HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 2 - LAN Switching Co - Page 66

The device uses the max age to determine whether a stored configuration BPDU has expired and discards it if the max age is exceeded. RSTP RSTP achieves rapid network convergence by allowing a newly elected root port or designated port to enter the forwarding state much faster than STP. A newly elected RSTP root port rapidly enters the forwarding state if the old root port on the device has stopped forwarding data and the upstream designated port has started forwarding data. A newly elected RSTP designated port rapidly enters the forwarding state if it is an edge port (a port that directly connects to a user terminal rather than to another network device or a shared LAN segment) or it connects to a point-to-point link (to another device). Edge ports directly enter the forwarding state. Connecting to a point-to-point link, a designated port enters the forwarding state immediately after the device receives a handshake response from the directly connected device. MSTP STP and RSTP limitations STP does not support rapid state transition of ports. A newly elected port must wait twice the forward delay time before it transits to the forwarding state, even if it connects to a point-to-point link or is an edge port. Although RSTP supports rapid network convergence, it has the same drawback as STP. All bridges within a LAN share the same spanning tree, and the packets from all VLANs are forwarded along the same spanning tree, so redundant links cannot be blocked based on VLAN and traffic cannot be load-shared among VLANs. MSTP features Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP and RSTP. In addition to supporting rapid network convergence, it provides a better load sharing mechanism for redundant links by allowing data flows of different VLANs to be forwarded along separate paths. MSTP provides the following features: • MSTP divides a switched network into multiple regions, each of which contains multiple spanning trees that are independent of one another. • MSTP supports mapping VLANs to spanning tree instances by means of a VLAN-to-instance mapping table. MSTP can reduce communication overheads and resource usage by mapping multiple VLANs to one instance. • MSTP prunes a loop network into a loop-free tree, which avoids proliferation and endless cycling of packets in a loop network. In addition, it supports load balancing of VLAN data by providing multiple redundant paths for data forwarding. • MSTP is compatible with STP and RSTP. 57