TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Init: Communication with the neighbor has started, but bidirectional
communication has not been established. Specifically, a hello packet was
received from the neighbor, but the local router was not listed in the
neighbor's hello packet.
► 2-way: Bidirectional communication between the two routers has been
established. Adjacencies can be formed. Neighbors are eligible to be elected
as designated routers.
► ExStart: The neighbors are starting to form an adjacency.
► Exchange: The two neighbors are exchanging their topology databases.
► Loading: The two neighbors are synchronizing their topology databases.
► Full: The two neighbors are fully adjacent and their databases are
synchronized.
Network events cause a neighbor’s OSPF state to change. For example, when a
router receives a hello packet from a neighboring device, the OSPF neighbor
state changes from Down to Init. When bidirectional communication has been
established, the neighbor state changes from Init to 2-Way. RFC 2328 contains a
complete description of the events causing a state change.
OSPF virtual links and transit areas
Virtual links are used when a network does not support the standard OSPF
network topology. This topology defines a backbone area that directly connects
to each additional OSPF area. The virtual link addresses two conditions:
► It can logically connect the backbone area when it is not contiguous.
► It can connect an area to the backbone when a direct connection does not
exist.
A virtual link is established between two ABRs sharing a common non-backbone
area. The link is treated as a point-to-point link. The common area is known as a
transit area. Figure 5-18 on page 208 illustrates the interaction between virtual
links and transit areas when used to connect an area to the backbone.
Chapter 5. Routing protocols 207