RSTP详解：快速生成树协议在思科交换机中的应用

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"思科RSTP(快速生成树协议)技术文档" Rapid Spanning Tree Protocol (RSTP)，即快速生成树协议，是IEEE 802.1w标准定义的一种增强型STP（Spanning Tree Protocol）协议。RSTP的主要目标是提高网络的收敛速度，从而在出现链路故障时能更快地恢复网络的连通性，减少服务中断时间。思科公司在其Catalyst系列交换机中广泛支持RSTP，以提供更高效、稳定的网络环境。新引入的端口状态和端口角色是RSTP的核心特性之一。端口状态包括Discarding（丢弃）、Learning（学习）和Forwarding（转发），而端口角色则有Root Port（根端口）、Designated Port（指定端口）和Backup Port（备份端口）以及Alternate Port（预备端口）。这些新的状态和角色使得RSTP能够更快地判断并调整网络拓扑。 RSTP的新BPDU格式包含了更多的信息，如端口优先级和路径成本，以支持更智能的决策。新BPDU处理规则规定，BPDU每Hello-Time间隔就会发送一次，这比STP的发送频率更高，有助于更快检测到拓扑变化。此外，RSTP还引入了更快的信息老化机制，以便在网络拓扑发生变化时迅速更新状态。为了快速过渡到转发状态，RSTP引入了Proposal/Agreement序列。当一个端口想要成为根端口或指定端口时，它会发送Proposal BPDU，如果上游端口同意，就会回应Agreement BPDU，这样端口就可以直接进入转发状态，而无需经过学习状态，显著减少了收敛时间。边缘端口（Edge Ports）是RSTP的另一项重要特性，它们被用于终端设备连接，可以立即进入转发状态，无需参与生成树计算，进一步提高了效率。同时，RSTP还提供了与802.1D STP的兼容性，确保在混合环境中能够稳定工作。 RSTP改进了拓扑变更的检测和传播机制。当网络中发生拓扑变化时，通过新的Topol

Disabled Discarding No No

Blocking Discarding No No

Listening Discarding Yes No

Learning Learning Yes Yes

Forwarding Forwarding Yes Yes

Port Roles

The role is now a variable assigned to a given port. The root port and designated port roles remain, while

the blocking port role is split into the backup and alternate port roles. The Spanning Tree Algorithm

(STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). In order to simplify

matters, the thing to remember about a BPDU is there is always a method to compare any two of them

and decide whether one is more useful than the other. This is based on the value stored in the BPDU and

occasionally on the port on which they are received. This considered, the information in this section

explains very practical approaches to port roles.

Root Port Roles

The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest

to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged

network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other

bridge sends. The root bridge is the only bridge in the network that does not have a root port. All

other bridges receive BPDUs on at least one port.

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Designated Port Role

A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D

bridges link together different segments, such as Ethernet segments, to create a bridged domain.

On a given segment, there can only be one path toward the root bridge. If there are two, there is a

bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of

each and agree on the bridge that sends the best BPDU as the designated bridge for the segment.

The port on that bridge that corresponds is designated.

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Cisco - Understanding Rapid Spanning Tree Protocol (802.1w)

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