Ospf; Dynamic Routing - D-Link DFL-260E User Manual

4.5. OSPF

4.5. OSPF
The feature called Dynamic Routing is implemented with NetDefendOS using the OSPF
architecture.
This section begins by looking generally at what dynamic routing is and how it can be implemented.
It then goes on to look at how OSPF can provide dynamic routing followed by a description of how
a simple OSPF network can be set up.

4.5.1. Dynamic Routing

Before looking at OSPF in detail this section will discuss generally the concept of Dynamic routing
and what type of dynamic routing OSPF provides. It introduces important concepts in dynamic
routing and in OSPF.
Differences to Static Routing
Dynamic routing is different to static routing in that a routing network device, such as a NetDefend
Firewall, can adapt to changes of network topology automatically.
Dynamic routing involves first learning about all the directly connected networks and then getting
further routing information from other connected routers specifying which networks they are
connected to. All this routing information is then processed and the most suitable routes for both
locally connected and remotely connected destinations are added into local routing tables.
Dynamic routing responds to routing updates dynamically but has some disadvantages in that it can
be more susceptible to certain problems such as routing loops. One of two types of algorithms are
generally used to implement the dynamic routing mechanism:
• A Distance Vector (DV) algorithm.
• A Link State (LS) algorithm.
How a router decides the optimal or "best" route and shares updated information with other routers
depends on the type of algorithm used. The two algorithm types will be discussed next.
Distance Vector Algorithms
A Distance vector algorithm is a decentralized routing algorithm that computes the best path in a
distributed way.
Each router in a network computes the "costs" of its own attached links, and shares routing
information only with its neighboring routers. Each router determines the least-cost path to a
destination by iterative computation and also using information exchanged with its neighbors.
Routing Information Protocol (RIP) is a well-known DV algorithm for router information exchange
and operates by sending regular update messages and reflecting routing changes in routing tables.
Path determination is based on the "length" of the path which is the number of intermediate routers
(also known as "hops") to the destination.
After updating its own routing table, the router immediately begins transmitting its entire routing
table to neighboring routers to inform them of changes.
Link State Algorithms
In contrast to DV algorithms, Link State (LS) algorithms enable routers to keep routing tables that
reflect the topology of the entire network.
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Chapter 4. Routing