Qtech QSW-3900-24-SFP-AC [96/245] Ospf area partition and route summarization

QTECH Software Configuration Manual

8-95

· 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 LSA is flooded into the local

subnet, the Type 10 is flooded into the local area, and the Type 11 is flooded throughout the whole AS.

8.1.1.6 Neighbor and Adjacency

In OSPF, the “Neighbor” and ”Adjacency” are two different concepts.

Neighbor : Two routers that have interfaces to a common network. Neighbor relationships are maintained

by, and usually dynamically discovered by, OSPF's hello packets. When a router starts, it sends a hello packet via the

OSPF interface, and the router that receives the hello packet checks parameters carried in the packet. If parameters of

the two routers match, they become neighbors.

Adjacency : A relationship formed between selected neighboring routers for the purpose of exchanging

routing information. Not every pair of neighboring routers become adjacent, which depends on network types. Only

by synchronizing the LSDB via exchanging DD packets and LSAs can two routers become adjacent.

8.1.2 OSPF Area Partition and Route Summarization

8.1.2.1 Area partition

When a large number of OSPF routers are present on a network, LSDBs may become so large that a great

amount of storage space is occupied and CPU resources are exhausted by performing SPF computation.

In addition, as the topology of a large network is prone to changes, enormous OSPF packets may be created,

reducing bandwidth utilization. Each topology change makes all routers perform route calculation.

To solve this problem, OSPF splits an AS into multiple areas, which are identified by area ID. The

boundaries between areas are routers rather than links. A network segment (or a link) can only reside in one area, in

other words, an OSPF interface must be specified to belong to its attached area, as shown in the figure below.