Qtech QSW-3900-24-SFP-AC [95/245] Basic concepts

QTECH Software Configuration Manual

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8.1.1 Basic Concepts

8.1.1.1 Autonomous System

A set of routers using the same routing protocol to exchange routing information constitute an Autonomous

System (AS).

8.1.1.2 OSPF route computation

OSPF route computation is described as follows :

· Based on the network topology around itself, each router generates Link State Advertisements (LSA) and

sends them to other routers in update packets.

· 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.

8.1.1.3 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 them.

· If no loopback interface is configured, select the highest IP address among addresses of active interfaces on

the router.

8.1.1.4 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. In this case,

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).

8.1.1.5 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.