CRC错误时的交换表 [英] Swich table in case of CRC error

问题描述

我对更新交换机表的基本概念如何工作感到非常困惑。
如果CRC错误，它是否仍会更新SMAC以便下次使用？也许是因为可能存在错误（SMAC），所以交换机使用消息丢弃了SMAC？

I'm really confused about how the basic idea of updating the switch table works. In the case that the CRC is wrong it still updates the SMAC to use it next time maybe? Maybe because the error could be there (SMAC) the switch drops the SMAC with the message?

另一件事是首先发生的事情，更新表或首先发送消息

Another thing is what happened first, update the table or first send the message in case of store&forward mode?

推荐答案

首先看这张图片

该场景是PC1 10.0.0.2 尝试Ping PC3 10.0.0.4 ：

The Scenario is PC1 10.0.0.2 try to Ping PC3 10.0.0.4 :

1. PC1发送ARP消息（BroadCast）消息，并且该交换机是一个广播域，这意味着它将接收广播消息，然后将所有消息发送到与它发送消息的所有接口上与其连接的任何设备。 PC2然后PC3正常，然后询问谁是 10.0.0.4 ，然后 PC3 会回答，然后将答案再次发送到交换机并说多数民众赞成我这是怎么发生的？

1. PC1 Send ARP Message (BroadCast) Message and the switch is one broadcast domain it mean it will receive broadcast msg then send all messages to any device connected to it on all interfaces it will send message to PC2 then PC3 ok then ask who is 10.0.0.4 then the PC3 will answer then send the answer again to the switch and say thats me how this happen ?

注意：您可以在Wireshark上看到这些消息

Note: You Can See These Messages on Wireshark

ARP消息（广播）包含： SIP（​​源IP） | DIP（目标IP） | SMAC（源Mac） | FF：FF：FF：FF：FF：FF（目标MAC〜> BroadCast消息）。

ARP Message (Broadcast) contains : SIP (Source IP) | DIP (Destination IP) | SMAC(Source Mac) | FF:FF:FF:FF:FF:FF (Destination MAC ~> BroadCast Message) .

确定PC3如何回答？

ok how the PC3 Answer ?

ARP协议会回答然后发送 SMAC（11:11： 11：11：11：11） | DMAC（33：33：33：33：33：33）

the ARP Protocol will Answer then send SMAC (11:11:11:11:11:11) | DMAC (33:33:33:33:33:33)

所以现在 Switch 会将它们都保存在 CAM TABLE 中，如图所示：

so now the Switch will Save both of them in CAM TABLE Like this picture :

确定如何发送的帧..它基于帧检查序列（FCS），它是通信协议中的额外检测代码。帧用于发送上限-层数据，最终是来自源到目的地的应用程序数据，但检测并不意味着错误恢复只是定义错误的帧然后将其删除，因为以太网不采取任何措施来重新传输，因此 FSC 字段包含一个数字，该数字由源节点根据帧中的数据计算得出，该数字加到目标节点收到FCS帧时发送的帧末尾重新计算数字并与之比较如果帧中包含的FCS编号不同（如果两个数字不同），则会发生错误，并且该帧将被丢弃，发送主机将在整个帧上计算 CRC 并附加此预告片像将其标记为数据，然后接收主机重新计算帧上的CRC，然后将其与接收到的FCS进行比较，在这种情况下，它可以检测到任何丢失或更改的数据，因此无需更新 CAM TABLE ，因为它不接受任何损坏的帧，您可以通过 clear mac address清除或刷新 CAM TABLE 动态或静态表，请详细了解FCS和CRC以及帧发送消息的方式 https://en.wikipedia.org/wiki/Frame_check_sequence

ok how the frame sent .. it based on the Frame Check Sequence (FCS) its an extra detecting code in a communications protocol Frames are used to send upper-layer data and ultimately the application data from a source to a destination but the detection doen't mean error recovery just define the errored frame then discared it because ethernet takes no action to be retransmitted so FSC field contains a number that calculated by the source node based on the data in the frame this number added to the end of a frame that is sent when the destination node receives the frame the FCS number is recalculated and compared with the FCS number which included in the frame if the two numbers are different an error occurred and the frame will be discard with sending host computing the CRC on the entire frame and append this trailer like tagging it to the data then the receiving host recomputes the CRC on the frame then compares it to the received FCS in this case it can detect any data lost or altered in transit so it will not be necessary to update the CAM TABLE because it will not accept any frame that corrupted you can clear or flush CAM TABLE via clear mac address-table dynamic or static please read more about FCS and CRC and how the frame sending the message https://en.wikipedia.org/wiki/Frame_check_sequence

已更新

如果广播未回复，交换机将保存SMAC吗？

what if the brodcast not reply ?the switch save the SMAC?

让我向您展示一个实际示例
首先显示 CAM TABLE 空

let me show you a practical example first go and show the CAM TABLE its empty

好吧，如果 PC1 尝试ping不存在的IP地址，例如 10.0.0.5

ok what if PC1 try to ping non-existent ip address for example 10.0.0.5

确定，现在再次检查 CAM TABLE

ok Check Now the CAM TABLE again

仅存储 SMAC

the switch storing just the SMAC

确定，再次尝试ping pc3 现在将重播

Ok Again go and ping pc3 now it will replay

现在检查 CAM TABLE 再次

now check the CAM TABLE again

好吧，如果要刷新 CAM TABLE ，请尝试 clear mac-address-table dynamic 或静态

ok what if you want to flush your CAM TABLE try clear mac-address-table dynamic or static

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