What is unicast flooding?

[watermelonyu]

unicast flooding 是什麼? 何時會產生 unicast flooding?

[watermelonyu]

Hi :

LAN switches use forwarding tables (Layer 2 (L2) tables, Content Addressable Memory (CAM) tables) to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. When there is no entry corresponding to the frame's destination MAC address in the incoming VLAN, the (unicast) frame will be sent to all forwarding ports within the respective VLAN, which causes flooding.



For more detail, please reference



http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml



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[watermelonyu]

Sorry, 有點看不太懂...



When there is no entry corresponding to the frame's destination MAC address in the incoming VLAN, the (unicast) frame will be sent to all forwarding ports within the respective VLAN, which causes flooding.



這意思是說, 當目的端 MAC 不在 switch 的 MAC address table 中時, 就會把這個 frame 傳到 VLAN 中的每個 port..... 有點不懂, 請問當destination MAC address 不在 switch 的 MAC address table 中時, 不是應該先用 broadcast 方式找到 destination MAC address 嗎?

[watermelonyu]

> 不是應該先用 broadcast 方式找到 destination MAC address 嗎?

No, this is in Link layer.

I wonder you are mix up ARP/RARP in IP IP layer.

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[watermelonyu]

"當destination MAC address 不在 switch 的 MAC address table 中時, 不是應該先用 broadcast 方式找到 destination MAC address 嗎? "

你說的沒錯會用broadcast去找,可是就是找不到對應的MAC阿,也不可能直接丟掉阿,所以直接廣播給所有人,有用的人就會收去用,不用的人就需要drop