路由更新的可达性问题-CCNA中文版PPT下载

路由更新的可达性问题 R1 R2 R3 S0 S0 E0 S0 E0 E0 帧中继 22.1.1.2/24 33.1.1.3/24 11.1.1.1/24 12.1.1.3/24 12.1.1.1/24 dlci:102 dlci:103 dlci:201 dlci:301 部分连接：R1和R2、R3直接相连，R2和R3没有直接相连 厦门微思网络http://www.xmws.cn * Purpose: This figure 续inues the discussion that leads into the need for subinterfaces. Emphasize: Partial mesh Frame Relay networks must deal with the case of split horizon not allowing routing updates to be retransmitted on the same interface from which they were received. Split horizon cannot be disabled for certain protocols such as AppleTalk. Split horizon issues are overcome through the use of logical subinterfaces assigned to the physical interface connecting to the Frame Relay network. A physical interface can be divided into multiple, logical interfaces. Each logical interface is individually configured and is named after the physical interface. A decimal number is included to distinguish it. The logical port names 续ain a decimal point and another number indicating these are subinterfaces of interface serial 0 (S0). Subinterfaces are configured by the same configuration commands used on physical interfaces. A broadcast environment can be Frame Relay-created by transmitting the data on each individual circuit. This simulated broadcast requires significant buffering and CPU resources in the transmitting router, and can result in lost user data because of 续ention for the circuits. Reference: Interconnections by Radia Perlman is also a good reference on split horizon. Note: Subinterfaces are particularly useful in a Frame Relay partial-mesh NBMA model that uses a distance vector routing protocol. Instead of migrating to a routing protocol that supports turning off split horizon, subinterfaces can be used to overcome the split horizon problem.