Dynamic Routing; Dynamic Routing Overview - D-Link NetDefend DFL-210 User Manual

4.4. Dynamic Routing

4.4. Dynamic Routing

4.4.1. Dynamic Routing overview

Dynamic routing is different to static routing in that the D-Link Firewall will adapt to changes of
network topology or traffic load automatically. NetDefendOS first learns of all the directly
connected networks and gets further route information from other routers. Detected routes are sorted
and the most suitable routes for destinations are added into the routing table and this information is
distributed to other routers.
Dynamic Routing responds to routing updates on the fly but has the disadvantage that it is more
susceptible to certain problems such as routing loops. In the Internet, two types of dynamic routing
algorithm are used: the Distance Vector(DV) algorithm and the 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.
Distance Vector Algorithms
The Distance vector (DV) algorithm is a decentralized routing algorithm that computes the "best"
path in a distributed way. Each router computes the costs of its own attached links, and shares the
route information only with its neighbor routers. The router will gradually learns the least-cost path
by iterative computation and information exchange with its neighbors.
The Routing Information Protocol (RIP) is a well-known DV algorithm and involves sending
regular update messages and reflecting routing changes in the routing table. Path determination is
based on the "length" of the path which is the number of intermediate routers {also known as
"hops"}. 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. 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.
Open Shortest Path First
Open Shortest Path First (OSPF) is a widely used LS algorithm. An OSPF enabled router first
identifies the routers and subnets that are directly connected to it and then broadcasts the
information to all the other routers. Each router uses the information it receives to build a table of
what the whole network looks like. With a complete routing table, each router can identify the
subnetworks and routers that lead to any destination. Routers using OSPF only broadcast updates
that inform of changes and not the entire routing table.
OSPF depends on various metrics for path determination, including hops, bandwidth, load and
delay. OSPF can provide a great deal of control over the routing process since its parameters can
finely tuned.
Comparing Dynamic Routing Algorithms
Due to the fact that the global link state information is maintained everywhere in a network, LS
algorithms 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 operate within a hierarchy, whereas RIP has no knowledge of
sub-network addressing. NetDefendOS uses OSPF as its dynamic routing algorithm because of the
many advantages it offers.
Routing metrics
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Chapter 4. Routing