9 Replies Latest reply: Aug 13, 2015 6:41 PM by Gregory RSS

    Switch Static and Dynamic mac-address

    thiyagarajankalaiselvan

      As per my understanding Switches maintain a database of MAC addresses both static and Dynamic,

       

      What does Static MAC address mean?

      What does dynamic MAC address mean?

       

      If a switch learns the mac-address from other switche,Which criteria this will match of the above?

       

      Can anybody elaborate on this.

       

      Thanks a ton in advance.

       

       

       

        • 1. Re: Switch Static and Dynamic mac-address
          rosscourtnell

          A Static MAC address is one that has been manualy input (typed via a command) into the CAM or MAC address table.

          A dynamic MAC address is one that has been learned via an arp request.

           

          If a switch learns the MAC address from another device then it has dynamically sourced the MAC address.

           

          Hope that helps

           

          Ross

          • 2. Re: Switch Static and Dynamic mac-address
            samychihi21

            Hi;

             

            Actually when we are talking about MAC address and switchs, we have to notice that th mac table in the switch help this device to locate another host.

            Mac address table command is :

             

            Switch#show mac-address-table
                      Mac Address Table
            -------------------------------------------

             

            Vlan    Mac Address       Type        Ports
            ----    -----------       --------    -----

             

               1    00e0.b058.d801    DYNAMIC     Fa0/1

             

            As you can see in this example, this MAC address is dynamic so it is lost after a while or when you reload the switch.

            The static MAC addresses are not lost (i think even after reload-but you have to check that)

             

            Regards.

            Samy

            • 3. Re: Switch Static and Dynamic mac-address
              Lorquet Dorce

              Also to expound on what rosscourtnell & samychihi21 stated, when you do run that show mac-address-table command be carefully not to mistake a static entry for that of the switch its self.

               

              Best of Luck

              Lorquet Dorce

              • 4. Re: Switch Static and Dynamic mac-address
                currah

                Hey found this post while studying my CCNA just want to update for thoose using google. In response to rosscourtnell post. Switches dont use ARP tables, since ARP is used for IP address to MAC resolution it operates at a layer above the switch in layer 3. A switches MAC table actually looks at the Source MAC in the Frame and adds it to the MAC address table. Which is what dynamic MAC address is.

                • 5. Re: Switch Static and Dynamic mac-address
                  Darrel

                  I was hoping someone could help me out. I received a practice question and I am having a hard time getting to the bottom line. I just wanted to double check my understanding and hopefully someone could point out what or if I am missing something. When a switch is first powered up and at its default configuration the MAC table is empty. Say that Host A pings Host B. The switch learns Host A's MAC address. Since the table is empty it will flood the frames and Host B will reply with a uni cast message, in turn having the switch learn Host B's MAC address. This creates 2 entries in the MAC address table correct? Below is question I am having a hard time with followed by the answers. I chose B C E G and obviously I am missing the 5th one. If anyone can help I would appreciate it.

                   

                  Assume that the switch has a factory default configuration and has just been powered on. Host A pings Host B successfully. Which of the following are true?

                  Select 5.

                   

                  A. If Host A pings host C, 1 new MAC table entry will be built.

                  B. If Host B pings host C, 1 new MAC table entry will be built.

                  C. If Host A pings host B, 1 new MAC table entry will be built.

                  D. If Host B pings host D, 2 new MAC table entries will be built.

                  E. If Host C pings host A, 1 new MAC table entry will be built.

                  F. If Host C pings host D, 2 new MAC table entries will be built.

                  G. If Host D pings host A, 1 new MAC table entry will be built.

                  H. If Host A pings host B, 2 new MAC table entries will be built

                  • 6. Re: Switch Static and Dynamic mac-address
                    Umar

                    Hi Darrel,

                     

                    (First as the question states, A pings B successfuly) this means the switch has already learnt 2 mac addresses, one corrected to A and other connected to B. So after that if A and B ping each other , the switch will not be learning any thing.

                     

                    OK, I think following are the answers with arguments:

                     

                    A: When A pings C, we know A is already known to switch so only C is learnt that is only 1 new MAC table entry

                    B: same argument as to A

                    E: same argument as to A but in reverse

                    F: C is pinging D, both are new so 2 MACs learnt.

                    G: same arbument as A but in reverse.

                     

                    Hope my answer war clear, take care,

                    Umar

                    • 7. Re: Switch Static and Dynamic mac-address
                      Alex

                      yes as Umar, I get A, B, E, F, G.

                       

                      the thing to remember is that a successful ping includes a reply, so the switch actually gets both mac's when one host pings another successfully.

                      • 8. Re: Switch Static and Dynamic mac-address
                        Jeff

                        I know this is an old thread but there must be a correction made to the answered previously given to Darrell.

                         

                        There is only one correct answer and it is H.  Unless there is a network Hosts C and D are not mentioned in the question so answers including hosts C and D are irrelevant.

                         

                        The MAC of Host A is learned/added to MAC table by the switch when the ARP req. (which is a broadcast using a destination MAC of FFFF:FFFF:FFFF) is received by the switch and flooded out all ports.

                         

                        The MAC of Host B is learned/added to MAC Table by the switch when it receives the Unicast reply sent from Host B (using destination IP, and Destination MAC of Host A),

                        • 9. Re: Switch Static and Dynamic mac-address
                          Gregory

                          Hi Jeff,

                           

                          I see where you are coming from with your answer, however I would like to direct you to the following detail (key to answering ALL Cisco exams - the little details). The question in this example asks for 5 correct responses:

                           

                          "Assume that the switch has a factory default configuration and has just been powered on. Host A pings Host B successfully. Which of the following are true?"

                          Select 5.

                           

                          The answer from Umar posted above is indeed the correct response to be able to answer the question fully - as those are the correct 5 responses to the given scenario. This question is geared to ascertain a students understanding of how MAC learning works and is a terrific example to be evaluated by.