與作為 IEEE 802.3 規(guī)范演變的基礎(chǔ)的令牌環(huán)網(wǎng)絡(luò)不同，實(shí)際上禁止將以太網(wǎng)配置為真正的環(huán)。以太網(wǎng)中的任何循環(huán)都將導(dǎo)致在無(wú)限循環(huán)中轉(zhuǎn)發(fā)的數(shù)據(jù)包重復(fù)，從而迅速降低網(wǎng)絡(luò)的帶寬和效率。沒(méi)有專門(mén)處理以太網(wǎng)環(huán)管理的可用通用標(biāo)準(zhǔn)。專有的網(wǎng)絡(luò)市場(chǎng)解決方案提供產(chǎn)品差異化和各種技術(shù)可用于網(wǎng)絡(luò)環(huán)管理的實(shí)施。然而，在尋找基于標(biāo)準(zhǔn)的環(huán)管理解決方案時(shí)，生成樹(shù)或快速生成樹(shù)協(xié)議可能符合要求。

生成樹(shù)協(xié)議 (STP) 在 IEEE 802.1d 中定義，通過(guò)阻止重復(fù)鏈接在任意兩個(gè)網(wǎng)絡(luò)節(jié)點(diǎn)之間形成單個(gè)活動(dòng)路徑，在以太網(wǎng)交換機(jī)的網(wǎng)狀網(wǎng)絡(luò)中創(chuàng)建“生成樹(shù)”。如果任何活動(dòng)鏈路發(fā)生故障，網(wǎng)絡(luò)中的冗余可以提供自動(dòng)備份路徑。

冗余鏈路的這種管理對(duì)于減少工業(yè)網(wǎng)絡(luò)的停機(jī)時(shí)間是非常可取的。例如，圖 2 中所示的網(wǎng)絡(luò)包含鏈路 2、3、4 和 5 之間的環(huán)路。這里 STP 將強(qiáng)制這些鏈路之一進(jìn)入阻塞狀態(tài)以中斷環(huán)路。在這種情況下，鏈路 4 被阻塞，然后可以充當(dāng)備用鏈路。如果任何其他活動(dòng)鏈路 2、3 或 5 隨后發(fā)生故障，協(xié)議將禁用阻塞狀態(tài)以啟用交換機(jī)之間的完全連接。請(qǐng)注意，在此示例中，鏈路 1 沒(méi)有冗余備份。

STP 由網(wǎng)絡(luò)中的每個(gè)交換機(jī)定期交換網(wǎng)絡(luò)狀態(tài)信息，每?jī)擅脒M(jìn)行一次。此信息在稱為網(wǎng)橋協(xié)議數(shù)據(jù)單元 (BPDU) 的特殊數(shù)據(jù)包中承載，由以太網(wǎng)多播地址 01:80:C2:00:00:00 標(biāo)識(shí)。協(xié)議的運(yùn)行分為三個(gè)階段：

選舉根交換機(jī)；

查找到根交換機(jī)的所有路徑并確定“成本低”的路徑；

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Unlike a token-ring network, which has been the basis for much of the IEEE 802.3 specification evolution, it is in fact forbidden to configure Ethernet as a true ring. Any loops within an Ethernet network will result in the duplication of packets that are forwarded in endless loops, quickly degrading the bandwidth and efficiency of a network. There is no available common standard that specifically deals with the management of Ethernet rings. Proprietary network market solutions provide product differentiation and various techniques can be used for the implementation of network ring management. However, when looking for a standards-based solution for ring management, Spanning Tree or Rapid Spanning Tree Protocol may fit the bill.

Spanning Tree Protocol (STP) is defined in IEEE 802.1d and creates a ‘spanning tree’ within a mesh network of Ethernet switches by blocking duplicate links to form a single active path between any two network nodes. The redundancy in the network can provide automatic backup paths if any of the active links fail.

This management of the redundant links is very desirable to reduce the down time industrial networks. For example, the network shown in Fig. 2 contains a loop between links 2, 3, 4 and 5. Here STP will force one of these links into a blocked state to break the loop. In this case link 4 is blocked, which can then act as the backup link. If any of the other active links 2, 3 or 5 subsequently fail, the protocol will disable the blocked state to enable full connectivity between the switches. Note in this example there is no redundancy backup for link 1.

STP is conducted by each switch in the network regularly exchanging information on the status of network, every two seconds. This information is carried in special data packets called Bridge Protocol Data Units (BPDUs), identified by the Ethernet multicast address 01:80:C2:00:00:00. There are three stages of operation of the protocol:

Electing a root switch;

Finding all paths to the root switch and determining the ‘least cost’ paths;