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Chapter 13
OSPF and EIGRP
- Distance-Vector routing protocols were designed to use little memory and processing power, as
routers had slow processors, less memory and processing power, and were connected with slow
links. They advertise just the basic routing information in order to conserve bandwidth.
- Link-State and Balanced Hybrid routing protocols were designed under the assumptions of faster
links and more processing power in the routers. By sending more information and more
processing cycles on the routers, faster convergence can be achieved.
- DV protocols say nothing about the routers beyond the neighboring router in a routing update.
LS protocols advertise a large amount of the network topology information which describes the
whole network topology in the routing updates. The routers will also perform some
CPU-intensive computations upon the topology information.
Open Shortest Path First (OSPF)
- Dijkstra Shortest Path First (SPF) algorithm is used by LS protocols to determine the best path
to a network. An OSPF router contains the map of an area (the topology of the network or area)
in its topology database. It then runs SPF algorithm to process all the topology information to
come up with the best route to each network, based on the metrics or costs of all potential paths
to a network in the same area. SPF calculations are not performed for routes from other areas.
- LS protocols must calculate the metric instead of simply using what have been received in
routing updates as with DV protocols. The metric of a route to a network is calculated by
totaling the costs associated with all links to the network as saved in the topology database.
- Cisco OSPF cost calculation is 10 8 / bandwidth (in bps). 100Mbps FE 1, 10Mbps 10.
Ex: An interface with bandwidth of 64000 1563.
- LS protocols need to use a process to dynamically discover neighbors instead of just start
broadcasting topology information out every interface upon initialization.
- Neighbors are routers running the same LS protocol and have an interface on a common subnet.
As soon as routers know that they are neighbors, they can exchange their own copies of topology
information as saved in the topology database, and then run SPF to calculate new routes.
- The process of identifying neighbors can be complicated, and it must happen before exchanging
any topology information. CCNP syllabus covers more details about neighbor relationships.
- Adjacency is a relationship between 2 OSPF routers that allows the exchange of routing updates.
OSPF routers only exchange routes with neighbors that have established adjacencies.
Adjacency depends upon both the network type and router configuration. Unlike EIGRP
which directly exchanges routes with all neighbors, OSPF is very choosy in forming adjacencies.
Ex: Hello and dead timer values must be the same in order for routers to form adjacency.
- Link-State Updates (LSUs) are the routing updates sent by OSPF to routers that have
established adjacencies. Link-State Advertisements (LSAs) are the items sent in an LSU.
An individual LSA describes the subnet number and mask, the cost (metric), and link type
about a particular subnet. OSPF sends out self-originated LSAs every 30 mins (LS refresh time).
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