ccna exam hk

ccna exam hk.2Learning: This state occurs during stable active topologies and during topology synchronization and changes. The learning state receives data frames to populate the MAC table to limit flooding of unknown unicast frames.3Forwarding ( Forwarding): This state occurs only in a stable active topology. The switch port in the forwarding state determines the topology. After a topology change occurs or during synchronization, data frames are forwarded only when the propose and agree process is complete.14.2.2 RSTP Proposal/Agreement MechanismThe root port and designated port in the RSTP port role are determined in the same way as STP. For non-designated ports, they are further divided into Alternate and Backup ports, as shown in Figure 14.7. Alternate ports are ports that are blocked because they receive better BPDUs from other bridges , and Backup ports are ports that are blocked because they receive better BPDUs from other ports on this switch. In Figure 14-7, the FaO/1 port of S3 is the designated port of this segment, and S2 will receive the better BPDUs sent by S3 from the FaO/1 port, so the FaO/1 port of S2 is the Alternate port; the FaO/2 port of S3 will receive the better BPDUs sent by S3's own FaO/1 port, so it is the Backup port. When the root port of S2 fails, the alternate port of S2 will immediately enter the forwarding state; and when the designated port of S3 fails, the backup port of S2 will immediately enter the forwarding state, thus greatly reducing the convergence time.RSTP uses the Proposal/Agreement handshake mechanism (see Figure 14-8) to accomplish fast convergence. In Figure 14-8, assume that S2 has a new link connected to the root bridge, and when the link is up, port p0 of S1 and port pl of S2 enter the designated blocking state at the same time, and then S1 sends a proposal BPDU from port p0o Since S2 receives a better BPDU from port pl, S2 starts synchronizing the new message to the other ports. port p2 is the alternative port and remains unchanged in synchronization. Port p3 is the designated port and must be blocked in synchronization; port p4 is the edge port and remains unchanged in synchronization. S2 sends an agree BPDU to root bridge S1 through port pl, port p0 and port pl handshake successfully, port pl becomes S2's new root port, and ports p0 and pl directly enter the forwarding state. At this time, port p3 is the designated port and is still in the blocking state. Similarly, for port p0 and port pl, port p3 completes fast convergence on its link according to the propose/agree handshake mechanism. The propose/agree handshake mechanism converges quickly and does not rely on any timers in the port state transition..224.Cisco Networking Lab CCNA Lab Guide (Release 2)Figure 14-8 RSTP using the propose/agree handshake mechanism14.2.3 Introduction to MSTPIn PVST, the switch builds an STP tree for each VLAN, which not only puts a lot of load on the CPU (especially the low end of the switch), but also takes up a lot of bandwidth. MSTP (MultipleSpanningTreeProtocol), on the other hand, maps multiple VLANs to a single STP instance, with each instance running RSTP, thus reducing the waste of resources.The concepts of Instance and Region are introduced in MSTP. An instance is a collection of multiple VLANs, and this approach saves communication overhead and resource usage by bundling multiple VLANs into a single instance. The calculation of the topology of each instance of MSTP is independent, and load balancing can be achieved by controlling STP elections on these instances. The domain consists of the domain name ( Configuration Name), the revision level ( Revision Level), the format selector (Configuration Identifier Format Selector), and the mapping relationship between the VLAN and the instance, where the domain name, format selector, and revision level are in the BPDU packet with The domain name, format selector and revision level are all related fields in the BPDU packet, while the mapping relationship between VLAN and instance is represented in the BPDU packet in the form of MDS digest information (ConfigurationDigest), which is a 16-byte signature calculated according to the mapping relationship. Only switches that are identical and interconnected as described above are considered to be in the same MSTP domain. By default, all VLANs are mapped to instance 0. Instance 0 of MSTP has a special role and is called C1ST (CommonInternaland SpanningTree), i.e., Common and Internal Spanning Tree, and the other instances are called MSTI (MultipleSpanningTree Instance ), i.e., Multiple Spanning Tree Instance.