- [Man] Until we have one routing protocol
to rule them all,
there's a need to have multiple routing protocols
peacefully coexist on the same network.
Perhaps Company A runs EIGRP,
and Company B runs OSPF,
and the two companies merge.
Until the newly combined IT staff agrees on
a standard protocol to use if they ever agree on one.
The routes known to OSPF need to be advertised
in the EIGRP and vice versa.
This kind of scenario is possible,
thanks to route redistribution.
And that's the focus of this video.
Other reasons that we might need to perform
a route redistribution include,
having different parts of your network
under different administrative control.
Or perhaps we want to connect out
with a network of a business partner.
And also when we're connecting out to the internet,
we might want to advertise our IGP routes,
or interior gateway protocol routes,
into BGP or vice versa.
Those are the types of things that route redistribution
can help us with.
Now, consider the basic topology we see here.
Here we're wanting the OSPF and EIGRP
to advertise routes that they know about into one another.
And this concept is called "mutual route redistribution".
And since router R2 has one interface in OSPF,
and one interface in EIGRP,
it's its responsibility to do the route redistribution,
that's where all the configuration is going to be done.
And we're going to be doing that configuration
coming up in a bit.
But the primary challenge that we run into
when redistributing routes
between different routing protocols,
is the different approaches routing protocols use
to measure their metrics.
For example, OSPF uses a metric of cost,
which is based entirely on bandwidth.
While EIGRP, uses a metric that's by default
based on bandwidth and delay.
And this issue isn't as simple as something like,
converting currencies between two countries,
because in that scenario,
there's a ratio describing the relationship
of the two currencies.
But with route redistribution,
we don't have a relationship like that.
So what are we supposed to do?
Well, we as administrators can configure the metric assigned
to routes coming in or from one autonomous system,
which are being redistributed
into another autonomous system.
And if we don't bother to do that manually,
if we don't do that configuration,
there are defaults.
Those default metrics are called "seed metrics".
And here we see some of those.
If you take a look at this table,
we can see that by default,
a route redistributed into OSPF,
will be assigned a metric of 20.
Unless that route is being redistributed into OSPF from BGP.
In that case, it's going to be given a metric of one.
Now interestingly, both RIP and EIGRP,
have a default seed metric of infinity.
Meaning that a route redistributed
into those routing protocols,
will be considered unreachable by default.
So when we're redistributing into RIP and EIGRP especially,
we want to manually configure
what metric gets assigned to those routes.
The BGP is a bit different,
it's going to use whatever the initially
interior gateway protocol metric was to begin with.
Because remember, BGP is looking at just a single metric.
Instead, it goes through a fairly lengthy process
of different things it's going to check like,
weight and local preference
and several other things.
And that's a look at the basic theory
of route redistribution.
So next, let's take a look at a configuration example
of mutual route redistribution.