L2 switch's behavior|
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Posted by Mark on March 30, 2009, 10:48 pm
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I wonder is it a normal switch's behavior, when it, being connected to a LAN, but not transmitting/receiving any traffic, starts learning process and fills its MAC address table. I understand that the switch is in promiscious mode upon reset, but I though it would learn MACs only for a traffic travelling through it. Please clarify my doubts. Thanks. -- Mark | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Albert Manfredi on March 31, 2009, 10:11 am
Please log in for more thread options > fills its MAC address table. I understand that the switch is in promiscio=
us
> mode upon reset, but I though it would learn MACs only for a traffic
> travelling through it. The switch inspects the MAC source address of frames going through each port, yes. If it sees a frame with a MAC destination address it hasn't yet learned, it floods it to all ports. That should be the default behavior. Of course, you can modify that behavior in a managed switch. For example, allow only certain MAC addresses at a given port, block ports, etc. Bert | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Rich Seifert on March 31, 2009, 11:01 am
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> > Hello
> > > > I wonder is it a normal switch's behavior, when it, being connected to a > > LAN, but not transmitting/receiving any traffic, starts learning process and > > fills its MAC address table. I understand that the switch is in promiscious > > mode upon reset, but I though it would learn MACs only for a traffic > > travelling through it. >
A switch will learn the MAC_address-to-port mapping for all frames it hears, not just those traversing the switch. -- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX Send replies to: usenet at richseifert dot com | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Mark on March 31, 2009, 8:05 pm
Please log in for more thread options Hello
>> > I wonder is it a normal switch's behavior, when it, being connected to
>> > a >> > LAN, but not transmitting/receiving any traffic, starts learning >> > process and >> > fills its MAC address table. I understand that the switch is in >> > promiscious >> > mode upon reset, but I though it would learn MACs only for a traffic >> > travelling through it. >>
>
> A switch will learn the MAC_address-to-port mapping for all frames it > hears, not just those traversing the switch. Id this is a standard behavior, is there any rationale for it? For the first sight it looks not very reasonable, as a switch can't be sure the addresses it has heard and learned, will be ever used in actual transmission across the switch, and these addresses will only occupy precious MAC address table :-) Od course aging procedure will clean them out, but anyway it's sort of waste of resources, isn't it? -- Mark | |||||||||||||||||||||||||||||||||||||||||||
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Posted by Rich Seifert on April 1, 2009, 11:50 am
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> Hello
> > >> > I wonder is it a normal switch's behavior, when it, being connected to
> >> > a > >> > LAN, but not transmitting/receiving any traffic, starts learning > >> > process and > >> > fills its MAC address table. I understand that the switch is in > >> > promiscious > >> > mode upon reset, but I though it would learn MACs only for a traffic > >> > travelling through it. > >> > >
> > A switch will learn the MAC_address-to-port mapping for all frames it > > hears, not just those traversing the switch. >
> Id this is a standard behavior, is there any rationale for it? See my earlier post regarding frames not traversing the switch. > For the first
> sight it looks not very reasonable, as a switch can't be sure the addresses > it has heard and learned, will be ever used in actual transmission across > the switch, By definition, every address the switch hears was used in an actual transmission; the addresses are gleaned from ordinary frames. It is the switch's job to determine the relative location of every SA it hears, i.e., through which port that station is reachable, and then to forward or discard subsequent frames sent to that station as a DA, so that traffic is isolated to only those portions of the Spanning Tree needed to communicate between SA and DA. > and these addresses will only occupy precious MAC address table
These days, MAC address table storage is not considered "precious". > :-) Od course aging procedure will clean them out, but anyway it's sort of
> waste of resources, isn't it? > Not at all. This is precisely the job of the switch. If it didn't filter frames via the address table, it would be a repeater. -- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX Send replies to: usenet at richseifert dot com | |||||||||||||||||||||||||||||||||||||||||||
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>
> I wonder is it a normal switch's behavior, when it, being connected to a
> LAN, but not transmitting/receiving any traffic, starts learning process =