What information can be found in a MAC address table on a switch?

show mac-address
[vlan <vlan-id> ]
[<PORT-LIST> ]
[ mac-addr ]

Table of Contents Show

• Why MAC address is unique for all devices?
• Command to find the MAC address in a device:
•  MAC Address Table:
• Need of MAC Address Table:
• Steps to Check MAC Address Table:
• Frame Forwarding (1.2.1)
• Switching Domains (1.2.2)

Listing all learned MAC addresses on the switch, with the port number on which each MAC address was learned

Listing all learned MAC addresses on one or more ports, with their corresponding port numbers

For example, to list the learned MAC address on ports A1 through A4 and port A6:

Listing all learned MAC addresses on a VLAN, with their port numbers

This command lists the MAC addresses associated with the ports for a given VLAN. For example:

	NOTE: The switches operate with a multiple forwarding database architecture.

Finding the port on which the switch learned a specific MAC address

For example, to find the port on which the switch learns a MAC address of 080009-21ae84:

This feature lets you determine which switch port on a selected VLAN is being used to communicate with a specific device on the network.

From the Main Menu, select:

1. The switch then prompts you to select a VLAN.

2. Use the Space bar to select the VLAN you want, and then press [Enter].

   The switch then displays the MAC address table for that VLAN (Example of the address table.)

   Example of the address table

   To page through the listing, use Next page and Prev page.

This feature uses a device's MAC address that you enter to identify the port used by that device.

1. Proceeding from Example of the address table, press [S] (for Search), to display the following prompt:

   Enter MAC address: _

2. Enter the MAC address you want to locate and press [Enter].

   The address and port number are highlighted if found (Example of menu indicating located MAC address.) If the switch does not find the MAC address on the currently selected VLAN, it leaves the MAC address listing empty.

   Example of menu indicating located MAC address

3. Press [P] (for Prev page) to return to the full address table listing.

This feature displays and searches for MAC addresses on the specified port instead of for all ports on the switch.

1. From the Main Menu, select:

   Listing MAC addresses for a specific port

2. Use the Space bar to select the port you want to list or search for MAC addresses, then press [Enter] to list the MAC addresses detected on that port.

Proceeding from step 2, above:

1. Press [S] (for Search), to display the following prompt:

   Enter MAC address: _

2. Enter the MAC address you want to locate and press [Enter].

   The address is highlighted if found. If the switch does not find the address, it leaves the MAC address listing empty.

3. Press [P] (for Prev page) to return to the previous per-port listing.

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MAC stands for Media Access Control it is a physical address that uniquely identifies a device and hardware manufacturer on a network. Most commonly used in  (IEEE 802) networking technologies, like Ethernet, WiFi, and Bluetooth technology.

A mac address is globally unique so suppose two devices are on the internet they can not have the same MAC address. It works on the Data link layer of the OSI model. It is also known as the Ethernet address are 6 bytes or 28 bits in length typically written in hexadecimal format.

A MAC address is generally assigned to the Network Interface Card of each and every device that is connected to the internet.

Why MAC address is unique for all devices?

If a network has a topology of two or more devices with the same MAC address then the network will not work. This is very crucial when a number of large devices are connected together within a single organization.

Example:

In an Organization, there are 2 devices A and B are connected through a router or a switch. the MAC address of the devices should be  00000ABB98FC, and 00000ABB58FC, respectively.

A MAC address can be expressed in 3 general formats:

• Hyphen-hexadecimal Notation
• Colon-Hexadecimal Notation
• Period-Separated Hexadecimal Notation

Command to find the MAC address in a device:

 MAC Address Table:

The switch maintains an address table called the MAC address table in order to efficiently switch frames between interfaces. So basically a switch stores information about the other (Ethernet interfaces) to which it is connected on a network. when a switch receives a frame, it associates the MAC address of the sending device with the switch port on which it was received.

Need of MAC Address Table:

The MAC address table is a way to map each and every port to a MAC address. The MAC address table consists of two types of entries. 

• Static Entries: Static entries have high priority than dynamic entries and remain active they can be changed or removed by the switch administrator as they are manually added by the switch administrator.
• Dynamic Entries: Dynamic entries are added to the table automatically with a process called MAC learning here the switch fetches the source MAC address of each Ethernet frame received on the port. Dynamic entries are automated they are automatically deleted

Steps to Check MAC Address Table:

• Click Start or click in the search box and type cmd.

• Then click on the command prompt.
• After that type arp -a on prompt.

One of the most exciting functions of networking is the switched environment because businesses are always adding devices to the wired network, and they will do so through a switch. Learning how switches operate is important to someone entering the networking profession.

Frame Forwarding (1.2.1)

On Ethernet networks, frames contain a source MAC address and a destination MAC address. Switches receive a frame from the source device and quickly forward it toward the destination device.

Switching as a General Concept in Networking and Telecommunications (1.2.1.1)

The concept of switching and forwarding frames is universal in networking and telecommunications. Various types of switches are used in LANs, WANs, and the public switched telephone network (PSTN). The fundamental concept of switching refers to a device making a decision based on two criteria:

• Ingress port
• Destination address

The decision on how a switch forwards traffic is made in relation to the flow of that traffic. The term ingress is used to describe a frame entering a device on a specific port. The term egress is used to describe frames leaving the device through a particular port.

When a switch makes a decision, it is based on the ingress port and the destination address of the message.

A LAN switch maintains a table that it uses to determine how to forward traffic through the switch.

In the animated example:

• If a message enters switch port 1 and has a destination address of EA, then the switch forwards the traffic out port 4.
• If a message enters switch port 5 and has a destination address of EE, then the switch forwards the traffic out port 1.
• If a message enters switch port 3 and has a destination address of AB, then the switch forwards the traffic out port 6.

The only intelligence of the LAN switch is its capability to use its table to forward traffic based on the ingress port and the destination address of a message. With a LAN switch, there is only one master switching table that describes a strict association between addresses and ports; therefore, a message with a given destination address always exits the same egress port, regardless of the ingress port it enters.

Cisco LAN switches forward Ethernet frames based on the destination MAC address of the frames.

Dynamically Populating a Switch MAC Address Table (1.2.1.2)

Switches use MAC addresses to direct network communications through the switch to the appropriate outbound port toward the destination. A switch is made up of integrated circuits and accompanying software that controls the data paths through the switch. For a switch to know which port to use to transmit a frame, it must first learn which devices exist on each port. As the switch learns the relationship of ports to devices, it builds a table called a MAC address table, or content addressable memory (CAM) table. CAM is a special type of memory used in high-speed searching applications.

LAN switches determine how to handle incoming data frames by maintaining the MAC address table. A switch builds its MAC address table by recording the MAC address of each device connected to each of its ports. The switch uses the information in the MAC address table to send frames destined for a specific device out the port, which has been assigned to that device.

An easy way to remember how a switch operates is the following saying: A switch learns on “source” and forwards based on “destination.” This means that a switch populates the MAC address table based on source MAC addresses. As frames enter the switch, the switch “learns” the source MAC address of the received frame and adds the MAC address to the MAC address table or refreshes the age timer of an existing MAC address table entry.

To forward the frame, the switch examines the destination MAC address and compares it to addresses found in the MAC address table. If the address is in the table, the frame is forwarded out the port associated with the MAC address in the table. When the destination MAC address is not found in the MAC address table, the switch forwards the frame out of all ports (flooding) except for the ingress port of the frame. In networks with multiple interconnected switches, the MAC address table contains multiple MAC addresses for a single port connected to the other switches.

The following steps describe the process of building the MAC address table:

Step 1. The switch receives a frame from PC 1 on Port 1 (Figure 1-13).

Figure 1-13 Building a MAC Address Table: PC1 Sends Frame to Port 1

Step 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, it associates the source MAC address of PC 1 with the ingress port (Port 1) in the MAC address table (Figure 1-14).

  

  Figure 1-14 Building a MAC Address Table: S1 Adds MAC Address Heard Through Port 1

• If the MAC address table already has an entry for that source address, it resets the aging timer. An entry for a MAC address is typically kept for five minutes.

Step 3. After the switch has recorded the source address information, the switch examines the destination MAC address.

• If the destination address is not in the MAC table or if it’s a broadcast MAC address, as indicated by all Fs, the switch floods the frame to all ports, except the ingress port (Figure 1-15).

  

  Figure 1-15 Building a MAC Address Table: S1 Broadcasts the Frame

Step 4. The destination device (PC 3) replies to the frame with a unicast frame addressed to PC 1 (Figure 1-16).

Figure 1-16 Building a MAC Address Table: PC3 Sends a Reply Frame

Step 5. The switch enters the source MAC address of PC 3 and the port number of the ingress port into the address table. The destination address of the frame and its associated egress port is found in the MAC address table (Figure 1-17).

Figure 1-17 Building a MAC Address Table: S1 Adds the MAC Address for PC3

Step 6. The switch can now forward frames between these source and destination devices without flooding because it has entries in the address table that identify the associated ports (Figure 1-18).

Figure 1-18 Building a MAC Address Table: S1 Sends the Frame to Port 1

Switch Forwarding Methods (1.2.1.3)

Commonly, in earlier networks, as they grew, enterprises began to experience slower network performance. Ethernet bridges (an early version of a switch) were added to networks to limit the size of the collision domains. In the 1990s, advancements in integrated circuit technologies allowed for LAN switches to replace Ethernet bridges. These LAN switches were able to move the Layer 2 forwarding decisions from software to application-specific-integrated circuits (ASICs). ASICs reduce the packet-handling time within the device, and allow the device to handle an increased number of ports without degrading performance. This method of forwarding data frames at Layer 2 was referred to as store-and-forward switching. This term distinguished it from cut-through switching.

As shown in the online video, the store-and-forward method makes a forwarding decision on a frame after it has received the entire frame and then checked the frame for errors.

By contrast, the cut-through switching method, as shown in the online video, begins the forwarding process after the destination MAC address of an incoming frame and the egress port has been determined.

Store-and-Forward Switching (1.2.1.4)

Store-and-forward switching has two primary characteristics that distinguish it from cut-through: error checking and automatic buffering.

Error Checking

A switch using store-and-forward switching performs an error check on an incoming frame. After receiving the entire frame on the ingress port, as shown in Figure 1-19, the switch compares the frame-check-sequence (FCS) value in the last field of the datagram against its own FCS calculations. The FCS is an error checking process that helps to ensure that the frame is free of physical and data-link errors. If the frame is error-free, the switch forwards the frame. Otherwise, the frame is dropped.

Figure 1-19 Store-and-Forward Switching

Automatic Buffering

The ingress port buffering process used by store-and-forward switches provides the flexibility to support any mix of Ethernet speeds. For example, handling an incoming frame traveling into a 100 Mb/s Ethernet port that must be sent out a 1 Gb/s interface would require using the store-and-forward method. With any mismatch in speeds between the ingress and egress ports, the switch stores the entire frame in a buffer, computes the FCS check, forwards the frame to the egress port buffer and then sends the frame.

Store-and-forward switching is Cisco’s primary LAN switching method.

A store-and-forward switch drops frames that do not pass the FCS check, therefore it does not forward invalid frames. By contrast, a cut-through switch may forward invalid frames because no FCS check is performed.

Cut-Through Switching (1.2.1.5)

An advantage to cut-through switching is the capability of the switch to start forwarding a frame earlier than store-and-forward switching. There are two primary characteristics of cut-through switching: rapid frame forwarding and invalid frame processing.

Rapid Frame Forwarding

As indicated in Figure 1-20, a switch using the cut-through method can make a forwarding decision as soon as it has looked up the destination MAC address of the frame in its MAC address table. The switch does not have to wait for the rest of the frame to enter the ingress port before making its forwarding decision.

Figure 1-20 Cut-Through Switching

With today’s MAC controllers and ASICs, a switch using the cut-through method can quickly decide whether it needs to examine a larger portion of a frame’s headers for additional filtering purposes. For example, the switch can analyze past the first 14 bytes (the source MAC address, destination MAC, and the EtherType fields), and examine an additional 40 bytes in order to perform more sophisticated functions relative to IPv4 Layers 3 and 4.

The cut-through switching method does not drop most invalid frames. Frames with errors are forwarded to other segments of the network. If there is a high error rate (invalid frames) in the network, cut-through switching can have a negative impact on bandwidth; thus, clogging up bandwidth with damaged and invalid frames.

Fragment Free

Fragment free switching is a modified form of cut-through switching in which the switch waits for the collision window (64 bytes) to pass before forwarding the frame. This means each frame will be checked into the data field to make sure no fragmentation has occurred. Fragment free mode provides better error checking than cut-through, with practically no increase in latency.

With a lower latency speed advantage of cut-through switching, it is more appropriate for extremely demanding, high-performance computing (HPC) applications that require process-to-process latencies of 10 microseconds or less.

Switching Domains (1.2.2)

Two commonly misunderstood terms used with switching are collision domains and broadcast domains. This section tries to explain these two important concepts that affect LAN performance.

Collision Domains (1.2.2.1)

In hub-based Ethernet segments, network devices compete for the medium, because devices must take turns when transmitting. The network segments that share the same bandwidth between devices are known as collision domains, because when two or more devices within that segment try to communicate at the same time, collisions may occur.

It is possible, however, to use networking devices such as switches, which operate at the data link layer of the OSI model to divide a network into segments and reduce the number of devices that compete for bandwidth. Each port on a switch is a new segment because the devices plugged into the ports do not compete with each other for bandwidth. The result is that each port represents a new collision domain. More bandwidth is available to the devices on a segment, and collisions in one collision domain do not interfere with the other segments. This is also known as microsegmentation.

As shown in the Figure 1-21, each switch port connects to a single PC or server, and each switch port represents a separate collision domain.

Figure 1-21 Collision Domains

Broadcast Domains (1.2.2.2)

Although switches filter most frames based on MAC addresses, they do not filter broadcast frames. For other switches on the LAN to receive broadcast frames, switches must flood these frames out all ports. A collection of interconnected switches forms a single broadcast domain. A network layer device, such as a router, can divide a Layer 2 broadcast domain. Routers are used to segment both collision and broadcast domains.

When a device sends a Layer 2 broadcast, the destination MAC address in the frame is set to all binary ones. A frame with a destination MAC address of all binary ones is received by all devices in the broadcast domain.

The Layer 2 broadcast domain is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host.

When a switch receives a broadcast frame, the switch forwards the frame out each of the switch ports, except the ingress port where the broadcast frame was received. Each device connected to the switch receives a copy of the broadcast frame and processes it, as shown in the top broadcast domain in Figure 1-22. Broadcasts are sometimes necessary for initially locating other devices and network services, but they also reduce network efficiency. Network bandwidth is used to propagate the broadcast traffic. Too many broadcasts and a heavy traffic load on a network can result in congestion: a slowdown in the network performance.

Figure 1-22 Broadcast Domains

When two switches are connected together, the broadcast domain is increased, as seen in the second (bottom) broadcast domain shown in Figure 1-22. In this case, a broadcast frame is forwarded to all connected ports on switch S1. Switch S1 is connected to switch S2. The frame is then also propagated to all devices connected to switch S2.

Alleviating Network Congestion (1.2.2.3)

LAN switches have special characteristics that make them effective at alleviating network congestion. First, they allow the segmentation of a LAN into separate collision domains. Each port of the switch represents a separate collision domain and provides the full bandwidth to the device or devices that are connected to that port. Second, they provide full-duplex communication between devices. A full-duplex connection can carry transmitted and received signals at the same time. Full-duplex connections have dramatically increased LAN network performance and are required for 1 Gb/s Ethernet speeds and higher.

Switches interconnect LAN segments (collision domains), use a table of MAC addresses to determine the segment to which the frame is to be sent, and can lessen or eliminate collisions entirely. Table 1-2 shows some important characteristics of switches that contribute to alleviating network congestion.

Table 1-2 Switch Characteristics That Help with Congestion

Page 2

We have seen that the trend in networks is toward convergence using a single set of wires and devices to handle voice, video, and data transmission. In addition, there has been a dramatic shift in the way businesses operate. No longer are employees constrained to physical offices or by geographic boundaries. Resources must now be seamlessly available anytime and anywhere. The Cisco Borderless Network architecture enables different elements, from access switches to wireless access points, to work together and allow users to access resources from any place at any time.

The traditional three-layer hierarchical design model divides the network into core, distribution, and access layers, and allows each portion of the network to be optimized for specific functionality. It provides modularity, resiliency, and flexibility, which provides a foundation that allows network designers to overlay security, mobility, and unified communication features. In some networks, having a separate core and distribution layer is not required. In these networks, the functionality of the core layer and the distribution layer are often collapsed together.

Cisco LAN switches use ASICs to forward frames based on the destination MAC address. Before this can be accomplished, the switch must first use the source MAC address of incoming frames to build a MAC address table in content-addressable memory (CAM). If the destination MAC address is contained in this table, the frame is forwarded only to the specific destination port. In cases where the destination MAC address is not found in the MAC address table, the frames are flooded out all ports except the one on which the frame was received.

Switches use either store-and-forward or cut-through switching. Store-and-forward reads the entire frame into a buffer and checks the CRC before forwarding the frame. Cut-through switching only reads the first portion of the frame and starts forwarding it as soon as the destination address is read. Although this is extremely fast, no error checking is done on the frame before forwarding.

Every port on a switch forms a separate collision domain allowing for extremely high-speed full-duplex communication. Switch ports do not block broadcasts, and connecting switches together can extend the size of the broadcast domain often resulting in degraded network performance.

Page 3

The following activities provide practice with the topics introduced in this chapter. The Class Activities are available in the companion Routing and Switching Essential Lab Manual (978-1-58713-320-6). The Packet Tracer Activities PKA files are found in the online course.

8. Class Activities | Next Section Previous Section