Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

How loops are formed in layer 2 switch?

HI

 I what to know how loops are formed in layer 2 switch ?

I have attached a example explain with these example when frame sending from pc1 to pc0 or explain with a different example ?

 

9 REPLIES
Hall of Fame Super Gold

The answer to your school

The answer to your school work question is Spanning-Tree.

 

Click on the link for a better explanation.

New Member

I know that I am asked about

I know that I am asked about how loops are formed in a layer 2 switch?

Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Loops are formed when the same broadcast domain has at least one circular L2 path.

In your example, if your 3 switches were "dumb" or, if "smart", the same VLAN was on all 3 connections, you've created a L2 loop.

When pc1 sends its first frame to pc0, sw1 will "record" the source mac was seen on the ingress port and then flood the frame to all other ports (as it doesn't yet know the dest mac port).

When sw2 and sw3 receive the flooded frames from sw1, each will record the src mac and flood to all other ports; one of which being pc0's.

However, sw2 and sw3 will then get the flooded frame from each other.  Now, though, they will see the src mac from a different ingress port, and also again, flood to all other ports.  Sw1 will receive the newly flooded frames from ports to sw2 and sw3, and it too will flood to all other ports.  I.e. the frame is looping through the L2 ring.  NB: for L2, there's no timeout, i.e. the frame will continue to loop.

New Member

Thanq to Joseph DohertyAnd in

Thanq to Joseph Doherty

And in these reply i have question ?

when flooded frame receives sw3 it will also flood pc mac address or not ?

Super Bronze

isclaimerThe Author of this

isclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, the PC MAC will be flooded, as its the source MAC for the original frame.

New Member

Can you please explain with

Can you please explain with routing table in every switch ? 

Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

(L2) Switches don't have routing tables.

New Member

can u please explain with mac

can u please explain with mac address table?

 

Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Sure, add to my original post, every time a frame enters a switch, the switch will record the source MAC and the ingress port it arrived on in the MAC table.

154
Views
0
Helpful
9
Replies
CreatePlease to create content