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New Member

Switch MAC table entries

General mac table question.


Scenario. Two switches connected. No trunking. VLAN 1 only.
Alll PCs and switches have been sending data successfully.

There is one connection between the switches.

20 PCs ----- SW1 ---------- SW2 ------------ 20 PCs

I know the mac table on SW1 has all the MACs of the PCs connected to SW1.

But if all the PCs connected to SW2 also send data through SW1 are their MACs also listed in SW1 MAC table?

If so are they all assigned to one port of SW1 the one connecting the two switches?

Thx in advance

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Blue

Re: Switch MAC table entries

kbullard00 wrote:

General mac table question.


Scenario. Two switches connected. No trunking. VLAN 1 only.
Alll PCs and switches have been sending data successfully.

There is one connection between the switches.

20 PCs ----- SW1 ---------- SW2 ------------ 20 PCs

I know the mac table on SW1 has all the MACs of the PCs connected to SW1.

But if all the PCs connected to SW2 also send data through SW1 are their MACs also listed in SW1 MAC table?

If so are they all assigned to one port of SW1 the one connecting the two switches?

Thx in advance

Yes, all the mac-addresses for the PCs on SW2 will be in the mac-address table of SW1 and the outgoing port will be the port connecting to SW2.

Jon

Re: Switch MAC table entries

Yes, The Mac Addresses of all the PC's connected to the SW2 will show up on SW1 on the port that is connecting SW1 to SW2. This is because SW1 will maintain a list of MAC Addresses reachable via the port connecting it to SW2 so that it can send data just to that port rather than doing a Flood.

Manish

5 REPLIES
Hall of Fame Super Blue

Re: Switch MAC table entries

kbullard00 wrote:

General mac table question.


Scenario. Two switches connected. No trunking. VLAN 1 only.
Alll PCs and switches have been sending data successfully.

There is one connection between the switches.

20 PCs ----- SW1 ---------- SW2 ------------ 20 PCs

I know the mac table on SW1 has all the MACs of the PCs connected to SW1.

But if all the PCs connected to SW2 also send data through SW1 are their MACs also listed in SW1 MAC table?

If so are they all assigned to one port of SW1 the one connecting the two switches?

Thx in advance

Yes, all the mac-addresses for the PCs on SW2 will be in the mac-address table of SW1 and the outgoing port will be the port connecting to SW2.

Jon

New Member

Re: Switch MAC table entries

In this Boson question that came with my Cisco book is that what they are saying.

It seems to me they are not. see attach

Thx a bunch for the reply

Re: Switch MAC table entries

Yes, The Mac Addresses of all the PC's connected to the SW2 will show up on SW1 on the port that is connecting SW1 to SW2. This is because SW1 will maintain a list of MAC Addresses reachable via the port connecting it to SW2 so that it can send data just to that port rather than doing a Flood.

Manish

New Member

Re: Switch MAC table entries

thanks a lot for the quick reply. that is what I thought. Then going through the Boson exam that came with my ICND1 book I came across this question

that confused me.

am I reading this wrong.

see attachment.

Hall of Fame Super Blue

Re: Switch MAC table entries

kbullard00 wrote:

thanks a lot for the quick reply. that is what I thought. Then going through the Boson exam that came with my ICND1 book I came across this question

that confused me.

am I reading this wrong.

see attachment.

The ICND question is correct, the packet from PC A arrives at sw1 and because it has no record in it's mac-address table it floods the frame out all ports, except fa0/1. Note as well as sw2, PC B also sees this frame. The frame arrives at sw2 and because sw2 has a record of PC D it only sends it out that port.

However the difference from your question was that with your question we assumed that the PCs on sw2 had already communicated with devices on sw1 so sw1 knew about their mac-addresses. In the ICND question the assumption is that PC D has never communicated with a device on sw1 so sw1 does not have a record of PC D mac against it's gi0/1 interface.

So sw1 has to flood it. Once PC D responds to PC A there will then be a record of PC D mac-address against sw1 gi0/1 and so to continue the communication between PC A and PC D sw1 no longer needs to flood the frame ie PC B will not receive any of the other packets.

Jon

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