H3C S7500E Series Operation Manual page 215

Operation Manual – MSTP
H3C S7500E Series Ethernet Switches
Note:
In RSTP, a newly elected root port can enter the forwarding state rapidly if this
condition is met: The old root port on the device has stopped forwarding data and
the upstream designated port has started forwarding data.
In RSTP, a newly elected designated port can enter the forwarding state rapidly if
this condition is met: The designated port is an edge port or a port connected with a
point-to-point link. If the designated port is an edge port, it can enter the forwarding
state directly; if the designated port is connected with a point-to-point link, it can
enter the forwarding state immediately after the device undergoes handshake with
the downstream device and gets a response.
Although RSTP support rapid network convergence, it has the same drawback as STP
does: All bridges within a LAN share the same spanning tree, so redundant links cannot
be blocked based on VLANs, and the packets of all VLANs are forwarded along the
same spanning tree.
2) Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and
RSTP. In addition to support for rapid network convergence, it also allows data flows of
different VLANs to be forwarded along their own paths, thus providing a better load
sharing mechanism for redundant links. For description about VLANs, refer to VLAN
Configuration.
MSTP features the following:
MSTP supports mapping VLANs to MST instances by means of a
VLAN-to-instance mapping table. MSTP can save communication overheads and
resource usage by mapping multiple VLANs to one MST instance.
MSTP divides a switched network into multiple regions, each containing multiple
spanning trees that are independent of one another.
MSTP prunes loop networks into a loop-free tree, thus avoiding proliferation and
endless recycling of packets in a loop network. In addition, it provides multiple
redundant paths for data forwarding, thus supporting load balancing of VLAN data
in the data forwarding process.
MSTP is compatible with STP and RSTP.
II. Basic concepts in MSTP
Assume that all the four switches in
diagram, the following paragraphs will present some basic concepts of MSTP.
Figure 1-4 are running MSTP. In light with the
1-12
Chapter 1 MSTP Configuration