Forwarding of common L2 Switches

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Filtering: whether the switch determines whether a frame should be forwarded to an interface or whether it should be discarded. For example, port + Mac binding is used as a filter condition. Forwarding: determines the interface to which a frame is redirected and moves the frame interface to the switch function of these interfaces. Network devices use MAC addresses to uniquely identify themselves, and the switch must know which port is connected to which device for communication between devices, therefore, a table with one-to-one MAC address and port number is required to implement layer-2 data forwarding within the switch. This is the layer-2 forwarding table, that is, the switch table. Vswitch filtering and forwarding are completed by means of the switch table.

The vswitch table contains some nodes on a LAN but does not have to be a table item for all nodes. A table item in the vswitch table includes: 1. the MAC address of the node; 2. the vswitch interface that arrives at the node; 3. The time when the table item for the node is placed in the table;
To understand the process of filtering and forwarding a vswitch, assuming that the frame with the destination address DD-DD-DD-DD-DD-DD arrives from the port x of the vswitch interface, the vswitch indexes its table with the MAC address DD-DD-DD-DD-DD-DD. There are three possible cases:

1. There are no table items for the DD-DD-DD-DD-DD-DD in the table. In this case, the switch forwards the copy of the frame to the output cache before all interfaces except interface X. That is, if there is no table entry for the destination address, the switch broadcasts the frame.

2. There is a table item in the table that associates the DD-DD-DD-DD-DD-DD with interface X. In this case, the frame comes from the lan cidr block that includes the adapter DD-DD-DD-DD-DD-DD. You do not need to forward the frame to any other interface. The switch can filter the frame by dropping it.

3. There is a table item in the table that associates the DD-DD-DD-DD-DD-DD with interface y not equal to X. In this case, the frame needs to be forwarded to the LAN network segment connected to interface y. The switch places the frame in the output cache area before interface y to complete forwarding.