Amer Networks E5Web GUI - Manual (Page 299)

and to determine the optimal path. The principal metrics used include:
Path length 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.
Item Bandwidth The traffic capacity of a path, rated by "Mbps".
Load The usage of a router. The usage can be evaluated by CPU utilization and
throughput.
Delay 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.
4.6.2. OSPF Concepts
Overview
Open Shortest Path First (OSPF) is a routing protocol developed for IP networks by the Internet
Engineering Task Force (IETF). The cOS Core OSPF implementation is based upon RFC 2328, with
compatibility to RFC 1583.
OSPF functions by routing IP packets based only on the destination IP address found in the IP
packet header. IP packets are routed "as is", in other words they are not encapsulated in any
further protocol headers as they transit the Autonomous System (AS).
The Autonomous System
The term Autonomous System refers to a single network or group of networks with a single,
clearly defined routing policy controlled by a common administrator. It forms the top level of a
tree structure which describes the various OSPF components.
In cOS Core, an AS corresponds to an OSPF Router object. This must be defined first when setting
up OSPF. In most scenarios only one OSPF router is required to be defined and it must be defined
separately on each Clavister Security Gateway involved in the OSPF network. This cOS Core
object is described further in Section 4.6.3.1, “OSPF Router Process”.
OSPF is a dynamic routing protocol as it quickly detects topological changes in the AS (such as
router interface failures) and calculates new loop-free routes to destinations.
Link-state Routing
OSPF is a form of link-state routing (LS) that sends Link-state Advertisements (LSAs) to all other
routers within the same area. Each router maintains a database, known as a Link-state Database,
which maps the topology of the autonomous system (AS). Using this database, each router
constructs a tree of shortest paths to other routers with itself as the root. This shortest-path tree
yields the best route to each destination in the AS.
Authentication.
All OSPF protocol exchanges can, if required, be authenticated. This means that only routers with
the correct authentication can join an AS. Different authentication schemes can be used and
with cOS Core the scheme can be either a passphrase or an MD5 digest.
Chapter 4: Routing
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