Ospf Area Partition And Route Summarization - H3C S3610-28P Operation Manual

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
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.
VI. 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.

3.1.2 OSPF Area Partition and Route Summarization

I. 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.
3-4
Chapter 3 OSPF Configuration