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In the case when the output buffer becomes empty before the whole packet has arrived to the switch, this gathering generates a store-and-forward delay at the switch, a delay which contributes to the total end-to-end delay. When a packet is forwarded through a store-and-forward packet switch, the packet is first gathered and stored in its entirety before the switch begins to transmit it on the outbound link. The two switching techniques only differ when the output buffer is empty. When there are packets in the output buffer, there is absolutely no difference between store-and-forward and cut-through switching. When the packet arrives, there may or may not be other packets in the outbound link’s output buffer.
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The packet arrives to the switch on a inbound link and leaves the switch on a outbound link. To understand this difference consider a packet that is being forwarded through a packet switch (i.e., a router, a bridge, or an Ethernet switch). The difference between store-and-forward and cut-through switching is subtle. In addition to large numbers of interfaces, support for multitudes of physical media types and transmission rates, and enticing network management features, Ethernet switch manufacturers often tout that their switches use cut-through switching rather than store-and-forward packet switching, used by routers and bridges.
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Filter If the destination MAC address comes from the same port on which it was received, (in another words, source mac address and destination mac address have the same exit port) then there is no need to forward it, and it is discarded.This action is known as unicast flooding. Flood If the destination MAC address is not in the MAC address table, then the frame needs to be flooded and is sent to all ports except for the port through which it arrived.Forward If the destination MAC address comes from another port within the switch, then the frame is sent to the identified port for transmission.Then the MAC address table is searched using the destination MAC address to determine which action to take. If the MAC address was already in the table, its associated aging countdown timer is reset (300 seconds by default). When a frame is received on a port, the source MAC address is placed in the MAC address table, along with the port ID of the port on which it was received. When a switch is first powered on, the MAC table is empty. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy. Loop avoidance If multiple connections between switches are created for redundancy purposes, network loops can occur. The frame is only forwarded out the specified destination port. There are three distinct functions of layer 2 switching: address learning, forward/filter decisions, and loop avoidance.Īddress learning Layer 2 switches and bridges remember the source MAC address of each frame received on an interface, and enter this information into a MAC table called a forward/filter table.įorward/filter decisions When a frame is received on an interface, the switch looks at the destination MAC address and finds the exit interface in the MAC table.