H3C Technologies H3C SecPath F1000-E - Manual (Page 297)

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• Each OSPF router collects LSAs from other routers to compose a LSDB (Link State Database). An LSA
describes the network topology around a router, so the LSDB describes the entire network topology
of the AS.
• Each router transforms the LSDB to a weighted directed graph, which actually reflects the topology
architecture of the entire network. All the routers have the same graph.
• Each router uses the SPF algorithm to compute a Shortest Path Tree that shows the routes to the
nodes in the autonomous system. The router itself is the root of the tree.
Router ID
An OSPF process running on a router must have its own router ID, which is a 32-bit unsigned integer, the
unique identifier of the router in the AS.
OSPF packets
OSPF uses five types of packets:
• Hello packet: Periodically sent to find and maintain neighbors, containing the values of some timers,
information about the DR, BDR and known neighbors.
• DD packet (database description packet): Describes the digest of each LSA in the LSDB, exchanged
between two routers for data synchronization.
• LSR (link state request) packet: Requests needed LSAs from the neighbor. After exchanging the DD
packets, the two routers know which LSAs of the neighbor are missing from the local LSDBs. Then,
they send an LSR packet to each other, requesting the missing LSAs. The LSA packet contains the
digest of the missing LSAs.
• LSU (link state update) packet: Transmits the needed LSAs to the neighbor.
• LSAck (link state acknowledgment) packet: Acknowledges received LSU packets. It contains the
headers of received LSAs (a packet can acknowledge multiple LSAs).
LSA types
OSPF sends routing information in LSAs, which, as defined in RFC 2328, have the following types:
• Router LSA: Type-1 LSA, originated by all routers, flooded throughout a single area only. This LSA
describes the collected states of the router's interfaces to an area.
• Network LSA: Type-2 LSA, originated for broadcast and NBMA networks by the designated router,
flooded throughout a single area only. This LSA contains the list of routers connected to the network.
• Network Summary LSA: Type-3 LSA, originated by ABRs (Area Border Routers), and flooded
throughout the LSA's associated area. Each summary-LSA describes a route to a destination outside
the area, yet still inside the AS (an inter-area route).
• ASBR Summary LSA: Type-4 LSA, originated by ABRs and flooded throughout the LSA's associated
area. Type 4 summary-LSAs describe routes to ASBR (Autonomous System Boundary Router).
• AS External LSA: Type-5 LSA, originated by ASBRs, and flooded throughout the AS (except stub and
NSSA areas). Each AS-external-LSA describes a route to another AS.
• NSSA LSA: Type-7 LSA, as defined in RFC 1587, originated by ASBRs in NSSAs (Not-So-Stubby
Areas) and flooded throughout a single NSSA. NSSA LSAs describe routes to other ASs.
• Opaque LSA: A proposed type of LSA, the format of which consists of a standard LSA header and
application specific information. Opaque LSAs are used by the OSPF protocol or by some
application to distribute information into the OSPF routing domain. The opaque LSA includes three
types, Type 9, Type 10 and Type 11, which are used to flood into different areas. The Type 9 opaque