H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 237)

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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.
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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, 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.