- [Instructor] It's important that we understand
administrative distances and the
three different kinds of routing protocols.
But let's start with understanding
the routing protocols using with the CCENT
and the protocols used within our IGPs for the CCENT
are RIP version two, enhanced IGRP,
and open shortest path first.
Let's make sure we have a fundamental understanding
of the difference between classful and classless.
Classful routing protocols, for example
RIP version one and then old IGRP
which isn't even supported,
don't include the subnet mask information
with the route advertisements.
Classless routing protocols include the subnet mask
with the route advertisements and allow us
to do things like VLSM.
And these are RIP version two,
EIGRP, OSPF, and another routing protocol
not covered is the IS-IS.
So it's important to understand administrative distance
or trustworthiness of a routing protocol.
The administrative distance is used
to rate the trustworthiness of routing information
received on a router from a neighbor router.
An administrative distance is a value
from zero to two five five
where zero is the most trusting
and two five five means no traffic
will be passed via this route.
If a router receives two updates
listing the same remote network,
the first thing the router checks
is the AD or administrative distance.
If one of the advertised routes
has a lower AD than the other,
then the route with the lowest AD
will be chosen and placed in the routing table.
Taking a look at the graphic here,
we can see that our router has received
updates from EIGRP and from OSPF.
Now by understanding our default administrative distances,
let's take a look.
Our directly connected is a value of zero.
That's the most trusted.
Static routes have administrative distance of one,
so if we use static routes,
I don't care what you turn on.
The only thing in a routing table will be static routes.
RIP has a value of 120,
which is only about 50% trusted, right?
IGRP was 100, but what we care about here
is RIP, EIGRP and OSPF.
EIGRP is 90, OSPF is 110.
So I can see that I'm being advertised
hey, to get to 172.17.8.0,
I have an administrative distance of 90.
OSPF comes here and says hey,
to get to 172.17.8.0 use me.
And my administrative distance is 110.
Well what's going to happen?
The router is going to discard the OSPF update.
Now I know what you're thinking.
What if the OSPF route was a better route?
It doesn't matter.
Trustworthiness, administrative distance,
ranks over anything else.
That's the first thing that's checked.
Then we get into metrics.
So if both advertised routes to the same network
have the same AD though,
so the default on all routing protocols
is for equals cause path.
But with some configuration, you could lowbounce
over many more paths.