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OSPF Basic Topology and the Link State Database (LSDB)
(Page 3 of 3)
LSDB Information Storage and Propagation
An important point to remember about
the LSDB is that even though each router maintains it, the database
isn't constructed from the perspective of the individual router. A router's
LSDB represents the topology of the entire AS, including links between
routers that may be rather distant from it. So, for example, RA
would keep the entire database in its storage area, including information
about RC and RD, to which it does not connect directly.
Since in basic topology all the routers
are peers and maintain information for the entire AS, they in theory
should have the exact same LSDB contents. When a router is first turned
on it may in fact have different LSDB information than its neighbors,
but this will be corrected through the exchange of update messages containing
link-state advertisements (LSAs). Eventually all routers should converge
to the same information. We will see how this works in the
topic on OSPF messaging.
OSPF, as an interior routing protocol,
is of course used only within the autonomous system. In most cases the
AS will be connected to other ASes through one or more of its routers.
The routers that connect the AS to other ASes are often called boundary
routers. These devices will use OSPF to communicate within the AS,
and an exterior routing protocol (typically BGP)
to talk to routers outside the AS. The name boundary router refers
to the fact that these devices are usually located on the periphery
of the AS.
Key Concept: In basic OSPF topology, each of the routers running OSPF is considered a peer of the others. Each maintains a link-state database (LSDB) that contains information about the topology of the entire autonomous system. Each link between a router and network or between two routers is represented by an entry in the LSDB that indicates the cost to send data over the link. The LSDB is updated regularly through the exchange of OSPF link-state advertisements (LSAs).
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Version 3.0 - Version Date: September 20, 2005
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