Switch ports connected with cross cable on same switch ..

Answered Question
Jan 27th, 2008

Hi experts,

Just got a doubt.

If two ports of a standalone switch are connected through a cross cable,how will be the stp behaviour with respect to this scenario..


thanks

arun


Correct Answer by shrikar.dange about 9 years 4 weeks ago

hi arun,

As all know switches use following criteria for applying STP:

1) Lowest root BID

2) Lowest path cost to root bridge

3) Lowest sender BID

4) Lowest port priority

5) Lowest port id.


So in your senario up to number 4 allvalues will be equal and there will be a tie.So for tie-breaker port id will be used and the port with higher port number will be blocked and lower port id will be in fwding state.


Example:

If you crossed cable the e1/1 and e1/2 on the same switch then 1/1 will b in fwding state while 1/2 will b in blocking state.


HTH,



regards,


shri :)

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Correct Answer
shrikar.dange Sun, 01/27/2008 - 20:05

hi arun,

As all know switches use following criteria for applying STP:

1) Lowest root BID

2) Lowest path cost to root bridge

3) Lowest sender BID

4) Lowest port priority

5) Lowest port id.


So in your senario up to number 4 allvalues will be equal and there will be a tie.So for tie-breaker port id will be used and the port with higher port number will be blocked and lower port id will be in fwding state.


Example:

If you crossed cable the e1/1 and e1/2 on the same switch then 1/1 will b in fwding state while 1/2 will b in blocking state.


HTH,



regards,


shri :)

s.arunkumar Sun, 01/27/2008 - 20:10

Hi shrikar

Thanks 4 the reply..

but what i doubt is the switch itself will be the root bridge and all the connected ports of the root bridge should be in forwarding mode (it will be designated port)..so how will be the stp topology build in this case..??????????

or is it that rule is not valid in this case!!!!!

arun

shrikar.dange Sun, 01/27/2008 - 20:32

hi arun,


Yes you are right that all ports of the root should be designated ports and should be in forwarding state.

But what i have understood is that when the port is in learning state it processes the BPDU.While processing this BPDU it will come to know that the sender BID, root BID and the BID of the switch is same so it will detect the loop and put the higher number port in to blockin state.


Any comments experts?


regards,


HTH,


shri :)

s.arunkumar Sun, 01/27/2008 - 20:36

That was a good one.... :)

Hope this is the way...lets waits for others too.....

arun


Jon Marshall Sun, 01/27/2008 - 23:30

Hi Shri


Yes one of the ports will have to block so this is one of those times when "all ports on a root bridge are designated ports and forwarding" is not strictly true.


It is still a good general rule though as it is failry uncommon to connect a switch to itself with a cross-over cable :)


Jon

s.arunkumar Mon, 01/28/2008 - 00:41

Obviously Jon,I just asked to know how it will behave in that case...

anyway thanks Jon for conforming the same...


arun

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