[09-02-06]catelyst 交换机STP中的 backbonefast 特性
看了很久的backbonefast特性的中文资料，越看看糊涂，越看越看不明白，一火之下真接找英文资料看了下，才发现其实这个特性也是挺简单的。不由得又感慨一句，中译文害人呀！！俺就把这段跟据咱自己的理解，重译一下，方便大伙参考，首先申明，俺英文水平有限，有不对的地方，欢迎指教！BackboneFast is a Catalyst feature that is initiated when a Root Port or blocked port on a switch receives inferior BPDUs from its Designated Bridge. An inferior BPDU identifies one switch as both the Root Bridge and the Designated Bridge. When a switch receives an inferior BPDU, it means that a link to which the switch is not directly connected (an indirect link) has failed. That is, the Designated Bridge has lost its connection to the Root Bridge. Under STP rules, the switch ignores inferior BPDUs for the configured Max Age (the default is 20 seconds).
The role of BackboneFast is essentially to cheat this 20-second delay. When the switch receives the inferior BPDU, the switch tries to determine if it has an alternative path to the Root Bridge.
backbonefast是catalyst交换机根端口或阻塞端口在接收到一个有问题的BPDU的时候才启动的一个特性，之所以说他这个BPDU有问题，是因为在上次STP计算过程中它已经是指定网桥了，现在它竟然还告诉我说他是根桥，这明显有问题，20秒都还没有过呢？当我是傻的呀！！既然我不傻，那就说明它出问题了，问题原因可能是它的根路径发生中断了，因为没有跟我直连接，所以我不知道。跟据STP规则，就算它丢了根路径，我也没办法去救它，因为我必须等到我这个接口上保存的最佳BPDU老化了才行，这个时间是20秒。但是我是谁？我是cisco公司生出来的儿子，熟话说，虎父无犬子，更何况，它是我的兄弟，所以我必需要想办法来规避这个20秒的等待时间。这就是我所具有的backbonefast特性，跟据这个特性，我将找出一条到达根桥的备选路径出来，只要我能找到这条到达根的备选路径，那就不用管什么20秒的等待，我可以真接老化，一秒都不用等。
There are two cases to consider:
重新找一条到根的路径，这不难。最大的问题在于，我自己必须要有去往根的备选路径才行。那么什么情况，我有备选路径呢？其实主要有两情况，如下：
*First, if the inferior BPDU arrives on a blocked port, the Root Port and other blocked ports on the switch become alternative paths to the Root Bridge.
第一，如果这个有问题的BPDU到达了我的阻塞端口，那么我的根端口和其他的阻塞端口将变成到达根的备选路径。
**Second, if the inferior BPDU arrives on the Root Port, all blocked ports become potential alternative paths to the Root Bridge.
第二，如果这个有问题的BPDU到达了我的根端口，那么我的所有阻塞端口都将变成潜在的到达根的备选路径。
If the inferior BPDU arrives on the Root Port and there are no blocked ports the switch assumes that it has lost connectivity to the Root Bridge causing the Max Age on the root to expire, and becomes the Root Bridge according to normal STP rules. If the switch has alternative paths to the Root Bridge it uses these alternative paths to transmit a new kind of Protocol Data Unit (PDU) called the Root Link Query PDU. The switch sends the Root Link Query PDU on all potential alternative paths to the Root Bridge.
如果这个有问题的BPDU到达我的根端口，并且我这没任何阻塞端口，那么根据上面讲的第二种情况，就是说我根本就没有了到达根的备选路径，这说明我到根也丢了，那这下好了，还帮什么帮，再跟据STP规则重新计算STP拓朴得了。再假如我有备选路径，无论是第一种情况，还是第二种情况，只要我有，那么我就会在这些备选路径上发送一种新的协议数据报文（PDU）叫做根连路查寻PDU，就是说我会把这个报文从所有的潜在备选路径上发到根桥去。
If the switch determines that it still has an alternative path to the root, it causes the Max Age on the ports on which it received the inferior BPDU to expire. The switch then makes all ports on which it received an inferior BPDU its Designated Ports and moves them out of the Blocking state (if they were in the Blocking state), through the Listening and Learning states, and into the Forwarding state. On the other hand, if the switch learns via the Root Link Query process that all the alternative paths to the Root Bridge have lost connectivity to the root, the switch causes the Max Age on the ports on which it received inferior BPDUs to expire, and a new STP topology is calculated.
假如我确认了还有可以到达根的备选路径径，那么我就真接把接到问题BPDU的端口老化掉，让我所有接到这个问题BPDU的端口全部进入指定端口角色，同时改变阻塞状态（如果原来是阻塞的话），时入侦听和学习状态，并最终进入转发状态。另一种情况是，通过RLQPDU了解到所有备选中径都丢失了到达根的连接，那么我也会把接到问题BPDU的端口老化，并重新计算STP拓朴。
网赚之路 zyl68一个直角三形的高是3.5米，它的一个角的度数是45度，一个平形四边形的
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