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The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling or disabling of these backup links. Spanning Tree Protocol (STP) was originally standardized as IEEE 802.1D, in 802.1d-1998, but much of the functionality (spanning tree, rapid spanning tree, multiple spanning tree) previously specified in 802.1D, 801.1s, 802.1w has been incorporated into IEEE 802.1Q-2014〔(Bridges and Bridged Networks )〕 As the name suggests, it creates a spanning tree within a network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. STP is based on an algorithm that was invented by Radia Perlman while she was working for Digital Equipment Corporation.〔 〕 ==Protocol operation== A local area network (LAN) can be depicted as a graph whose nodes are bridges and LAN segments (or cables), and whose edges are the interfaces connecting the bridges to the segments. To break loops in the LAN while maintaining access to all LAN segments, the bridges collectively compute a spanning tree. The spanning tree that the bridges compute using the Spanning Tree Protocol can be determined using the following rules. The example network at the right, below, will be used to illustrate the rules. Select a root bridge. The ''root bridge'' of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a configurable priority number and a MAC address; the bridge ID contains both numbers combined together - bridge priority + MAC (32768.0200.0000.1111). The bridge priority default is 32768 and can only be configured in multiples of 4096. When comparing two bridge IDs, the priority portions are compared first and the MAC addresses are compared only if the priorities are equal. The switch with the lowest priority of all the switches will be the root; if there is a tie, then the switch with the lowest priority and lowest MAC address will be the root. For example, if switches A (MAC=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 32768 then switch A will be selected as the root bridge. If the network administrators would like switch B to become the root bridge, they must set its priority to be less than 32768. Determine the least cost paths to the root bridge. The computed spanning tree has the property that messages from any connected device to the root bridge traverse a least cost path, i.e., a path from the device to the root that has minimum cost among all paths from the device to the root. The cost of traversing a path is the sum of the costs of the segments on the path. Different technologies have different default costs. An administrator can configure the cost of traversing a particular network segment. The property that messages always traverse least-cost paths to the root is guaranteed by the following two rules. ''Least cost path from each bridge.'' After the root bridge has been chosen, each bridge determines the cost of each possible path from itself to the root. From these, it picks one with the smallest cost (a least-cost path). The port connecting to that path becomes the ''root port'' (RP) of the bridge. ''Least cost path from each network segment.'' The bridges on a network segment collectively determine which bridge has the least-cost path from the network segment to the root. The port connecting this bridge to the network segment is then the ''designated port'' (DP) for the segment. Disable all other root paths. Any active port that is not a root port or a designated port is a ''blocked port'' (BP). Modifications in case of ties. The above rules over-simplify the situation slightly, because it is possible that there are ties, for example, the root bridge may have two or more ports on the same LAN segment, two or more ports on a single bridge are attached to least-cost paths to the root or two or more bridges on the same network segment have equal least-cost paths to the root. To break such ties: ''Breaking ties for root ports.'' When multiple paths from a bridge are least-cost paths, the chosen path uses the neighbor bridge with the lower bridge ID. The root port is thus the one connecting to the bridge with the lowest bridge ID. For example, in figure 3, if switch 4 was connected to network segment d instead of segment f, there would be two paths of length 2 to the root, one path going through bridge 24 and the other through bridge 92. Because there are two least cost paths, the lower bridge ID (24) would be used as the tie-breaker in choosing which path to use. ''Breaking ties for designated ports.'' When the root bridge has more than one port on a single LAN segment, the bridge ID is effectively tied, as are all root path costs (all equal zero). The designated port then becomes the port on that LAN segment with the lowest port ID. It's put into Forwarding mode while all other ports on the root bridge on that same LAN segment become non-designated ports and are put into blocking mode.〔802.1d-1998 section 8.3.1: The Designated Port for each LAN is the Bridge Port for which the value of the Root Path Cost is the lowest: if two or more Ports have the same value of Root Path Cost, then first the Bridge Identifier of their Bridges, and their Port Identifiers are used as tie breakers.〕 Not all bridge/switch manufacturers follow this rule, instead making all root bridge ports designated ports, and putting them all in forwarding mode. A final tie-breaker is required as noted in the section "The final tie-breaker." When more than one bridge on a segment leads to a least-cost path to the root, the bridge with the lower bridge ID is used to forward messages to the root. The port attaching that bridge to the network segment is the ''designated port'' for the segment. In figure 4, there are two least cost paths from network segment d to the root, one going through bridge 24 and the other through bridge 92. The lower bridge ID is 24, so the tie breaker dictates that the designated port is the port through which network segment d is connected to bridge 24. If bridge IDs were equal, then the bridge with the lowest MAC address would have the designated port. In either case, the loser sets the port as being blocked. ''The final tie-breaker.'' In some cases, there may still be a tie, as when the root bridge has multiple active ports on the same LAN segment (see above, "Breaking ties for designated ports") with equally low root path costs and bridge IDs, or, in other cases, multiple bridges are connected by multiple cables and multiple ports. In each case, a single bridge may have multiple candidates for its root port. In these cases, candidates for the root port have already received BPDUs offering equally-low (i.e. the "best") root path costs and equally-low (i.e. the "best") bridge IDs, and the final tie breaker goes to the port that received the lowest (i.e. the "best") port priority ID, or port ID.〔802.1d-1998 section 8.3.2 b) A Bridge that receives a Configuration BPDU on what it decides is its Root Port conveying better information (i.e. highest priority Root Identifier, lowest Root Path Cost, highest priority transmitting Bridge and Port), passes that information on to all the LANs for which it believes itself to be the Designated Bridge.〕 In summary, the sequence of events to determine the best received BPDU (which is the best path to the root) is * Lowest root bridge ID - Determines the root bridge * Lowest cost to the root bridge - Favors the upstream switch with the least cost to root * Lowest sender bridge ID - Serves as a tie breaker if multiple upstream switches have equal cost to root * Lowest sender port ID - Serves as a tie breaker if a switch has multiple (non-Etherchannel) links to a single upstream switch, where: * *Bridge ID = priority (4 bits) + locally assigned system ID extension (12 bits) + ID (address ) (48 bits); the default bridge priority is 32768, and * *Port ID = priority (4 bits) + ID (Interface number) (12 bits); the default port priority is 128. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spanning Tree Protocol」の詳細全文を読む スポンサード リンク
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