Netgear GS728TPP User Manual download pdf (Page 266)

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GS752TP, GS728TP, and GS728TPP Gigabit Smart Switches

MSTP

Spanning Tree Protocol (STP) runs on bridged networks to help eliminate loops. If a bridge

loop occurs, the network can become flooded with traffic. IEEE 802.1s Multiple Spanning

Tree Protocol (MSTP) supports multiple instances of spanning tree to efficiently channel

VLAN traffic over different interfaces. Each instance of the spanning tree behaves in the

manner specified in IEEE 802.1w, Rapid Spanning Tree, with slight modifications in the

working but not the end effect (chief among the effects is the rapid transitioning of the port to

the forwarding state).

The difference between the RSTP and the traditional STP (IEEE 802.1D) is the ability to

configure and recognize full-duplex connectivity and ports that are connected to end stations,

resulting in rapid transitioning of the port to the forwarding state and the suppression of

topology change notification. These features are represented by the parameters

point-to-point and edgeport. MSTP is compatible with both RSTP and STP and behaves

appropriately to STP and RSTP bridges.

An MSTP bridge can be configured to behave entirely as an RSTP bridge or an STP bridge.

So, an IEEE 802.1s bridge inherently also supports IEEE 802.1w and IEEE 802.1D.

The MSTP algorithm and protocol provide simple and full connectivity for frames assigned to

any given VLAN throughout a bridged LAN comprising arbitrarily interconnected networking

devices, each operating MSTP, STP, or RSTP. MSTP enables frames assigned to different

VLANs to follow separate paths, each based on an independent Multiple Spanning Tree

Instance (MSTI), within Multiple Spanning Tree (MST) regions composed of LANs and or

MSTP bridges. These regions and the other bridges and LANs are connected into a single

common spanning tree (CST). (IEEE DRAFT P802.1s/D13).

MSTP connects all bridges and LANs with a single Common and Internal Spanning Tree

(CIST). The CIST supports the automatic determination of each MST region, choosing its

maximum extent. The connectivity calculated for the CIST provides the CST for

interconnecting these regions, and an Internal Spanning Tree (IST) within each region.

MSTP ensures that frames with a given VLAN ID are assigned to only one of the MSTIs or

the IST within the region, that the assignment is consistent among all the networking devices

in the region, and that the stable connectivity of each MSTI and IST at the boundary of the

region matches that of the CST. The stable active topology of the bridged LAN with respect to

frames consistently classified as belonging to any given VLAN thus simply and fully connects

all LANs and networking devices throughout the network, though frames belonging to

different VLANs can take different paths within any region, per IEEE DRAFT P802.1s/D13.

All bridges, whether they use STP, RSTP or MSTP, send information in configuration

messages through bridge protocol data units (BPDUs) to assign port roles that determine

each port’s participation in a fully and simply connected active topology based on one or

more spanning trees. The information communicated is known as the spanning tree priority

vector. The BPDU structure for each of these different protocols is different. An MSTP bridge

transmits the appropriate BPDU depending on the received type of BPDU from a particular

port.

An MST region comprises of one or more MSTP bridges that have the same MST

configuration identifier, using the same MSTIs that have no bridges attached that cannot

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Netgear GS728TPP User Manual Page 266

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Netgear GS728TPP User Manual Page 266

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