IP-Chapter1-LANOverview要点.ppt

* Stallings DCC8e Figure 15.8 illustrates the action of a bridge connecting two LANs, A and B, using the same MAC protocol. In this example, a single bridge attaches to both LANs; frequently, the bridge function is performed by two half-bridges, one on each LAN. The functions of the bridge are few and simple: ? Read all frames transmitted on A and accept those addressed to any station on B. ? Using the medium access control protocol for B, retransmit each frame on B. ? Do the same for B-to-A traffic. * Several design aspects of a bridge are worth highlighting: ? The bridge makes no modification to the content or format of the frames it receives, nor does it encapsulate them with an additional header. Each frame to be transferred is simply copied from one LAN and repeated with exactly the same bit pattern on the other LAN. Because the two LANs use the same LAN protocols, it is permissible to do this. ? The bridge should contain enough buffer space to meet peak demands. Over a short period of time, frames may arrive faster than they can be retransmitted. ? The bridge must contain addressing and routing intelligence. At a minimum, the bridge must know which addresses are on each network to know which frames to pass. Further, there may be more than two LANs interconnected by a number of bridges. In that case, a frame may have to be routed through several bridges in its journey from source to destination. ? A bridge may connect more than two LANs. In summary, the bridge provides an extension to the LAN that requires no modification to the communications software in the stations attached to the LANs. * The IEEE 802.1D specification defines the protocol architecture for MAC bridges. Within the 802 architecture, the endpoint or station address is designated at the MAC level. Thus, it is at the MAC level that a bridge can function. Figure 15.9 shows the simplest case, which consists of two LANs connected by a single bridge. The LANs employ the same MAC and LLC protocols. T