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 connec-
ted 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 dis-
tributed to other routers. Dynamic Routing responds to routing updates on the fly but has the disad-
vantage 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 informa-
tion with other routers depends on the type of algorithm used.
4.4.1.1. 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 regu-
lar 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 ta-
ble to neighboring routers to inform them of changes.
4.4.1.2. Link State algorithms
Different from the 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 al-
gorithm 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 (OSPF) is a widely used LS algorithm. An OSPF enabled router first iden-
tifies 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.
4.4.1.3. Comparing dynamic routing algorithms
Due to the fact that the global link state information is maintained everywhere in a network, Link
State algorithms have a high degree of configuration control and scalability. Changes result in
broadcasts of just the updated information to others routers which results in 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 advantages it offers.
4.4.1.4. Routing metrics
Routing metrics are the criteria a routing algorithm uses to compute the "best" route to a destination.
A routing protocol relies on one or several metrics to evaluate links across a network and to determ-
ine the optimal path. The principal metrics used include:
80
Chapter 4. Routing