Working of a Switch explained practically

Now let us add one more switch to our topology and arrange two devices for each of them. Let’s observe how the switches are making entries in their CAM/MAC table for the devices connected to them.

Here we go:

 I just added Switch1 to our previous topology and connected both the switches (Switch0 & Switch1) with their Fa0/5 ports. And the devices are connected to the ports as shown in the pic.

Now let’s examine the MAC tables of both the switches:

On Switch0: “show mac-address-table”

On Switch1: “show mac-address-table”

The MAC tables of both switches have only one entry because none of the devices have communicated from the past 300 seconds.

Now the question here is whose mac is that, on both the switches?

            It is the MAC address of the Fa0/5 interfaces of each of them. Since Switch0 is connected to the Fa0/5 port of Switch1, Switch0 will have entry for Fa0/5 port of Switch1.

            Similarly since Switch1 is connected to the Fa0/5 port of Switch0, Switch1 will have entry for Fa0/5 port of Switch0.

Let’s  check it out practically:

            We will see the MAC address of Fa0/5 port of both the switches.

On Switch0: “show interfaces fastethernet 0/5”

The output is truncated. From the output we can see that the hardware/MAC address of the Fa0/5 port on Switch0 is: 0060.4731.5B05.

On Switch1: “show interfaces fastethernet 0/5”

The output shows that the hardware/MAC address of the Fa0/5 port on Switch1 is: 0060.5C4E.9605.

So it is clear now that since Switch1 through its Fa0/5 port is connected to Switch0. Switch0 is showing the MAC: 0060.54CE.9605 in its MAC table. 
The same thing with Switch1: Since Switch0 through its Fa0/5 port is connected to Switch1. Switch1 is showing the MAC: 0060.4731.5B05 in its MAC table.

Here you may get a doubt: Presently there is no communication between both the switches, then why they are having the MAC entries of the ports joining them ?

            The reason is, though the devices are not communicating to each other from our perspective. There is a protocol in Cisco called as “Cisco Discovery Protocol” which discovers the directly connected cisco devices. The CDP protocol will send the CDP messages for every 180 second’s.

            So here both the Switches are exchanging CDP messages through their Fa0/5 ports, that is the reason for having the MAC entries of the Fa0/5 ports of each other in their CAM tables respectively. (We will discuss more on CDP later).

Now let’s go back to our topology to understand practically more about the Switch functions:

 

For our convenience I labeled the ports joining both the switches with the MAC addresses.

Now let’s ping PC3 (192.168.1.3) on Switch1 from PC1 (192.168.1.1) on Switch0:

           

            Let us examine how both switches make entries for both the devices (PC1 & PC3) in their MAC tables.

The ping was successful. Now let’s check out the CAM table of both the Switches. By this time we should be able to say which MAC will be mapped to which port on both the switches. (Note down your answers on a paper and check them out)

On Switch0: “show mac-address-table”

Here two new entries have found. One is: 0060.70A7.005D (PC3) & other is: 0090.0C1C.4B41 (PC1).

            The MAC: 0090.0C1C.4B41 is associated to Fa0/1 port because it is of the PC1 which is connected to Fa0/1 port of Switch0.

            The MAC: 0060.70A7.005D is of the PC3 and is associated/mapped to Fa0/5 port because PC3 is located on Switch1 and from Switch0 the only way to Switch1 is through its Fa0/5 port, so Switch0 associated/mapped the MAC address of PC3 to its Fa0/5 port.

On Switch1: “show mac-address-table”

Coming to Switch1, here also there are two new entries. One is: 0060.70A7.005D (PC3) & other is: 0090.0C1C.4B41 (PC1).

            The MAC: 0060.70A7.005D is associated to Fa0/3 port because it is of the PC3 which is connected to Fa0/3 port of Switch1.

            The MAC: 0090.0C1C.4B41 is of the PC1 and is associated to Fa0/5 port because PC1 is located on Switch0 and from Switch1 the only way to Switch0 is through its Fa0/5 port, so Switch1 associated/mapped the MAC address of PC3 to its Fa0/5 port.

Let’s dig more into the above explanation using frames generated, step by step:

            When I try to ping 192.168.1.3 (PC3) from (PC1) 192.168.1.1:

1. The first frame generated will by by PC1 towards Switch0 on its Fa0/1 port.

 2. Now the switch0 will receive this frame on its Fa0/1 port and will perform its learning function.

Here is the PDU information from Switch0 for the first frame it received on its Fa0/1 port.

The above figure shows that a frame has been received on Fa0/1 port from the device: 0090.0C1C.4B41 (PC1) and is destined for the device with MAC: 0060.70A7.005D (PC3).

Let’s see whether the switch has performed its learning function or not.

On Switch0: “show mac-address-table”

       
From the above output we can confirm that the Swtich0 has  performed its learning function. It mapped/associated the PC1 MAC: 0090.0C1C.4B41 to its Fa0/1 port.

3. Now the next function of the switch0 is forwarding. So first it checks whether the destination MAC is in it’s CAM table or not. Since the destination MAC is not present in it’s CAM table, now the switch0 will flood the frame to all of its active ports.   

4. Now the switch0 is going to flood the frame to its active ports: Fa0/2 & Fa0/1. (Remember that switch doesn’t bother about the device connected to it, it just reads the MAC address of the NIC interface of the device) 

5. Now the flooded frame will be rejected by Wstation2 but it will be accepted by Switch1 because it is a switch & it will think that the destination device may be connected to me, so let me accept the frame. 
(Actually switch accepts a frame after validating few things, we will discuss them as we go on). For now the Switch1 will accept the frame. (Don’t worry about the position of the frame in the pic)

Wstation2 rejected the frame & Switch1 accepted the frame.

6. Now again, what is the first function of the Switch1 ? It is nothing but it has to perform its learning function. So Switch1 will first compare whether or not the Source MAC address:0090.0C1C.4B41 is present in the CAM table.
         Since it is not present, it will add it to its CAM table. Let check it.  

On Switch1: “show mac-address-table” 

Switch1 has mapped the PC1’s MAC: 0090.0C1C.4B41 to Fa0/5 port.

7. Now the next function of the switch1 is forwarding. So first it checks whether the destination MAC is in it’s CAM table or not. Since the destination MAC is not present in it’s CAM table, now the switch1 will flood the frame to all of its active ports.   

 Here is the PDU information:

8. Now the flooded will be received by Fa0/3 port (PC3) and Fa0/4 port (PC4). However PC3 will accept the frame because the destination MAC address in the frame matches its own mac address. And obviously PC4 will ignore the frame.

 
9. So PC3 has accepted the frame. Here is the PDU information on PC3 for the received frame.

The above information says that the destination MAC address has matched the receiving interface MAC address.

10. Now the PC3 is going to reply back to PC1. Now the frame will contain the PC1 as the Destination and PC3 as the source.

11. Now Switch1 has receive4d this reply frame with above fields from PC3.

12. Now the first function of the switch is to learn the source mac from the frame. Let’s check it out:
On Switch1: “show mac-address-table”

So from the output we confirm that Switch1 has learned the source mac from the frame and it has mapped/associated it to its Fa0/3 port.

13. Now the next function of the switch is to forward the frame to the destination MAC. The Switch1 will examine its CAM table and finds an entry for the destination MAC. So it is going to forward the frame to Fa0/5 port as per the CAM table.

 

14. Now the frame has been received by the Switch0 on its Fa0/5 port. Here is the PDU information.

  The above figure shows that a frame has been received on Fa0/5 port from the device: 0060.70A7.005D (PC3) and is destined for the device with MAC: 0090.0C1C.4B41 (PC1).

Let’s see whether the switch has performed its learning function or not.

On Switch0: “show mac-address-table”

 

From the above output we can confirm that the Swtich0 has  performed its learning function. It mapped/associated the PC3 MAC: 0060.70A7.005D to its Fa0/5 port.

15. Now the next function of the switch0 is forwarding. So first it checks whether the destination MAC is in it’s CAM table or not. Since the destination MAC is present in it’s CAM table, now the switch0 will forward the frame to Fa0/1 port as per the CAM table.

  

16. Finally PC1 will receive this reply frame of PC3.

This is how the switch helps the devices connected to it communicate each other.  

In the next post we will discuss about the different type of frames a switch will receive and how it process them.