CCNA2 capitulo 1.

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If a message enters the switch port 1 and the destination address is EA, the switch forwards traffic for port 4.

If a message enters the port 5 switch and the destination address is EE, the switch forwards traffic on port 1.

If a message enters the port 3 of the switch and the destination address is AB, the switch forwards traffic by port 6.

The only intelligence possessed by LAN switches is the ability to use the table to forward traffic based on the input port and the destination address of a message. With LAN switches, there is only one main switching table describing a strict association between addresses and ports; Therefore, a message with a particular destination address always come out the same output port, regardless of the input port by entering.

Cisco LAN switches Ethernet frames forwarded by the destination MAC address of frames.

Completed the dynamic MAC address table of a switch

The switches use MAC addresses to direct network communications through the switch to the appropriate port to the destination. A switch consists of integrated circuits and the supplemental software that controls the data paths through the switch. To determine which port to use to transmit a frame, the switch must first know what devices exist in each port. As the switch discovers the relationship between ports and devices, create a table called "MAC address table" or "table of content addressable memory" (CAM). CAM is a special type of memory that is used in applications of high-speed search.

LAN switches determine how to handle incoming data frames by MAC address table. The switch generates the MAC address table by recording the MAC address of each device connected to each port. The switch uses the information in the MAC address table to send frames destined to a specific device for the port that is assigned to that device.

The complete switch MAC address table according to the source MAC addresses. When the switch receives an incoming frame with a destination MAC address that is not in the MAC address table, it forwards the frame out all ports (saturation), except the input port of the plot. When the destination device responds, the switch adds the source MAC address of the frame and the port through which the frame was received to the MAC address table. In networks with multiple interconnected switches, the MAC address table contains multiple MAC addresses for a single port connected to other switches.

The following steps describe the process of creating a MAC address table:

1. The switch receives a frame from PC 1 port 1.

2. The switch examines the source MAC address and compares it to the MAC address table.

If the address is not in the MAC address table, the switch associates the source MAC address of the PC 1 to input port (port 1) in the MAC address table.

If the MAC address table already contains an entry for the source address, resets the timer expiration. Usually, tickets to the MAC addresses are stored for five minutes.

3. Once the information recorded switch the source address, examines the destination MAC address.

If the destination address is not in the MAC table or if it is a MAC broadcast address, all letters F indicated by the switch all ports saturate the plot, except the input port.

4. The target device (PC 3) meets the frame with a unicast frame addressed to the PC one.

5. The switch incorporates the source MAC address of the PC 3 and the input port number to the address table. In the MAC address table, they are the destination address of the frame and the associated output port.

6. Now the switch can forward these frames between source and destination devices without saturation, as it has entries in the address table that identify the associated ports.

Store and forward switching

The store and forward switching by two main features that differentiate it from cutting method: error checking and automatic storage buffer.

Error Checking

The switches by using store and forward switching perform error checking incoming frames. After receiving the complete frame at the port of entry, as shown in the illustration, the switch compares the value of frame check sequence (FCS) in the last field of the datagram with their own calculations of FCS. FCS is a process of error checking to help ensure that the frame does not contain physical and data link errors. If the frame has no errors, the switch forwards. Otherwise, it is the rule.

Automatic storage buffer

The buffering process input port using the store and forward switches provides the flexibility to support any mix of Ethernet speeds. For example, handling an incoming frame that moves to an Ethernet port 100Mb / s to be sent by an interface 1 Gb / s requires the use of the method of storage and shipping. For any incompatibility of the speeds of the input and output ports, the switch stores the entire frame in a buffer, calculates the FCS check, forwards it to buffer the output port and then sends it.

The switching for storage and shipping is the primary method of Cisco LAN switching.

The store and forward switches discarded frames that fail the FCS check and, therefore, do not forward invalid frames. Conversely, the switches using the cutting method can forward invalid frames, and not performing the verification FCS.

Cut-through switching

An advantage of cut-through switching is that the switch has the ability to initiate the forwarding of a frame before the switching for storage and shipping. The cut-through switching has two main characteristics: rapid frame forwarding and processing of invalid frames.

Fast forwarding frames

As shown in the figure, the switches using the cutting method can make a decision as soon as forwarding the MAC destination address of the frame in the MAC address table. The switch does not have to wait for the rest of the frame entering the port of entry before making a forwarding decision.

With the MAC ASIC and current controllers, switches using the cutting method can quickly decide if they need