Basic Ethernet Question

Answered Question
Sep 24th, 2010

Will a switch flood an ethernet frame with an unknown destination MAC address to ALL ports in that particular vlan, or only on the trunks and ports where the machine has been dormant (and therefore its MAC entry timed out)?

I would think its the latter because if the switch already has an entry for a MAC on a particular port, why should it send the unknown unicast to it, too?

Feedback?

Thanks

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 6 years 2 months ago

ex-engineer wrote:

Will a switch flood an ethernet frame with an unknown destination MAC address to ALL ports in that particular vlan, or only on the trunks and ports where the machine has been dormant (and therefore its MAC entry timed out)?

I would think its the latter because if the switch already has an entry for a MAC on a particular port, why should it send the unknown unicast to it, too?

Feedback?

Thanks

Joe

The switch will flood it to all ports within that vlan, including trunk ports, except the port it was received on. It has to because it has no way of knowing which port the machine could be on. Even if it had a mac-address entry for a particular port that port could have a hub hanging off it and the destination mac-address is connected to the hub.

So no, it doesn't filter who to send the unknown mac-address to, it simply sends it to all ports.

Jon

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.8 (6 ratings)
Loading.
Correct Answer
Jon Marshall Fri, 09/24/2010 - 14:57

ex-engineer wrote:

Will a switch flood an ethernet frame with an unknown destination MAC address to ALL ports in that particular vlan, or only on the trunks and ports where the machine has been dormant (and therefore its MAC entry timed out)?

I would think its the latter because if the switch already has an entry for a MAC on a particular port, why should it send the unknown unicast to it, too?

Feedback?

Thanks

Joe

The switch will flood it to all ports within that vlan, including trunk ports, except the port it was received on. It has to because it has no way of knowing which port the machine could be on. Even if it had a mac-address entry for a particular port that port could have a hub hanging off it and the destination mac-address is connected to the hub.

So no, it doesn't filter who to send the unknown mac-address to, it simply sends it to all ports.

Jon

ex-engineer Fri, 09/24/2010 - 15:12

Jon your explanation was spot on. Thanks for clarifying. You make a good point about the hub.

Rated 5, not a 3. That wasnt me.

Jon Marshall Fri, 09/24/2010 - 15:16

ex-engineer wrote:

Jon your explanation was spot on. Thanks for clarifying. You make a good point about the hub.

Rated 5, not a 3. That wasnt me.

Joe, no problem, glad to have helped. Didn't think it was you on the 3, thanks for the +5.

Jon

jasonfmic Fri, 09/24/2010 - 14:59

The MAC table is for the entire switch and there will only be one entry per MAC.  When a frame is received for an unknown port, that frame is broadcast to all ports except for the port on which the frame was received. It sounds like you were under the impression there is a MAC table per port? In the case of VLANs, think of each VLAN as a separate switch.

Now as to how learning occurs, when a switch receives a frame it records the MAC of the sender in its MAC table with the port that was the source of the frame. From then on, when a frame is received for this MAC the switch only sends to the port in its MAC table. If however, the same MAC sends a frame that comes in on a different port, the switch will delete the reference to the old port and add the new port to its MAC table. The switch never stores more than one source port per MAC. Again, here, think of VLANs as basically separate switches. In this case there is one entry per MAC & VLAN combination.

I hope that helps.

ex-engineer Fri, 09/24/2010 - 15:13

Jason, thanks for the ino. I was not under the impression that there is more than one MAC table.

Jon addressed the point I wasnt sure about.

Thanks

mlund Tue, 09/28/2010 - 05:28

Hi

The command " switchport block" stops the switch from forward unknown unicast or multicast frames out a port.

/Mikael 

Actions

This Discussion