R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide
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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.
• P2P—If the link layer protocol is PPP or HDLC, OSPF considers the network type as P2P. On a P2P
network, OSPF packets are multicast to 224.0.0.5.
The following are the differences between NBMA and P2MP networks:
• NBMA networks are fully meshed. P2MP networks are not required to be fully meshed.
• NBMA networks require DR and BDR election. P2MP networks do not have DR or BDR.
• On a 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
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, consuming
many system and bandwidth resources.
The DR and BDR mechanisms can solve this problem.
• 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
alone is time-consuming and prone to route calculation errors.
• 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 "DROthers." 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
DROther on another interface.
In Figure 18, s
olid lines are Ethernet physical links, and dashed lines represent OSPF adjacencies. With
the DR and BDR, only seven adjacencies are established.