HP FlexNetwork NJ5000, User Manual

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Introduction to RSTP
Developed based on the 802.1w standard of IEEE, RSTP is an optimized version of STP. It 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.
Introduction to 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 160 shows a switched network that comprises four MST regions, each MST region
comprising four MSTP devices.