Amer Networks E5Web GUI - Manual (Page 297)

In contrast to DV algorithms, Link State (LS) algorithms enable routers to keep routing tables that
reflect the topology of the entire network.
Each router broadcasts its attached links and link costs to all other routers in the network. When
a router receives these broadcasts it runs the LS algorithm and calculates its own set of least-cost
paths. Any change of the link state will be sent everywhere in the network, so that all routers
keep the same routing table information and have a consistent view of the network.
Advantages of Link State Algorithms
Due to the fact that the global link state information is maintained everywhere in a network, LS
algorithms, like that used in OSPF, offer a high degree of configuration control and scalability.
Changes result in broadcasts of just the updated information to other routers which means faster
convergence and less possibility of routing loops. OSPF can also function within a hierarchy,
whereas RIP has no knowledge of sub-network addressing.
The OSPF Solution
Open Shortest Path First (OSPF) is a widely used protocol based on an LS algorithm. Dynamic
routing is implemented in cOS Core using OSPF.
An OSPF enabled router first identifies the routers and sub-networks that are directly connected
to it and then broadcasts the information to all the other routers. Each router uses the
information it receives to add the OSPF learned routes to its routing table.
With this larger picture, each OSPF router can identify the networks and routers that lead to a
given destination IP and therefore the best route. Routers using OSPF then only broadcast
updates to inform others of any route changes instead of broadcasting the entire routing table.
OSPF depends on various metrics for path determination, including hops, bandwidth, load and
delay. OSPF can also provide a high level of control over the routing process since its parameters
can be finely tuned.
A Simple OSPF Scenario
The simple network topology illustrated below provides an excellent example of what OSPF can
achieve. Here we have two Clavister Security Gateways A and B connected together and
configured to be in the same OSPF area (the concept of area will be explained later).
Figure 4.11. A Simple OSPF Scenario
OSPF allows security gateway A to know that to reach network Y, traffic needs to be sent to
security gateway B. Instead of having to manually insert this routing information into the routing
tables of A, OSPF allows B's routing table information to be automatically shared with A.
In the same way, OSPF allows security gateway B to automatically become aware that network X
is attached to security gateway A.
Chapter 4: Routing
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