HP HPE FlexNetwork 7500 series, Configuration Manual

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Each router transforms the LSDB to a weighted directed graph that shows the topology of the
area. All the routers within the area have the same graph.
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Each router uses the SPF algorithm to compute a shortest path tree that shows the routes to the
nodes in the area. The router itself is the root of the tree.
OSPF network types
OSPF classifies networks into the following types, depending on different link layer protocols:
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Broadcast —By default, OSPF considers the network type as broadcast. On a broadcast
network, hello, LSU, and LSAck packets are multicast to 224.0.0.5 that identifies all OSPF
routers or to 224.0.0.6 that identifies the DR and BDR. DD packets and LSR packets are
unicast.
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NBMA —OSPF packets are unicast on an NBMA network.
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P2MP —No link is P2MP type by default. P2MP must be a conversion from other network types
such as NBMA. On a P2MP network, OSPF packets are multicast to 224.0.0.5.
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P2P —On a P2P network, OSPF packets are multicast to 224.0.0.5.
The following are the differences between NBMA and P2MP networks:
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NBMA networks are fully meshed. P2MP networks are not required to be fully meshed.
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NBMA networks require DR and BDR election. P2MP networks do not have DR or BDR.
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On an NBMA network, OSPF packets are unicast, and neighbors are manually configured. On a
P2MP network, OSPF packets are multicast by default, and you can configure OSPF to unicast
protocol packets.
DR and BDR
DR and BDR mechanism
On a broadcast or NBMA network, any two routers must establish an adjacency to exchange routing
information with each other. If n routers are present on the network, n(n-1)/2 adjacencies are
established. Any topology change on the network results in an increase in traffic for route
synchronization, which consumes a large amount of system and bandwidth resources.
Using the DR and BDR mechanisms can solve this problem.
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DR —Elected to advertise routing information among other routers. If the DR fails, routers on the
network must elect another DR and synchronize information with the new DR. Using this
mechanism without BDR is time-consuming and is prone to route calculation errors.
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BDR —Elected along with the DR to establish adjacencies with all other routers. If the DR fails,
the BDR immediately becomes the new DR, and other routers elect a new BDR.
Routers other than the DR and BDR are called DR Others. They do not establish adjacencies with
one another, so the number of adjacencies is reduced.
The role of a router is subnet (or interface) specific. It might be a DR on one interface and a BDR or
DR Other on another interface.
As shown in Figure 22 , solid lines are Ethernet physical links, and dashed lines represent OSPF
adjacencies. With the DR and BDR, only seven adjacencies are established.