2.1 Describe and verify switching concepts

Yahoo! we have now moved to switching. We have here different switching concepts need to discuss but the thing here is that I cannot discuss them separately or individually as switching concepts below relates to one another.

2.1.a MAC learning and aging
2.1.b Frame switching
2.1.c Frame flooding
2.1.d MAC address table




Let me explain the switching concept specially MAC address learning and aging. I have here a  simple topology that would help us to know switching concepts plus MAC-adddress table below



Switch>show mac-address-table

Mac Address Table


Vlan Mac Address Type Ports

—- ———– ——– —–

1 0001.43d7.a958 DYNAMIC Fa0/1>>>> pc1

1 0001.64c6.5ecd DYNAMIC Fa2/1>>>> PC2

1 0090.0c24.8c3e DYNAMIC Fa1/1>>> PC3

As you can see MAC- addresses are already learned by our switch, the question is how such  a switch learned these MAC-addresses?

MAC learning process goes like this:

Scenario PC3 would like to connect to PC1

Step1 PC3  would send an arp request to PC1

This ARP request contains the source address and the destination MAC address:


DMAC: since PC3 doesn’t know the MAC address of PC1, it would send a broadcast address of FFFF.FFFF.FFFF.

STEP 2  Switch will check the source MAC address with this conversation:

Switch: Do I already know your source mac address?

No, let me add you first to my MAC-address table

Yes, Ok will process with the next step

Step 3. Since the destination MAC address is a broadcast address, the switch will flood the broadcast address which is the receiver should only be the PC1 but it will be flood to all hosts connected to the switch. But PC1 will the only one who will response and the other hosts will just drop the packet. The concept of flooding goes into this scenario.

Step4 Since PC1 is the only one who will response, it will send an ARP reply to PC3 by sending its source MAC address and destination MAC address:



Step 5 Switch will check if it alredy knows the source MAC address of PC1 with this conversation:

Switch: Do I already know your source mac address?

No, let me add you first to my MAC-address table

Yes, Ok will process with the next step

Step6 Same process Switch will check if it already knows the  destination MAC address which happened to be the MAC address of PC3 which is already part of the MAC-address table.

So PC3 was able to get a response and communicate with PC1

So I was able to explain MAC learning, flooding and MAC address table.

Aging refers to the time that an entry in MAC-address table remains over a period of time.

*****************************END OF BLOG*******************************



2 thoughts on “2.1 Describe and verify switching concepts”

  1. How would this initial communication be established in the first place? For instance if PC1 wants to communicate with PC2 but doesn’t know its MAC then how does the destination machine (PC2) know the information is intended for itself when the broadcast is received?

    For example PC1 is called Bob and pc 2 is called Rob (MAC addresses respectively)

    PC1 doesn’t know Rob’s name so sends a broadcast out with what information exactly?

    When Rob gets the broadcast info how does it know it is the intended recipient? Surely Bob needs to know Rob’s MAC initially and if the switch doesn’t know where to find it the switch sends out a broadcast to update it’s table.

    Hope this makes sense.


    1. Hi Joe, thank you for that feedback, it helps me a lot to make improvements on writing blogs. Now, with your question, I hope I understand your point. I just would like to clarify that your question is that how will Rob ( pc2) know that he will be the intended recipient instead of dropping off the packet? If I got your question correctly, ARP request is the protocol involved with that scenario. ARP is a protocol that resolves or retrieves MAC address when IP address is present. So on our scenario, Rob (PC2) would know that he is the recipient of the broadcast address because at the first place the encapsulation should be( Source IP address: BOB PC1 , Destination IP address: rob pc2, source MAC: MAC address of PC1, DMAC: suppossedly MAC address of PC2 but since it is unknown it will be replaced by broadcast address to know PC2’s MAC address.. Apologies, if I overlooked unto the details and if it is not clearly explained. Link below is the difference of ARP and RARP protocols http://techdifferences.com/difference-between-arp-and-rarp.html


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