5.1 Creating Multiple OSPF Areas
5.1.1 Issues with large one-area OSPF networks
Thus far, you have seen how OSPF operates within a single area. What issues would arise if this single area ballooned into 400 networks? The issues shown in the main figure, at a minimum, would need to be addressed.

The issues shown in the Figure are described as follows:

  • Frequent SPF calculations --- With such a large network, network changes are inevitable, so the routers will spend many more CPU cycles recalculating the routing table.
  • Large routing table --- Each router needs to maintain at least one entry for every network; that is, at least 400 networks. Assuming there are multiple paths to just 25 percent of the networks adds another 100 entries!
  • Large link-state table --- Because the link-state table includes the complete topology of the network, each router needs to maintain an entry for every network in the area, even routes that are not used.

Luckily, OSPF was created to solve many of these network scaling issues previously left unanswered by earlier routing protocols. OSPF allows large areas to be separated into smaller, more manageable areas that can still exchange routing information. By splitting the network into manageable pieces, the routers can handle the traffic and the network can grow efficiently.