Allied Telesis AT-S83 - Manual (Page 34)

Chapter 2: Introduction
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the metric value of the path by one. (This is referred to as hop count.) The
router recognizes the IP-address-advertising router at the next hop, then it
sends the routing updates to other routers. A maximum allowable hop
count is 15. If a router reaches a metric of 16 or above, the destination is
defined as unreachable. This feature avoids indefinite routing loops. The
split horizon and hold down features are used to avoid propagating
incorrect routing information. The route becomes invalid when the route
time-out timer expires. It remains in the table until the route-flush timer
expires.
OSPF
A link-state routing protocol, OSPF, is an Interior Gateway Protocol (IGP)
that uses the SPF Dijsktra algorithm for the Internet. OSPF sends link-
state advertisements (LSAs) to all other routers within the same
hierarchical area. Data on attached interfaces, metrics used, and other
variables are included in OSPF LSAs. As OSPF routers accumulate link-
state data, they use the SPF algorithm to calculate the shortest path to
each node.
An Autonomous System (AS) or domain is defined as a group of networks
with a common routing infrastructure. OSPF can work in one AS. Or, it can
receive and send routes from (or to) different AS systems. An AS system
consist of areas which is a group of neighboring networks or attached
hosts.
All backbone OSPF area routers use the same procedures and algorithms
to maintain routing information within the backbone as any other area
router. The backbone topology is invisible to all routers within an area. The
individual area topologies are invisible to the backbone. Sometimes the
backbone is not a contiguous area. Virtual links function as if they were
direct links and are configured between backbone routers that share a link
to a non backbone area.
During boot-up, an OSPF router initializes its routing-protocol-specific data
structures and tables. When the lower-layer protocols with which it
interfaces are functional, it sends the OSPF Hello protocol packets to find
neighboring routers. A router sends Hello packets as keep-alive packets,
informing other routers about its continuing functionality.
Two routers are considered adjacent when their link state databases are
synchronized. Multi-access networks have more than two routers. On
multi-access networks, the hello protocol chooses a designated router and
a designated backup router. The designated router generates LSAs for the
entire multi-access network, and reduces network traffic and the size of
the topological database. The designated router also determines the
adjacency of routers and the synchronization of their topological
databases. The data on a router’s adjacencies or state changes are
provided by periodic transmission of an LSA. Failed routers are detected,
and topology is changed quickly by comparing adjacencies to link states.