7-9
For this reason, as a mechanism for state transition in STP, the newly elected root ports or designated
ports require twice the forward delay time before transiting to the forwarding state to ensure that the new
configuration BPDU has propagated throughout the network.
z
Hello time is the time interval at which a bridge sends hello packets to the surrounding bridges to
ensure that the paths are fault-free.
z
Max age is a parameter used to determine whether a configuration BPDU held by the bridge has
expired. A configuration BPDU beyond the max age will be discarded.
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 quicker under certain conditions than in STP.
z
In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: the
old root port on the bridge has stopped forwarding data and the upstream designated port has
started forwarding data.
z
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 bridge
undergoes handshake with the downstream bridge and gets a response.
Introduction to MSTP
Why MSTP
1) Weakness of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or designated port must
wait twice the forward delay time before transiting to the forwarding state, even if it is a port on a
point-to-point link or an edge port, which directly connects to a user terminal rather than to another
bridge or a shared LAN segment.
Although RSTP supports 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
VLAN, and the packets of all VLANs are forwarded along the same spanning tree.
2) Features
of
MSTP
Developed based on IEEE 802.1s, MSTP overcomes the shortcomings of STP and RSTP. In addition to
the support for rapid network convergence, it also allows data flows of different VLANs to be forwarded
along separate paths, thus providing a better load sharing mechanism for redundant links. For more
information about VLANs, see
VLAN
in the
Layer 2 – LAN Switching Configuration Guide
.
MSTP features the following:
