The basic premise behind stub areas in OSPF is that if routers want to communicate to another area they need to send packets to the Area Border Router (ABR), a router that connects OSPF areas.
So, by using a default route that points to ABRs address the routers inside an area can reach all the destinations outside their area without the need to have all the specific routes in their routing tables.
In the last blog post about different OSPF area types we noted that multiple types of OSPF areas exist. We also briefly explained each area type and why is it useful. Then we configured two of those types in GNS3: Stub areas & Totally Stubby Areas.
External Routes are not advertied into a stub area. Interarea routes are advertied.
Multi Area OSPF: Backbone Area
In a blog post about Multi Area OSPF we went over some pros of having multiple OSPF areas in your network.
- Reduce the size of the routing tables. Each OSPF area can contain multiple Routers and cover a portion of a network that can be summarized.
- Reduce traffic between different parts of a network
- Reduce execution of Shortest Path First algorithm
Continuing from a previous blog post about OSPF where we explored Multi Area OSPF, let's take a look at a cool OSPF feature: Stubby Areas.
There are sites inside the network that need to send the packets to the Internet. All such packets need to arrive at the router connected to the Internet.
Also, if routers want to communicate to the other area they must send the packets to the ABR, meaning they can use the default route to do that.
Continuing from a previous blog post about OSPF, in this blog post I will try to show you some advantages of having multiple OSPF areas, and then we'll design and configure our network with a couple of areas.
The need for a dynamic routing protocol
As you probably know, in order for a Router to be able to forward the packets to the network, it must know how to reach that network. The information how to reach a network is stored in a router's routing table. If the router doesn't have a network in it's routing table it simply discards the packet.