Layer 2 switching loop

hello and this is sunny welcome back

today I will talk about a problem of a

switched Network layer 2 switching loop

in a typical local area network it is a

very common that multiple switches are

used and interconnected for redundancy

redundancy means that more than one path

is possible between two switches and the

redundancy is good because the network

will not feel completely even when some

link breaks

however the interconnected switches

would have a one series potential


layer 2 switching loop

let's say an example we have a four

interconnected switches a couple devices

are connected to those switches like

computer a is connected to switch four

and a web server is connected to switch

one suppose computer a wants to talk to

a web server it is saints a requester

frame to the server to understand the

switching loop which I will talk about

in a moment you need to know how a

switch uses is set a table to forward or

deliver a frame to the destination

please check my video how a switch bills

sauce adjustable dynamically if you are

not familiar with how it works I put a

video link below just a for your

reference otherwise please continue

switch for guests the frame first and it

checks the destination MAC address

against is set a table and it does not

see the web servers marriages sweetie

for with a broadcast to all it supports

except the source port that's why the

frame is the same to both sweetie to and

a switch 3 when they get in the frame

both switch 2 and a switch 3 checks

their set table they do not see the

destination MAC address in there set

table because the webserver is not

attached to any of their ports

therefore the broadcast to all other

posts except the source port now let's

pause for moment to check what's

happening now switch to sends the frame

to three directions computer B switch 1

and switch 3 switch 3 sense

same frame to three directions to

computer C switch one and switch to we

might see a problem already at this

point switch one switch it to and a

switch three get the same frame from

different sources

switch to gets it from sweetie three and

a sweetie three cases from switch it to

and a switch one gets two copies from

both sweetie to and sweetie three now

that's a focus on switch one only we

switch one guess

two friends either checks is a set table

and the finds the webserver is the

destination and the day it delivers to

same frames to the web server one copy

is unnecessary though but the problem is

still trivial as mo bigger problems are

coming soon

now that's a focus on switch to and a

switch III we switch to and a switch

three gather the frame from each other

they both check the set table they do

not see the destination marriages in

their table next step the broadcaster

again they send the same frame to all

the airports except the source port thus

you will see a messy picture now from

switch to the broadcaster goes to three

directions computer B switch one and a

switchy full from switch three the

broadcaster goes to three directions to

computers C switch one and switch full

what's happening now two frames arrived

and a switch one and switch one would

deliver them to the web server by now

the web server has got four copies of

the same frame now let's turn our

attention to switch full and we would

say an even bigger problem occurs as a

switch folk has two copies from both

sweetie - and a sweetie 3 either checks

is set table against the destination

marriages in the broadcast again but at

this time even the computer a their

frame originator will get his own frame

and twice besides switch fault also

sends the frame to both sweetie - and a

sweetie 3 does it look familiar

that was how the frame start his journey

at the very beginning the second around

of loop starts during this one cycle the

web server has got four copies and even

the computer a has got two copies of his

own message

the frame will start circulating the

network in a loop and it to make things

worse it would be an endless loop it is

not difficult to imagine how crazy and

messy the network would become with a

just one frame not to mention multiple

devices are talking at the same time it

would be absolutely a disaster

ultimately the network home would


any solution the answer would be yes

layer 2 switching loop can be prevented

by using spanning tree protocol and

what's the spanning tree protocol how

does it work it would be another topic

in my next video I hope this video is

helpful if you want to learn Network

systematically please check out my

playlists they are organized by topics

thank you very much and see you next