3Com 4800G Series, Configuration Manual

Page: 275 / 1246

Introduction to OSPF 275
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LSU (link state update) packet: Transmits the needed LSAs to the neighbor.
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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:
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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.
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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.
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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).
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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).
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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.
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NSSA External 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.
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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.
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.