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A router's first step in OSPF operation
is to establish router adjacencies. Each of the three routers shown
in the figure attempts to become adjacent to another router on the
same IP network.
To become adjacent with another
router, RTB sends hello packets, advertising its own router ID.
Because no loopback interfaces are present, RTB chooses its highest
IP address, 10.6.0.1, as its router ID.
Assuming that RTB is appropriately
configured, RTB multicasts hello packets out both S0 and E0. RTA and
RTC should both receive the hello packets. These two routers then
add RTB to the Neighbor ID field of their respective hello packets
and enter the Init state with RTB.
RTB receives hello packets from both
of its neighbors and sees its own ID number (10.6.0.1) in the
Neighbor ID field. RTB declares a Two-Way state between itself and
RTA, and a Two-Way state between itself and RTC.
At this point, RTB determines which
routers to establish adjacencies with, based on the type of network
that a particular interface resides on. If the network type is
point-to-point, the router becomes adjacent with its sole link
partner. If the network type is multiaccess, RTB enters the election
process to become a DR or BDR, unless both roles are already
established (as advertised in the hello packet header).
If an election is necessary, OSPF
routers will proceed as described in the next section, Step 2: Elect
a DR and a BDR. However, if an election is not necessary, the
routers will enter the ExStart state, as described in the section,
Step 3: Discover Routes.
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