STP Questions

[erkange005]

Hello People,

I am stumped on a question from the practice quiz from Wendell Odom's book Routing and Switching 200-120 Official Cert Guide Library in the ICND2 section Chapter 2 question 5 which states:

 Switch SW3 is receiving only two hello BPDUs, both from the same root switch, received on the two interfaces listed as follows: 

SW3# show interfaces status

Port Name Status Vlan Duplex Speed Type

Fa0/13 connected 1 a-half a-100 10/100BaseTX

Gi0/1 connected 1 a-full a-1000 1000BaseTX 

SW3 has no STP-related configuration commands. The hello received on Fa0/13 lists root cost 10, and the hello received on Gi0/1 lists root cost 20. Which of the following is true about STP on SW3?

a. SW3 will choose Fa0/13 as its root port.

b. SW3 will choose Gi0/1 as its root port.

c. SW3’s Fa0/13 will become a designated port.

d. SW3’s Gi0/1 will become a designated port.

The answer is B.  Could someone please explain why the Gi0/1 port (at a cost of 20) will be the root port when it's cost is higher than Fa0/13's port at 10.  

2nd question: If Gi0/1 is the root port would that not make Fa0/13 the designated port which would be answer C?  In which case why is there only 1 correct answer?

[erkange005]

can anyone help?

[Jon Marshall]

For the root port election the switch adds the cost of the interface the BPDU was received on to the cost in the BPDU so -

fast Ethernet 100Mbps = cost of 19

gigabit cost = 4

which means -

fa0/13 = 10 + 19 = 29

gi0/1 = 20 + 4 = 24

so gi0/1 has the lowest cost path to root.

The above are costs for standard STP ie. not RSTP.

In terms of the designated port that is a port that leads away from the root switch not towards it.

It does depend on the topology of the network as to whether that port could become designated but from the description it sounds like the BPDUs are being received from the same upstream switch.

Jon 

[erkange005]

Thanks for the reply Jon.  Your answer makes sense.  I do have another question, can STP work on a half-duplex interface because I thought they usually run on full-duplex modes only.  Of course these are my thoughts after I posted this thread because I'm still researching STP which covers a lot of information. 

Thank you in advance

[Jon Marshall]

Yes STP can work on half duplex but you usually find most switch interconnects are full duplex.

In addition if one end is set to half duplex but the other end to full this can cause an STP loop because if the full duplex link is continually transmitting traffic the half duplex end never gets access to the shared media (shared from it's perspective) and so cannot send BPDUs.

 With RSTP a half duplex link can also impact on the convergence times when a link fails.

Jon

[erkange005]

Ok so what about my other question in the original post,

If Gi0/1 is the root port would that not make Fa0/13 the designated port which would be answer C?  In which case why is there only 1 correct answer?

[Jon Marshall]

Ok so what about my other question in the original post,

I answered that question also in my original response.

Just because the port is not elected root does not make it designated.

Designated ports lead away from the root. So imagine a switch with two connections to an upstream switch which then connected to the root switch.

This switch would receives BPDUs on both ports and elect a root port but that doesn't make the other port a designated port, it is simply blocked.

Like I said before it all depends on the topology of the network as to whether a port that is not elected the root port could become a designated port.

The question from the book is open to interpretation unless there was a diagram with it ie. both ports could be receiving BPDUs from the same upstream switch. In this case whether the upstream switch was the root switch or whether it lead to the root switch what I said above applies.

But each port could also be connected to totally different switches which eventually lead back to the same root switch in which case it is not as clear cut.

Can't say for sure without knowing more details.

Jon