什么是NBMA网络？

原文
NBMA (non-broadcast multiple access) is one of four network types in the OSPF (Open Shortest Path First) communications protocol. NBMA is used to accurately model X.25 and frame relay environments in multiple-access networks where there are no intrinsic broadcast and multicast capabilities. The other OSPF network types are: broadcast, point-to-point, and point-to-multipoint. In an NBMA configuration, OSPF sends HELLO packets (packets sent periodically to establish and confirm neighbor relationships between routers) to each router one at a time rather than multicasting them. The HELLO timer (which tells the router how often to send HELLO packets) is extended from 10 to 30 seconds and the dead router timer (which tells the router how long to wait before it decides that a neighboring router is not functioning) is extended from 40 to 120 seconds.
译文:
NBMA（非广播多路访问网络）是OSPF（开放最短路径优先）通信协议中四种网络的一种。NBMA用于精确模型X2.5和帧延迟环境，这些模型不具备内部广播和多点传送能力。其他的OSPF网络类型有：广播、点对点和点对多点。在NBMA设置里，OSP