Specifications

Spanning Tree Folder
Extreme Networks EAS 100-24t Switch Software Manual
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processed quickly and completely throughout interconnected bridges utilizing any of the three spanning
tree protocols (STP, RSTP or MSTP).
This protocol will also tag BPDU packets so receiving devices can distinguish spanning tree instances,
spanning tree regions and the VLANs associated with them. An MSTI ID will classify these instances.
MSTP will connect multiple spanning trees with a Common and Internal Spanning Tree (CIST). The
CIST will automatically determine each MSTP region, its maximum possible extent and will appear as
one virtual bridge that runs a single spanning tree. Consequentially, frames assigned to different VLANs
will follow different data routes within administratively established regions on the network, continuing
to allow simple and full processing of frames, regardless of administrative errors in defining VLANs
and their respective spanning trees.
Each switch utilizing the MSTP on a network will have a single MSTP configuration that will have the
following three attributes:
1 A configuration name defined by an alphanumeric string of up to 32 characters (defined in the MST
Configuration Identification window in the Configuration Name field).
2 A configuration revision number (named here as a Revision Level and found in the MST
Configuration Identification window) and;
3 A 4094-element table (defined here as a VID List in the MST Configuration Identification window),
which will associate each of the possible 4094 VLANs supported by the Switch for a given instance.
To utilize the MSTP function on the Switch, three steps need to be taken:
1 The Switch must be set to the MSTP setting (found in the STP Bridge Global Settings window in
the STP Version field)
2 The correct spanning tree priority for the MSTP instance must be entered (defined here as a Priority
in the STP Instance Settings window when configuring MSTI ID settings).
3 VLANs that will be shared must be added to the MSTP Instance ID (defined here as a VID List in
the MST Configuration Identification window when configuring an MSTI ID settings).
Rapid Spanning Tree
The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree
Protocol (MSTP) as defined by the IEEE 802.1Q-2005, the Rapid Spanning Tree Protocol (RSTP) as
defined by the IEEE 802.1D-2004 specification and a version compatible with the IEEE 802.1D-1998 STP.
RSTP can operate with legacy equipment implementing IEEE 802.1D-1998; however the advantages of
using RSTP will be lost.
The Rapid Spanning Tree Protocol (RSTP) evolved from the STP standard. RSTP was developed in order
to overcome some limitations of STP that impede the function of some recent switching innovations, in
particular, certain Layer 3 functions that are increasingly handled by Ethernet switches. The basic
function and much of the terminology is the same as STP. Most of the settings configured for STP are
also used for RSTP. This section introduces some new Spanning Tree concepts and illustrates the main
differences between the two protocols.
Port Transition States
An essential difference between the three protocols is in the way ports transition to a forwarding state
and in the way this transition relates to the role of the port (forwarding or not forwarding) in the
topology. MSTP and RSTP combine the transition states disabled, blocking and listening used in STP
and creates a single state Discarding. In either case, ports do not forward packets. In the STP port
transition states disabled, blocking or listening or in the RSTP/MSTP port state discarding, there is no
functional difference, the port is not active in the network topology. The table below compares how the
three protocols differ regarding the port state transition.