3Com 5500-EI PWR Install Manual page 289

Hello packet:
Hello packets are most commonly used OSPF packets, which are periodically sent by a router to its
neighbors. A Hello packet contains the values of some timers, DR, BDR and known neighbors.
DD packet:
When two routers synchronize their databases, they use database description (DD) packets to describe
their own LSDBs, including the summary of each LSA. The summary refers to the header of an LSA
which uniquely identifies the LSA. This reduces the size of traffic transmitted between the routers
because the header of an LSA only occupies a small portion of the LSA. With the header, the peer
router can judge whether it has the LSA or not.
LSR packet:
After exchanging DD packets, the two routers know which LSAs of the peer router are lacked in the local
LSDB, and send link state request (LSR) packets requesting for the lacked LSAs to the peer. These
LSR packets contain the digest of the needed LSAs.
LSU packet:
Link state update (LSU) packets are used to transmit the needed LSAs to the peer router. An LSU
packet is a collection of multiple LSAs (complete LSAs, not LSA digest).
LSAck packet
Link state acknowledgment (LSAck) packets are used to acknowledge received LSU packets. An
LSAck contains the header(s) of LSA(s) to be acknowledged (One LSAck packet can acknowledge
multiple LSAs).
LSA Types
1) Five basic LSA types
As described in the preceding sections, LSAs are the primary source for OSPF to calculate and
maintain routes. RFC 2328 defines five types of LSAs:
Router-LSA: Type-1 LSAs, generated by every router to describe the router's link states and costs,
and advertised only in the originating area.
Network-LSA: Type-2 LSAs, generated by the DRs on a broadcast or NBMA network to describe
the link states of the current network segment, and are advertised only in the originating area.
Summary-LSA: Type-3 and Type-4 LSAs, generated by ABRs and advertised in the areas
associated with the LSAs. Each Summary-LSA describes a route to a destination in another area of
the AS (also called inter-area route).Type-3 Summary-LSAs are for routes to networks (that is, their
destinations are segments), while Type-4 Summary-LSAs are for routes to ASBRs.
AS-external-LSA: Type-5 LSA, also called ASE LSA, generated by ASBRs to describe the routes
to other ASs and advertised to the whole AS (excluding stub areas and NSSA areas). The default
AS route can also be described by AS-external-LSAs.
2) Type-7 LSAs
In RFC 1587 (OSPF NSSA Option), Type-7 LSA, a new LSA type, is added.
As described in RFC 1587, Type-7 LSAs and Type-5 LSAs mainly differ in the following two ways:
Type-7 LSAs are generated and advertised in an NSSA, where Type-5 LSAs will not be generated
or advertised.
4-3