- [Instructor] While the terms routed protocol
and routing protocol are often said
in close proximity, they are two different things.
A routed protocol is something
that defines a standard structure
for information that would be forwarded by a router.
Say for example IPv4 or IPv6.
Routing protocols are collections
of rules by which routers share information
between others, most notably routes.
Examples are RIP, OSPF, and BGP.
All routing protocols share the same basic functions.
First, advertise routing info to neighboring routers
sharing in the routing protocol.
Second, learn routing information
from neighboring routers sharing in the routing protocol.
Third, find the best routes among all
the learned routing information and install
them into the routing table.
Fourth, react expeditiously
when the network topology changes.
Convergence is a term to describe
what happens in that fourth step.
In essence, convergence is the means
by which all routers running the same routing protocol
realize there's a change and react
to it by choosing a new best path for the given subnets.
Convergence may be due to link or router failure,
but can also be caused when a new subnet
is added to the network.
As far as routing protocol types go,
there are really two main categories, IGP and EGP.
Interior Gateway Protocols are those intended
to be used inside of an autonomous system
while Exterior Gateway Protocols
are those designed to be used between autonomous systems.
For EGP, think Border Gateway Protocol or BGP,
and everything else are IGPs.
Above, I used the term autonomous system
which refers to a network under the administrative control
of a single organization.
For example, if I set up an office network
for a one-off customer, then that network
would be considered an autonomous system.
If I set up a network for a car dealership
that has four locations that were interconnected,
they all could be considered as one autonomous system.
If I'm an ISP that has hundreds
of routers spread across multiple states
or countries, this could still be considered
a single autonomous system.
BGP being the only protocol in use today for EGP
uses the concept of AS numbers.
These are numbers used to represent
all the devices within a single autonomous system.
These numbers are used on the global internet router tables
and are assigned by regional internet numbers registries.
IGPs come in a few primary flavors.
Distance vector, advanced distance vector, and link state.
Distance vector was developed first
through the likes of Routing Information Protocol, or RIP.
These protocols were simplistic and slow to converge.
Link-state protocols like OSPF
and Intermediate System to Intermediate System
were later developed to overcome some
of the shortcoms of distance vector protocols.
Not only do link-state protocols require more effort
when planning networks, they also use more CPU
and memory on routers.
Cisco also created a proprietary protocol called
Enhanced Interior Gateway Routing Protocol, or EIGRP.
Since it uses mostly distance vector logic
with a little link-state sprinkled in,
it's considered an advanced distance vector protocol.
Each routing protocol uses some set of parameters
to determine which paths are the best.
These are known as metrics.
RIP uses hop count or how many routers away something is.
OSPF uses cost, which is an aggregate value
of the cost of each link along the way.
While EIGRP has a calculation based
on the link speeds and delay along the way.
Cisco primarily focuses on OSPF in the new CCNA exams.
So pay particular attention when reviewing it.