H3C Technologies H3C S12500 Series Switches - Manual (Page 77)

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A Type-2 external route has low credibility. OSPF considers the cost from the ASBR to the destination of
the Type-2 external route is much greater than the cost from the ASBR to an OSPF internal router. The cost
from the internal router to the destination of the Type-2 external route = the cost from the ASBR to the
destination of the Type-2 external route. If two routes to the same destination have the same cost, OSPF
takes the cost from the router to the ASBR into consideration to determine the best route.
OSPF network classification
OSPF network types
OSPF classifies networks into the following types depending on different link layer protocols:
• Broadcast—If the link layer protocol is Ethernet or FDDI, OSPF considers the network type as
broadcast by default. On a broadcast network, hello, LSU, and LSAck packets are multicast to
224.0.0.5 that identifies all OSPF routers or 224.0.0.6 that identifies the DR, while DD packets and
LSR packets are unicast.
• NBMA (Non-Broadcast Multi-Access)—If the link layer protocol is Frame Relay, ATM, or X.25,
OSPF considers the network type as NBMA by default. OSPF packets are unicast on a NBMA
network.
• P2MP (point-to-multipoint)—By default, OSPF considers no link layer protocol as P2MP, which is a
conversion from other network types such as NBMA. On a P2MP network, OSPF packets are
multicast to 224.0.0.5.
• P2P (point-to-point)—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.
NBMA network configuration guidelines
Typical NBMA networks include ATM and Frame Relay networks.
Because NBMA interfaces cannot broadcast hello packets, you must specify neighbors manually and
configure DR priorities for the neighbors.
An NBMA network is fully meshed, which means any two routers in the NBMA network have a direct
virtual circuit for communication. If direct connections are not available between some routers, the
network type of interfaces associated should be configured as P2MP. If such an interface has only one
neighbor, configure its network type as P2P.
The following are the differences between NBMA and P2MP networks:
• NBMA networks are fully meshed, non-broadcast, and multi access. P2MP networks are not
required to be fully meshed.
• NBMA networks require DR and BDR election. P2MP networks do not have DR or BDR.
• NBMA is the default network type. P2MP is a conversion from another network type, such as
NBMA.
• On a NBMA network, OSPF packets are unicast, and neighbors are configured manually on routers.
On a P2MP network, OSPF packets are multicast.
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 required.
Any topology change on the network results in an increase in traffic for route synchronization, consuming
many system and bandwidth resources.