D-Link DES-3828P — обзор протоколов Spanning Tree: STP, RSTP и MSTP [57/260]

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DES-3550 Fast Ethernet Layer 2 Switch

802.1w Rapid Spanning Tree

The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree Protocol (MSTP) as

defined by the IEEE 802.1s, the Rapid Spanning Tree Protocol (RSTP) as defined by the IEEE 802.1w specification and a

version compatible with the IEEE 802.1d STP. RSTP can operate with legacy equipment implementing IEEE 802.1d,

however the advantages of using RSTP will be lost.

The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1d 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 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 discard-

ing, there is no functional difference, the port is not active in the network topology. Table 6-1 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 is this absence of immediate feedback from adjacent bridges.

802.1d MSTP 802.1w RSTP 802.1d STP Forwarding Learning

Discarding Discarding Disabled No No

Discarding Discarding Blocking No No

Discarding Discarding Listening No No

Learning Learning Learning No Yes

Forwarding Forwarding Forwarding Yes Yes

Table 6- 1. Comparing Port States

RSTP is capable of a more rapid transition to a forwarding state - it no longer relies on timer configurations - RSTP

compliant bridges are sensitive to feedback from other RSTP compliant bridge links. Ports do not need to wait for the

topology to stabilize before transitioning to a forwarding state. In order to allow this rapid transition, the protocol introduces

two new variables: the edge port and the point-to-point (P2P) port.

Edge Port

The edge port is a configurable designation used for a port that is directly connected to a segment where a loop cannot be

created. An example would be a port connected directly to a single workstation. Ports that are designated as edge ports

transition to a forwarding state immediately without going through the listening and learning states. An edge port loses its

status if it receives a BPDU packet, immediately becoming a normal spanning tree port.

P2P Port

A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under RSTP/MSTP, all

ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden through configuration.