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What is the purpose of MAC table refresh time

Can someone please let me what is the purpose of mac table expire timer ?

And also what is the default time (I guess 300 seconds).

And also what is the command to  see the present mac-table expire threshold time.

Thanks,

Chandu

Regards, Chandu
3 ACCEPTED SOLUTIONS

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Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi,

Can someone please let me what is the purpose of mac table expire timer ?

To age out inactive (and maybe invalid) entries. It can happen when you relocate a server to a different port or different device on the same LAN. Or even if hosts move from port to port / device. Some switches may be unable to relearn this address on different port simply because the hosts are not communicating through them anymore, but they still have an entry in mac address table stating, that this mac address resides on that port of the switch (which is not true anymore). So if some other hosts tries to communicate with that relocated host or server, switch simply sends all the data to this black hole - it thinks, the host is still there, he is maybe just inactive or busy at the moment or doesn't need to respond at all. Well, to avoid this scenario, we have the aging time for each of the mapping. So even if the host relocate or so, after 5 minutes (default), the record in mac address table is flushed and if a frame with such destination mac address arives, it is broadcasted throughout the network - and hopefully learned on a different port.

And also what is the default time (I guess 300 seconds).

Yes, that's correct.

And also what is the command to  see the present mac-table expire threshold time.

I really can't think of any used on common low-end catalyst switches.

Best regards,

Jan

View solution in original post

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What is the purpose of MAC table refresh time

To End,

The command is :

#sh mac   address-table aging-time

Global Aging Time:  300

Vlan    Aging Time

----    ----------

Please rate all of my helpful posts, that is even more useful than a thanks

Regards

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC

View solution in original post

Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi,

Unfortunatelly more things won't work in PT, this is one of them.

And i believe that in real time people won't change ports or server locations or switches much frequently.

Still, you do need a mechanism that will deal with this type of things. And it is better to flush inactive mac addresses, so the table won't get full after some time.

And also what is the maximum number of MAC entries for a 2960 switch?

8024 entries

You can check the max number by issuing the command

Switch#show mac-address-table count

But I think that this command is not available in packet tracer either.

Best regards,

Jan

View solution in original post

17 REPLIES 17
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What is the purpose of MAC table refresh time

Hello Chandra,

As you might now there is a maximum amout of MAC addresses that you can store in your mac-address table and also for tracking purposes you want to make sure you learn the mac addresses of devices that are actually in your network.

What happens if you learn the mac address of PC1 and after a few minutes, that pc is disconnected from the network.

Your switch MAC address table will still be showing that MAC entry on the table, so with the 300 timer if we do not receive any other packet from that source MAC address we will remove it from the table.

Do you follow me?

Regards,

follow me on http://laguiadelnetworking.com

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi,

Can someone please let me what is the purpose of mac table expire timer ?

To age out inactive (and maybe invalid) entries. It can happen when you relocate a server to a different port or different device on the same LAN. Or even if hosts move from port to port / device. Some switches may be unable to relearn this address on different port simply because the hosts are not communicating through them anymore, but they still have an entry in mac address table stating, that this mac address resides on that port of the switch (which is not true anymore). So if some other hosts tries to communicate with that relocated host or server, switch simply sends all the data to this black hole - it thinks, the host is still there, he is maybe just inactive or busy at the moment or doesn't need to respond at all. Well, to avoid this scenario, we have the aging time for each of the mapping. So even if the host relocate or so, after 5 minutes (default), the record in mac address table is flushed and if a frame with such destination mac address arives, it is broadcasted throughout the network - and hopefully learned on a different port.

And also what is the default time (I guess 300 seconds).

Yes, that's correct.

And also what is the command to  see the present mac-table expire threshold time.

I really can't think of any used on common low-end catalyst switches.

Best regards,

Jan

View solution in original post

Highlighted

What is the purpose of MAC table refresh time

To End,

The command is :

#sh mac   address-table aging-time

Global Aging Time:  300

Vlan    Aging Time

----    ----------

Please rate all of my helpful posts, that is even more useful than a thanks

Regards

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC

View solution in original post

Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi Julio,

but this command only displays the configured aging-time value (5min default), not the actual value for each of the addresses, doesn't it?

Best regards,

Jan

Highlighted

Re: What is the purpose of MAC table refresh time

Where did he ask for a timeout for each mac address entry??

And if that's the case I have not seen it just like you

Can someone please let me what is the purpose of mac table expire timer ?

And also what is the default time (I guess 300 seconds).

And also what is the command to  see the present mac-table expire threshold time.

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

Re: What is the purpose of MAC table refresh time

Hi Julio,

I didn't want to sound rude, I was just curious if there is a way to see each entry's timeout, not just the max value itself.

Btw I hope you don't mind that I answered the thread after you. By the time I finished writing, your post was already here.

Best regards,

Jan

Highlighted

What is the purpose of MAC table refresh time

Hello Jan,

No problem my friend sorry if I actually sound rude.

Regards,

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Thank you all for your information.

However in PT i can't see the aging time. That command is not working in PT.

Switch2#show mac-address-table ?

  dynamic     dynamic entry type

  interfaces  interface entry type

  static      static entry type

 

And i believe that in real time people won't change ports or server locations or switches much frequently.

And also what is the maximum number of MAC entries for a 2960 switch?

Thanks in advance.

Regards,

Chandu

Regards, Chandu
Highlighted

What is the purpose of MAC table refresh time

Hello Chandra,

Expected behavior

Packe-tracer is just not 100 % real hehe

But trust me that is the command you are looking for.

Remember to rate all of the helpful posts

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi,

Unfortunatelly more things won't work in PT, this is one of them.

And i believe that in real time people won't change ports or server locations or switches much frequently.

Still, you do need a mechanism that will deal with this type of things. And it is better to flush inactive mac addresses, so the table won't get full after some time.

And also what is the maximum number of MAC entries for a 2960 switch?

8024 entries

You can check the max number by issuing the command

Switch#show mac-address-table count

But I think that this command is not available in packet tracer either.

Best regards,

Jan

View solution in original post

Highlighted

What is the purpose of MAC table refresh time

Hello Chandra,

And i believe that in real time people won't change ports or server locations or switches much frequently.

Really, so you have not work on a company where there are a lot of meeting rooms, training rooms, etc where you go and plug your laptop into a switch port and after the meeting/training you plug it off and go to your desktop?

This is really commom now day.s

And also what is the maximum number of MAC entries for a 2960 switch?

Jan already answered this

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Expecting some more inputs. That's the only reason for not marking this as answered.

Thanks for you both again.

Regards

Chandu

Regards, Chandu
Highlighted

What is the purpose of MAC table refresh time

Hello Chandra,

What do you mean some more inputs LOL.

I mean I do not know anyone with access to PT code so they can modify it so you can see that output hehe.

That being said we (Jan and I) have answered you the 3 questions.

For future users having the same question would be great if you mark it as answered.

Note: If U still do not think we have answered it properly please rent a real cisco switch and check the commands we provided so you can see it

Regards,

Jcarvaja

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Highlighted
Enthusiast

What is the purpose of MAC table refresh time

Hi,

It i not like you guys didn't answered properly. The only reason that i don't want to mark it as answered right now because if i mark it as answered more people will not provide more info right ?

That is the only reason. It was great information from you both.

Regards,

Chandu

Regards, Chandu
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