OSPF Summarization

OSPF summarization is pretty easy to implement. It can only be done between areas meaning we can only summarize at the ABR and ASBR where routes are being shared into another area. Due to the area database needing to be consistent we can only place summarization at these positions in the network as opposed to EIGRP where you may summarize at any position you wish. 

Virtual Links

What is a virtual link and why should anyone care? Well, they have a purpose in select scenarios only. Every OSPF area must contain a router that has a physical or logical connection to area 0. In the case of virtual links – this is the logical connection. We might use one in an “emergency” scenario, or joining a differing area into area 0, but area 0 is already burdened and up against recommended design constraints. Virtual links are an effective way of doing such things. However it is only meant to be temporary.

OSPF in General

OSPF Quick Facts:

OSPF Stub Areas

There are numerous stub areas. Stub areas are pretty easy to configure and easier to implement than EIGRP stub areas if you ask me. The point of stub areas is to reduce table sizes, look up times, LSA propagation, and allows for easy summarization at the ABR and ASBRs.

OSPF LSAs

OSPF LSA (link state advertisement) are used to advertise all known routes within a domain and ensure each router database in that area is consistent. Based on this database, we can essentially construct the topology without even having it available. LSAs give a router “the full picture” of an area so it is aware of the topology beyond it’s neighbor, unlike EIGRP.

OSPF Load Balancing & Cost Manipulation

OSPF load balances with 4 paths by default. So if we have 4 equal cost links to the same destination, they will all be used. If we want to achieve such a scenario we can give each of the links to said destination equal cost. OSPF, due to the databases requiring consistency across all routers in the area(s), it doesn’t do unequal cost load balancing – at least not without some intervention on your part in the way of route-maps.