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v Open Shortest Path First (OSPF) routing protocol is a
Link State protocol based on cost rather than hops or ticks (i.e. it is not a
vector based routing protocol).
v OSPF is an Interior Gateway Protocol (IGP) Protocol, uses flooding
of link state information and Dijkstra’s least-cost path algorithm.
v With OSPF, router constructs a complete topological map of the
entire autonomous system. The router then locally runs the Dijkstra’s
shortest-path algorithm to determine shortest-path tree to all networks with
itself as rootnode.
v The router’s routing table is then obtained from this
shortest-path tree.
v At steady state: All routers have same LS database, Know how many
routers in network, Interfaces & links between routers, Cost of each link
v Occasional Hello messages (10 sec) & LS updates sent (30 min)
Advantages
of OSPF:
v Changes in an OSPF network are propagated quickly.
v OSPF is hierarchical, using area 0 as the top of the hierarchy.
v OSPF is a Link State Algorithm.
v OSPF supports Variable Length Subnet Masks (VLSM).
v OSPF uses multicasting within areas.
v After initialization, OSPF only sends updates on routing table
sections which have changed, it does not send the entire routing table.
v
Using
areas, OSPF networks can be logically segmented to decrease the size of routing
tables. Table size can be further reduced by using route summarization.
v
OSPF
is an open standard, not related to any particular vendor.
Disadvantages of
OSPF:
v
OSPF
maintains multiple copies of routing information, increasing the amount of memory
needed.
v
Using
areas, OSPF can be logically segmented (this can be a good thing and a bad
thing).
v
OSPF
is not as easy to learn as some other protocols.
v
In
the case where an entire network is running OSPF and one link within it is
"bouncing" every few seconds, OSPF updates would dominate the network
by informing every other router every time the link changed state.
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