H3C Technologies H3C S12500-X Series Switches - Manual (Page 79)

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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.
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 root port rapidly enters the forwarding state.
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. 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
MSTP overcomes the following STP and RSTP limitations:
• STP 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.
• RSTP limitations—Although RSTP enables faster network convergence than STP, RSTP fails to
provide load balancing among VLANs. As with STP, all RSTP bridges in a LAN share one spanning
tree and forward packets from all VLANs along this spanning tree.
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.
MSTP basic concepts
Figure 18 shows a switched network that comprises four MST regions, each MST region comprising four
MSTP devices. Figure 19 shows the networking topology of MST region 3.