router# show ip route 10.1.103.0 Routing entry for 10.1.103.0/24
Known via "ospf 100", distance 110, metric 791, type intra area
Last update from 10.1.100.2 on Serial1/0:0.21, 01:09:25 ago
Routing Descriptor Blocks:
* 10.1.100.2, from 172.16.1.1, 01:09:25 ago, via Serial1/0:0.21
Route metric is 791, traffic share count is 1
But what exactly do these numbers mean? Let’s take a closer
look at each one.
Administrative distance (AD) is how a router determines
which source of routes it should use if it has two identical routes from
different sources. In other words, the router needs to be able to determine
which routes to trust if it’s receiving the same information from two different
sources. For a better idea, consider trying to decide which local news program,
all of which more or less cover the same events, is most trustworthy.
If you only have one router with one routing protocol and
one WAN circuit, or if you’re only using static routes, administrative distance
doesn’t affect your situation. But that doesn’t mean you shouldn’t be familiar
with its purpose.
But if you have a slightly more complex network—say you have
two WAN circuits or you’re using two routing protocols (even if one of them is
static routing)—administrative distance takes on more importance.
The sources of the routes aren’t only routing protocols,
such as RIP, OSPF, or BGP. Possible sources can also be connected routes (i.e.,
the interfaces on the router) and static routes (which you entered as network
The router determines which source is the most trustworthy
(i.e., reliable) according to the administrative distance. The lower the
administrative distance, the more trustworthy the routing source.
To help make this decision, routers contain a preprogrammed
table that lists all of the possible sources and their default administrative
distances. Listing A offers an example
of what this table looks like. (While administrators can change default
administrative distances by using the distance
command in Router Configuration Mode, this is usually not advisable.)
For example, if the router receives a route from OSPF and a
route from RIP, it chooses the OSPF route. OSPF’s administrative distance is
110, as compared to RIP’s 120.
Here’s another example: Let’s say your router received a
route from EIGRP Internal, whose administrative distance is 90, but you
mistakenly entered a static route to an IP address, which has an administrative
distance of 1. The router would use the static route and not the EIGRP route.
One last thought: Administrative distance is an important
topic on the CCNA test. If you’re preparing for this exam, make sure you know
the administrative distances for the common routing protocols.
A routing protocol uses a metric to determine which route to
include in the routing table when it has two available routes to the same
destination. The router will include the route with the smallest metric because
it considers this route to be the shortest—and therefore best.
As opposed to administrative distance, metrics involve a
single routing protocol. They have nothing to do with multiple sources for routes.
For example, here’s a look at the truncated output of a show ip eigrp topology command:
P 10.55.103.0/24, 1 successors, FD is 6049536
via 10.220.100.1 (6049536/5537536), Serial3/0
via 10.55.100.14 (52825600/281600), Tunnel55
Notice that this routing protocol, EIGRP, has two routes to
this network. However, the router will only include one of these routes—the one
with the best metric—in the routing table. Here’s an example of what the entry
in the routing table looks like:
Router# show ip route 10.55.103.0 Routing entry for 10.55.103.0/24
Known via "eigrp 100", distance 120, metric 6049536, type internal
Redistributing via eigrp 100
Last update from 10.220.100.1 on Serial3/0, 00:56:12 ago
Routing Descriptor Blocks:
* 10.220.100.1, from 10.220.100.1, 00:56:12 ago, via Serial3/0
Route metric is 6049536, traffic share count is 1
Total delay is 41000 microseconds, minimum bandwidth is 512 Kbit
Reliability 226/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
Different routing protocols calculate their metric in
different ways. RIP uses hops, OSPF uses bandwidth, and EIGRP uses a
combination of bandwidth, delay, load, and reliability.
For more information on what different routing protocols use
as their metrics, check out “Select the
right routing protocol for your network.”
David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.