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2.9.- Graph of Adjacencies |
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One can picture the collection of adjacencies on a network as forming an
indirect graph. Vertices are routers with edges joining two routers when
they are adjacent. The graph describes the flow of routing protocol
packets throughout the AS (called the control plane). |
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Two graphs are possible, depending on whether a DR is elected. On
P2P, P2MP and VL, neighboring routers become adjacent
whenever they can communicate directly. On broadcast and NBMA
networks, only the DR and the BDR become adjacent to all other
routers attached to the network. |
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These graphs are shown in figure 16 below. On the figure, for broadcast
and NBMA networks, router RT7 is the DR and router
RT3 the BDR. The BDR performs a lesser function than the
DR. That's the reason for the dashed lines connecting the RT3's
BDR with the rest of routers. |
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Protocol Packet Processing |
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It is very important that the routers' LS-databases remain
synchronized. For this reason, control plane should get preference
treatment over the data plane, both in sending and receiving. |
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With the exception of Hello packets, which are used to discover
adjacencies, the rest of the control plane flows only through
adjacencies. This means that all routing packets travel just a single hop,
except those sent over VLs. |
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