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Understand the significance of administrative distance and their metrics
when working with routers
When it comes to routing protocols and routes, administrative distance and
metrics are two important factors.
What exactly do these numbers mean?
Most routing protocols have metric structures and algorithms that are not
compatible with other protocols. In a network where multiple routing
protocols are present, the exchange of route information and the capability
to select the best path across the multiple protocols are critical.
Administrative distance is the feature used by routers to select the best
path when there are two or more different routes to the same destination
from two different routing protocols. Administrative distance defines the
reliability of a routing protocol. Each routing protocol is prioritized in
order of most to least reliable (believable) using an administrative
distance value.
Being knowledgeable about these two aspects can make all the difference in network performance,
reliability, and circuit selection.
If you are not familiar with administrative distance and metrics, you have 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 of a routing table route learned via OSPF:
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 is 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
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 administrator).
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. The table
below is an example of what it looks like.
Administrative distance is the first criterion that a router uses to determine which routing
protocol to use if two protocols provide route information
for the same destination. It is a measure of the trustworthiness of the
source of the routing information. Keep in mind that administrative distance
has only local significance; it is not advertised in routing updates.
Note: The smaller the administrative distance value, the more reliable the protocol.
For example, if a router receives a route to a certain network
from both Open Shortest Path First (OSPF) (default administrative - 110) and Interior Gateway
Routing Protocol (IGRP) (default administrative distance - 100),
the router will choose IGRP because it is more reliable.
This means the IGRP version of the route would be added to the routing table.
If you lose the source of the IGRP - derived information (for example, because of a power shutdown),
the software uses the OSPF - derived information until the IGRP - derived information reappears.
Default Distance Value Table
The table below lists the administrative distance default values of the protocols that Cisco supports.
SOURCE OF THE ROUTE |
DEFAULT DISTANCE |
Connected Interface or static route to an Interface |
0 |
Static route to an IP Address * |
1 |
EIGRP Summary |
5 |
BGP External |
20 |
EIGRP Internal |
90 |
IGRP |
100 |
OSPF |
110 |
IS-IS |
115 |
RIP |
120 |
EIGRP External |
170 |
BGP Internal |
200 |
Unknown Source ** |
225 |
* Static route pointing is always 1 regardless if it points to a next hop IP address or to an outgoing interface.
** If the administrative distance is 255, the router does not believe the source of that route
and does not install the route in its routing table.
When using route redistribution, occasionally there may be a need to modify the
administrative distance of a protocol so that it takes precedence.
For example, if you want the router to select RIP - learned routes (default
value 120) rather than IGRP - learned routes (default value 100) to the same
destination, you must increase the administrative distance for IGRP to 120+,
or decrease the administrative distance of RIP to a value less than 100.
You can modify the administrative distance of a protocol using the distance command in the routing process
subconfiguration mode, which specifies that the administrative distance is assigned to the routes learned from a
particular routing protocol. This procedure is generally used when the
network is being migrated from one routing protocol to another, the latter
having a higher administrative distance. Keep in mind, however, that
changing the administrative distance may lead to routing loops and black
holes. While administrators can change default administrative distances by using the
distance command in Router Configuration Mode, this is usually not advisable.
An 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.
Metrics
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 is 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 with bandwidth and delay being the default combination.
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