Simple question about STP port selection.

[Daitunas Kalibra]

Hello,

My friend asked my to look at this example:

A. DSW11 will be elected the root bridge.

B. DSW12 will be elected the root bridge.

C. ASW13 will be elected the root bridge.

D. P3/1 will be elected the nondesignated port.

E. P2/2 will be elected the nondesignated port.

F.

P3/2 will be elected the nondesignated port.

Answer: A and D

Guys, im totally confused about it. I think the question is bad. Because there is a two non-designated ports. If DSW11 is Root bridge, so

P3/1 is blocked port, because cost to the root is bigger, than in oposite direction. But P3/2 also will be non-desiganted port, cause it's a root port. So the answer F is correct too.

What do you think ? Thanks

[cadet alain]

Hi,

answer A is correct because DSW11 and DSW12 have same lower priority but MAC of DSW11 is lower which is the tie-breaker.

So nom DSW11 has all its ports as DP.

Next is RP election on non-root bridges: DSW12,ASW13 and ASW14

on ASW14 it is f1/4 as it is the only way to the root bridge

on ASW13 it is P3/1 as cost is lower than via P3/2

on DSW12 it is P2/1 for same reason

Now comes the DP election  on the segment DSW12-ASW13:

the cost is the same so we use next rule which is lower BID. AS DSW12 has lower BID it is Designated bridge and so port 2/2 is DP and on SW13 P3/2 is non DP so F is the correct answer.

Regards.

Alain.

[lgijssel]

A is obviously correct (lowest mac address), no discussions there.

For the other answer, this is indeed confusing because imho non-designated ports are not "elected"; they are what remains after root ports and designated ports are determined by the STP algorithm.

RP's are P2/1, P3/1 and P4/1 (root ports are ports which are receiving bpdu's)

DP's are P1/1, P1/2 and P2/2 (designated ports are the ports forwarding bpdu's on the segment)

This elimination leaves only P3/2 without a designation so for me it would have been A and F.

regards,

Leo

[Daitunas Kalibra]

Thanks for answer,

But i think that port P3/1 is not root port because the link is only 10mbps (no 100), so root on ASW13 is P3/2..

am right ?

[cadet alain]

Hi,

Yep you're right. well spotted  100 is higher than 2 X 19.

So Answer D is right, this port is non DP and so blocking   and port on DSW12 pointing to ASW13 is DP

Regards.

Alain.

[Jon Marshall]

For me the answer is A & D.

P3/1 cost to root = 100 (10Mbps link)

P3/2 cost to root = 38 (100Mbps link = 19, 2 x 19 = 38)

So P3/2 is the root port and p3/1 becomes blocking.

Edit -  i agree with Leo though in that the term elected non-designated is slightly confusing terminology ie. you elect root and DP ports. You don't elect blocking ports.

Jon