Help! This STP scenario is driving me crazy.

socraticmethod

Are the answers in this scenario correct? Why is it saying that SwitchA's Fa0/2 port is the designated port to reach SwitchC instead of SwitchB's Gi0/2? I understand that default costs for FastEthernet ports is 19 and Gig ports is 4. But what happens on a Segment with both FastEthernet and Gig ports?


1) Is SwitchA's cost to reach straight to SwitchC 19?

2) What is SwitchB's cost to reach SwitchC by going through SwitchA? Is it 4+19=23?

3) What about SwitchB's cost to reach SwitchC by going through SwitchD, is it 4+4=8?

4) Wouldn't that mean that SwitchB's Gi0/2 port should be DP on that segment because it has a lower cost to root?

5) Do you only add the cost of the outgoing interface on a path, or do you add the cost of the incoming ports too? AKA, from SwitchB's Gi0/2 path through SwitchD to SwitchC, do I add the cost of SwitchB's Gi0/1 (4) + SwitchD's Gi0/2 (4)? Or do I have to take into account SwitchC's Fa0/2(19) port as well?

Mooseboost

One piece of advise I will throw in here: Remember that cost is calculated on the receiving, not on the one it is sent out on.

socraticmethod

So what is the cost to reach SwitchB coming from both directions? Because either way you calculate it, SwitchB's Gi0/2 should be the DP on that segment, yet that's not what the answer is reflecting.

Alceo

For what i know:

D receives a BPDU from C with Root Cost=0
D adds the cost of gi0/2 (19, the if is using 100Mbps) and sends it to B
B receives the BPDU from D (cost 19), adds the cost of G0/1 -> Total cost 23

A receives a BPDU from C with Root Cost=0
A adds the cost of f0/1 = 19 and sends it to B
B receives the BPDU from A (cost 19), adds the cost of G0/2 (19, the if is using 100Mbps)-> Total cost 38

So G0/1 is the root port

Priston

socraticmethod wrote: »

1) Is SwitchA's cost to reach straight to SwitchC 19?
2) What is SwitchB's cost to reach SwitchC by going through SwitchA? Is it 4+19=23?
3) What about SwitchB's cost to reach SwitchC by going through SwitchD, is it 4+4=8?
4) Wouldn't that mean that SwitchB's Gi0/2 port should be DP on that segment because it has a lower cost to root?
5) Do you only add the cost of the outgoing interface on a path, or do you add the cost of the incoming ports too? AKA, from SwitchB's Gi0/2 path through SwitchD to SwitchC, do I add the cost of SwitchB's Gi0/1 (4) + SwitchD's Gi0/2 (4)? Or do I have to take into account SwitchC's Fa0/2(19) port as well?

1) Yes
2) No, It's 19+19. Switch A Fa0/2 is 100 mbps
3) No, It's 19+4. Switch C Fa0/2 is 100 mbps
4) No, Switch B's Gi0/2 has a higher cost and will be blocked.
5) While Switch D's Gi0/2 is a gigabit interface it is not running at 1gbps, it's running at 100mbps because switch C's Fa0/2 does not support 1gbps.

I hope this helps.

Priston

Alceo,

Switch D receives a BPDU from Switch C with a cost of 19
Switch B receives a BPDU from Switch D with a cost of 23

Alceo

Priston wrote: »

Alceo,
Switch D receives a BPDU from Switch C with a cost of 19
Switch B receives a BPDU from Switch D with a cost of 23

I think that this is not correct, quoting from Odom's book:
"For each received hello, the switch adds the cost listed in the hello BPDU to the cost of the incoming interface (the interface
on which the hello was received)."

So I think that C sends hello with cost 0 being the Root.
D receives this hello (cost =0) and adds the cost of its interface (100 Mbps), so it sends an hello with cost 19
B receives the hello from D (cost=19) and adds the cost of its interface (1000Mbps) -> Cost 23

Am I missing something?

HAMP

socraticmethod wrote: »

Are the answers in this scenario correct? Why is it saying that SwitchA's Fa0/2 port is the designated port to reach SwitchC instead of SwitchB's Gi0/2?
Sounds like you're studying to hard. It isn't saying SwitchB Gio/2 isn't the designated port on that segment. That is just an option with the wrong wording and you shouldn't select it.
SwitchB, Gi0/1, is the root, but it is not a choice to select
SwitchB, Gi0/2, is the designated, but again, it isn't a choice to select.

erstand that default costs for FastEthernet ports is 19 and Gig ports is 4. But what happens on a Segment with both FastEthernet and Gig ports?


1) Is SwitchA's cost to reach straight to SwitchC 19? Correct!!!

2) What is SwitchB's cost to reach SwitchC by going through SwitchA? Is it 4+19=23? Correct!!!

3) What about SwitchB's cost to reach SwitchC by going through SwitchD, is it 4+4=8? Correct!!!

4) Wouldn't that mean that SwitchB's Gi0/2 port should be DP on that segment because it has a lower cost to root? No, it isn't the lower cost to the root

In red.

Priston

Alceo wrote: »

I think that this is not correct, quoting from Odom's book:
"For each received hello, the switch adds the cost listed in the hello BPDU to the cost of the incoming interface (the interface
on which the hello was received)."

So I think that C sends hello with cost 0 being the Root.
D receives this hello (cost =0) and adds the cost of its interface (100 Mbps), so it sends an hello with cost 19
B receives the hello from D (cost=19) and adds the cost of its interface (1000Mbps) -> Cost 23

Am I missing something?

I guess your right, I must have learned it wrong.

socraticmethod

HAMP wrote: »

SwitchB, Gi0/2, is the designated, but again, it isn't a choice to select..

That makes sense, in the Answer key, it says that answer A "SwitchA, Fa0/2" is the DP on that segment. Which kinda goes with my original question, is this scenario wrong?

Priston

Switch C is the root bridge
Switch C Fa0/1 is a designated port
Switch C Fa0/2 is a designated port
Switch A Fa0/1 is a root port
Switch A Fa0/2 is a designated port
Switch B Gi0/1 is a root port
Switch B Gi0/2 is a blocking or alternate port (No it is not designated)
Switch D Gi0/1 is a designated port
Switch D Gi0/2 is a root port

When spanning tree stops a loop from forming, one side will be a designated port and the other side will be a blocked (or alternate) port.
Switch A's root cost is 19 and Switch B's root cost is 23, since switch A has a lower cost to the root bridge switch A's fa0/2 port will be in a forwarding state and switch B's gi0/2 port with go into a blocking or discarding state.

socraticmethod

Priston wrote: »

Switch C is the root bridge
Switch C Fa0/1 is a designated port
Switch C Fa0/2 is a designated port
Switch A Fa0/1 is a root port
Switch A Fa0/2 is a designated port
Switch B Gi0/1 is a root port
Switch B Gi0/2 is a blocking or alternate port (No it is not designated)
Switch D Gi0/1 is a designated port
Switch D Gi0/2 is a root port

When spanning tree stops a loop from forming, one side will be a designated port and the other side will be a blocked (or alternate) port.
Switch A's root cost is 19 and Switch B's root cost is 23, since switch A has a lower cost to the root bridge switch A's fa0/2 port will be in a forwarding state and switch B's gi0/2 port with go into a blocking or discarding state.

Finally, the scenario makes sense after reading your comment and the one above, "For each received hello, the switch adds the cost listed in the hello BPDU to the cost of the incoming interface (the interface on which the hello was received)."

My confusion was on port cost and where it was being added (incoming interface).

Thanks y'all!

techfiend

Priston's way of determining the cost of the link speed is correct? EG. F0/1 -> G0/1 = 19. It's the way I've always understood it but both Odom and Lammle didn't do a very good job explaining it.

Alceo

I too had doubts about the cost in this scenario, so I read again the chapter about STP in Odom's book and found this:
"On interfaces that support multiple speeds, Cisco switches base the cost on the current actual speed. So, if an interface negotiates to use a lower speed, the default STP cost reflects that lower speed."
That' s pretty clear, I don't know how I missed it the first time I studied.

[Deleted User]

Remember how STP does the root bridge election. Lowest priority then MAC address. Since the priority is not listed (assume 32,768 default), I would assume the priority is the same. From there, look at the MAC address. Switch C will have the lowest MAC address and therefor becomes the root bridge and will receive designated ports by default on the switch since it doesn't need root ports to send data to itself. From there, B is correct because it needs to send data to root bridge hence root port and same with answer choice F because of the cost for a fastethernet is 19 I believe and Gi is 4. It is remembering the fundamentals of STP and the theory behind it. Once you have that, you are good to go. Hope this helps you out. Good luck!

techfiend

In this same scenario with switch c as the root what would be the alternate port if it was switch C G0/2 and switch A G0/2?

Switch A F0/1 because it's cost is 19 vs. 12 going through B and C? Would a root designated port lead to a blocked port?

james43026

Priston wrote: »

1) Yes
2) No, It's 19+19. Switch A Fa0/2 is 100 mbps
3) No, It's 19+4. Switch C Fa0/2 is 100 mbps
4) No, Switch B's Gi0/2 has a higher cost and will be blocked.
5) While Switch D's Gi0/2 is a gigabit interface it is not running at 1gbps, it's running at 100mbps because switch C's Fa0/2 does not support 1gbps.

I hope this helps.

This is spot on.

aldoilici1

techfiend wrote: »

In this same scenario with switch c as the root what would be the alternate port if it was switch C G0/2 and switch A G0/2?

Switch A F0/1 because it's cost is 19 vs. 12 going through B and C? Would a root designated port lead to a blocked port?

switch A G0/2 (former Fa0/2) will be RP --> switchB G0/2 will be DP
switch A F0/1 will be BP

techfiend

Thanks, I thought I may have read that root switches always connect to forward ports but I guess I didn't.