Basic Concepts - 3Com MSR 50 Series Configuration Manual

918 C 58: OSPF C
HAPTER

Basic Concepts

ONFIGURATION
Multicast: Supports packet multicasting on some types of links.
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Autonomous System
A set of routers using the same routing protocol to exchange routing information
constitute an Autonomous System (AS).
OSPF route computation
OSPF route computation is described as follows:
Based on the network topology around itself, each router generates Link State
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Advertisements (LSA) and sends them to other routers in update packets.
Each OSPF router collects LSAs from other routers to compose a LSDB (Link
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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
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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
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the routes to the nodes in the autonomous system. The router itself is the root
of the tree.
Router ID
To run OSPF, a router must have a Router ID, which is a 32-bit unsigned integer,
the unique identifier of the router in the AS.
You may assign a Router ID to an OSPF router manually. If no Router ID is
specified, the system automatically selects one for the router as follows:
If the loopback interfaces are configured, select the highest IP address among
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them.
If no loopback interface is configured, select the highest IP address among
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addresses of active interfaces on the router.
OSPF packets
OSPF uses five types of packets:
Hello Packet: Periodically sent to find and maintain neighbors, containing the
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values of some timers, information about DR, BDR and known neighbors.
DD packet (Database Description Packet): Describes the digest of each LSA in
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the LSDB, exchanged between two routers for data synchronization.
LSR (Link State Request) Packet: Requests needed LSAs from the peer. After
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exchanging the DD packets, the two routers know which LSAs of the neighbor
routers are missing from the local LSDBs. In this case, they send LSR packets,
requesting the missing LSAs. The packets contain the digests of the missing
LSAs.
LSU (Link State Update) Packet: Transmits the needed LSAs to the peer router.
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LSAck (Link State Acknowledgment) Packet: Acknowledges received LSU
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packets. It contains the Headers of LSAs requiring acknowledgement (a packet
can acknowledge multiple LSAs).