So my home router is connected to the Internet
and can forward the traffic into the Internet
even though it doesn’t know all the routes in the Internet
Internet routing tables are growing all the time
so there are more than 500,000 routes on the Internet
and a small router wouldn't be able to handle that number of routes in its routing table.
Now it’s possible to telnet to live BGP routers on the Internet.
So in this example, I’m going to telnet to route-server.ip.att.net
I’m told that I can log in with this user name
which gives me read only access to the router
and I’m going to type show route summary.
As you can see here there are 8.5 million odd routes
in the routing table with 567,000 destinations
show bgp summary will show me the BGP routing table.
BGP is the routing protocol used on the Internet.
So total path is 8.5M active path is 567,000
as you can imagine a small router
is not going to be able to handle this number of routes in its routing table.
Here you can see examples of routes in the BGP routing table
on this router and how long thoses route have been in the routing table.
Small routers aren't not going to handle that number of routes
so you would typically use a default route pointing your router to a gateway of last resort.
So a default route is a special type of static route
pointing your device or router to a gateway of last resort.
This is similar to the concept of a default gate way on a PC or a device
such as an iPhone or iPad
when a router doesn’t know where to send the traffic
it simply sends it to the gateway of last resort.
In other words, it will send it to this IP address
as configured with the static default route.
Another advantage of static routes is that you as the administrator
determine explicitly where traffic flows.
So rather than a routing protocol making the decision for you
you decide and have control over where traffic goes
or where packets are routed but on the flip side
the burden of management and keeping things up to date also fall on your shoulders
so you have to administer the routing table, keep it up to date
make sure that routes are not pointing to non-existing devices
where networks go down, you would have to update the routing table
and that’s simply not scalable in large topologies
so hence dynamic routing protocols
such as OSPF or EIGRP are used to dynamically add or remove routes from a routing table.
BGP as mentioned is the routing protocol
used on the Internet for very large scale implementations.
The major advantage of dynamic routing protocols is that
there is the dynamic or automatic adjustment of the routing table
based on topology changes in your network.
So rather than you having to manually adjust to a topology change
the routing protocols update, insert or removed routes
from the routing table based on changing conditions in the network.
As soon as you enable a routing protocol
such as OSPF or EIGRP the routers will form a neighbor or peer relationships
with each other and exchange routes with one another.
The routers will does automatically learn about the networks available in the topology.