The switch sends a copy of the frame out all interfaces, except the interface on which the frame was received, identically to unknown unicasts. The negative impact would be more significant in a larger switched network with several switches and possibly hundreds of host. It is not difficult to understand that a lot of bandwidth is wasted here because the frame is sent to every host on the switched network. Only host A whose MAC address matches the destination MAC address of the frame would accept that frame while all other devices would reject it. Pay careful attention to the direction of arrows and note that the frame sent by host A is received by all hosts in the switched network including hosts connected to other switches. Host A yet again sends a frame destined to host B but this time intermediate switches do not yet have an entry for the MAC address of host B in their MAC address tables. The idea here is that the frame would ultimately reach all hosts and the host having the same MAC address as the destination address of the frame would accept it while all other hosts would reject the frame.įigure 7-2 describes how unknown unicasts are propagated in a switched network. The switch sends a copy of the frame out all interfaces, other than the interface on which the frame was received. The switch has no entry in its MAC address table for the destination MAC address in the frame. Host A sends a frame destined to host B which is forwarded by the intermediate switches to its final destination following the direction of the arrows. The switch forwards frame out the single interface associated with the destination MAC address in the frame.įigure 7-1 describes how known unicasts are propagated in a switched network. The switch already has an entry in its MAC address table for the destination MAC address in the frame so it knows exactly which interface leads to the destination of the frame. The forwarding logic differs based on the type of destination MAC address and on whether the destination address has been added to the MAC address table of the switch. The basic logic used by switches when forwarding frames is the key to understanding many enhanced switching concepts and is worth a quick review here. In the second case, multiple devices may be directly connected to the same switch port through a hub. In one case, the switch port may be connected to another switch which in turn has multiple devices connected to it.
There are two possible scenarios in which multiple MAC addresses may be associated with the same switch port. However, there may be more than one MAC addresses associated with the same switch port which means multiple devices are reachable off the same switch port. Each dynamically learned MAC address is associated with one and only one switch port. In the above output, you should be able to identify various dynamically learned MAC addresses and the switch ports those MAC addresses are associated with. Total Mac Addresses for this criterion: 27
The command show mac address-table, not surprisingly, is used to display the MAC address table of a switch we just talked about, as shown below:
#Cisco mac address table command windows
In this way, a switch dynamically builds an address table by using the source MAC address of the frames received.Īs a practice, try issuing command ipconfig /all on Windows CLI and show mac address-table on Cisco switch CLI to get warmed up for later examples. When the switch receives a frame, it associates the MAC address of the sending device with the switch port on which it was received. The switch maintains an address table called MAC address table in order to efficiently switch frames between interfaces. The term packet is used to describe the layer 3 header and data without the layer 2 header or trailer. The term frame refers to the bits and bytes that include the layer 2 header and trailer along with the data encapsulated by the header and trailer. Let’s make a distinction between frame and packet before moving forward as these terms are often used rather loosely. They are merely different representations of the same MAC address. A Microsoft Windows system would list a MAC address as 12-34-56-78-9A-BC whereas a Cisco switch would list it as 1234.5678.9abc.
Ethernet addresses, also known as MAC (Media Access Control) addresses, are 6 bytes or 48 bits in length, typically written in hexadecimal form. The ultimate goal of switches is to carry frames from the source to the appropriate destination based on the destination Ethernet address in the frame header.
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