Ospf - D-Link NetDefend DFL-210 User Manual

4.4.2. OSPF

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
determine the optimal path. The principal metrics used include:
Path length
Item Bandwidth
Load
Delay
4.4.2. OSPF
Overview
Open Shortest Path First (OSPF) is a routing protocol developed for IP networks by the Internet
Engineering Task Force (IETF). The NetDefendOS OSPF implementation is based upon RFC 2328,
with compatibility to RFC 1583.
The way OSPF works is that it routes IP packets based only on the destination IP address found in
the IP packet header. IP packets are routed "as is", that is they are not encapsulated in any further
protocol headers as they transit the Autonomous System (AS). OSPF is a dynamic routing protocol,
it quickly detects topological changes in the AS (such as router interface failures) and calculates
new loop-free routes after a period of time.
OSPF is a link-state routing protocol that calls for the sending of link-state advertisements (LSAs) to
all other routers within the same area. In a link-state routing protocol, each router maintains a
database describing the Autonomous System's topology. This database is referred to as the link-state
database. Each router in the same AS has an identical database. From the information in the
link-state database, each router constructs a tree of shortest paths with itself as root. This
shortest-path tree gives the route to each destination in the Autonomous System.
OSPF allows sets of networks to be grouped together, this is called an area. The topology of an area
is hidden from the rest of the AS. This information hiding reduces the amount of routing traffic
exchanged. Also, routing within the area is determined only by the area's own topology, lending the
area protection from bad routing data. An area is a generalization of an IP subnetted network.
All OSPF protocol exchanges can be authenticated. This means that only routers with the correct
authentication can join the Autonomous System. Different authentication schemes can be used, like
none, passphrase or MD5 digest. It is possible to configure separate authentication methods for each
Autonomous System.
OSPF Areas
The Autonomous System is divided into smaller parts called OSPF Areas. This section describes
what an area is, and its associated terms.
Areas
ABRs
The sum of the costs associated with each link. A commonly used value for
this metric is called "hop count" which is the number of routing devices a
packet must pass through when it travels from source to destination.
The traffic capacity of a path, rated by "Mbps".
The usage of a router. The usage can be evaluated by CPU utilization and
throughput.
The time it takes to move a packet from the source to the destination. The
time depends on various factors, including bandwidth, load, and the length
of the path.
An area consists of networks and hosts within an AS that have been grouped
together. Routers that are only within an area are called internal routers, all
interfaces on internal routers are directly connected to networks within the
area. The topology of an area is hidden from the rest of the AS.
Routers that have interfaces in more than one area are called Area Border
Routers (ABRs), these maintain a separate topological database for each area
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Chapter 4. Routing