一直以来在一个典型的交换三角网络内,存在这样的一个问题:一台没有接入任何一台服务器二层交换机的两个uplink口的RX方向上竟然存在很大的流量。通过在此交换机上抓这两个uplink口的数据包,发现存在很多unicast的包。而从三层交换机上面通过查找arp表和mac表后证明这些数据包不应该会下发到此交换机上。此问题查找了很久都没有找到原因。今天在某朋友的提示下,查看了block-unicast的命令解释。
Understanding How Unicast Flood Blocking Works
You can enable unicast flood blocking on any Ethernet port on a per-port basis. Unicast flood blocking provides you the option to drop the unicast flood packets on an Ethernet port that has only one host that is connected to the port. All the Ethernet ports on a switch are configured to allow unicast flooding; unicast flood blocking allows you to drop the unicast flood packets before they reach the port. Caution
You must have a static CAM entry that is associated with the Ethernet port before you enable unicast flood blocking. If you do not have a static CAM entry that is associated with the port, you will lose network connectivity if you enable unicast flood blocking. You can verify that a static CAM entry exists by entering the show cam static command.
---摘自 http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/8.x/configuration/guide/uniflood.html