Ospf Algorithm; Ospf Network Types - Zte ZXR10 5900E Series User Manual

ZXR10 5900E Series User Manual (IPv4 Volume)
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OSPF Algorithm

As OSPF is a link state protocol. OSPF router generates a routing
table by setting up a link state database, which contains the infor-
mation of all networks and routers. Routers use this information
to establish routing tables. To ensure reliability, all routers must
have the same link state database.
Link state database is built, based on Link State Advertisements
(LSAs) ,which are generated by all routers and spread over the
whole OSPF network. There are many types of LSAs, a complete
LSA set shows an accurate distribution diagram over the whole
network.
OSPF uses cost as the metric. The cost is distributed to each port
of a router. A port calculates the cost, that is based on a 100M
benchmark by default. The path cost to a particular destination is
the total cost of all links between the router and the destination.
To generate a routing table, based on the LSA database, a router
runs the Dijkstra SPF algorithm to construct a cost routing tree
with itself as the root of the routing tree. The Dijkstra algorithm
enables a router to calculate the lowest-cost path between itself
and any node on the network. Router saves the routes of the paths
in the routing table.
Different from RIP, OSPF does not simply broadcast all of its routing
information regularly. An OSPF router sends call messages to its
neighbors to let them know it is still alive. If a router does not
receive any message from a neighbor within a period of time then
the neighbor might not be alive.
OSPF routing is incrementally updated. Router sends the update
information only when topology changes. When the age of an LSA
reaches 1800 seconds, a new version of the LSA is resent.

OSPF Network Types

Type of the network connecting to a port is used to determine the
default OSPF behavior on that port. The network type affects the
adjacency relationship and how the router designates a timer to
the port.
There are five network types in OSPF, and they are as follows:
� Broadcast
�
Non-broadcast Multi-access (NBMA) networks
� Point-to-Point networks
� Point-to-multipoint networks
� Virtual links