3Com Switch 7750 Configuration Guide

306 CHAPTER 34: OSPF CONFIGURATION
■ The DR on a network segment is not necessarily the router with the highest
priority. Likewise, the BDR is not necessarily the router with the second-highest
priority.
OSPF Packets OSPF uses five types of packets:
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, the
DR, the BDR and the known peers.
DD packet
When two routers synchronize their databases, they use database description (DD)
packets to describe their own LSDBs, including the digest of each LSA. The digest
refers to the HEAD of an LSA which uniquely identifies the LSA. This reduces the
size of traffic transmitted between the routers because the HEAD of an LSA only
occupies a small portion of the LSA. With the HEAD, 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 HEAD(s) of LSA(s) to be acknowledged (one
LSAck packet can acknowledge multiple LSAs).
LSA Types 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 area where the router resides.
■ Network-LSA: Type-2 LSAs, generated by the DRs of broadcast or NBMA
network to describe the link states of the current network segment and are
advertised only in the area where the DRs reside.
■ 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.