xStack® DGS-3120 Series Layer 3 Managed Gigabit Ethernet Switch Web UI Reference Guide
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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
MSTI Config Information
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).
802.1D-2004 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 IEEE 802.1D-2004 Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D-1998 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 802.1D-1998 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. Table 7-3 below compares how the three protocols differ regarding the port state transition.
All three protocols calculate a stable topology in the same way. Every segment will have a single path to the root
bridge. All bridges listen for BPDU packets. However, BPDU packets are sent more frequently - with every Hello
packet. BPDU packets are sent even if a BPDU packet was not received. Therefore, each link between bridges is
sensitive to the status of the link. Ultimately this difference results in faster detection of failed links, and thus faster
topology adjustment. A drawback of 802.1D-1998 is this absence of immediate feedback from adjacent bridges.
802.1Q-2005 MSTP
802.1D-2004 RSTP
802.1D-1998 STP
Forwarding
Learning
Disabled
Disabled
Disabled
No
No
Discarding
Discarding
Blocking
No
No
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