STP Vs. RSTP – Answer 2
This next #CCNA STP vs RSTP question zeroes in on RSTP scenarios, but the question and answers also emphasizes some of the similarities between the two. As usual, the answer post gives the answers, and the reasons why each answer is either wrong or right.
Related posts:
- STP Vs. RSTP Question 1
- STP Puzzles
- Overview of how to approach STP puzzles
- Link to the ICND2 book (to learn more STP)
General Advice
Before listing the answers, let me summarize a few key points that can let you quickly rule out some of the answers as incorrect.
Ruling Out Port Roles that do NOT Apply to STP
One key difference between RSTP and STP relates to two port roles added by RSTP: the alternate port and backup port role. STP simply does not include these port roles. So, knowing this fact, any of the answers that list “STP” as the protocol, and either alternate port or backup port as the port role, cannot possibly be true.
Root switches Cannot have Root Ports
Both STP and RSTP use the same logic of choosing one switch to be the root switch. Then, both STP and RSTP have each non-root switch determine its port that’s part of the best path back towards the root switch. That port is that switch’s root port.
As a side effect of the above rules, one switch – the root switch – does not have a root port. So, any question that identifies the root switch cannot then claim a root port exists on that switch!
Correct and Incorrect Answers
Answers 1, 3, 5 have the same reasons with RSTP as with STP, because these answers mention port roles that apply to both STP and RSTP (the root port role and the designated port role).
RSTP, SW1, G0/1, Root Port – Incorrect. SW1 is the root switch, so it cannot have a root port.
RSTP, SW1, G0/2, Backup Port – Incorrect. Backup ports require the switch (SW1) to have >1 port connected to the same collision domain, and this kind of topology does not exist on SW1.
RSTP, SW2, G0/2, Designated Port – Incorrect. This is actually a pretty interesting case for STP and RSTP. With SW1 as the root switch, it will advertise a root cost of 0 on the link between SW1 and SW2. Even if SW2’s root port is SW2’s G0/1, SW2’s root cost will be larger/worse than SW1’s root cost, because SW1 is the root switch. So, there is no possibility that SW2’s G0/2, which connects directly to the root switch, to become a designated port.
RSTP, SW2, G0/1, Alternate Port – Correct. Any configuration that results in: a) SW2’s G0/2 (which connects to SW1) being it’s root port, and b) SW3 becoming the designated port on the link between SW2 and SW3, makes it so SW2 meet the requirements to choose SW2’s G0/1 as an alternate port.
RSTP, SW3, G0/1, Root Port – Correct. With default configuration, this port will be part of SW3’s least cost path to reach the root switch (SW1).
RSTP, SW3, G0/2, Backup Port – Incorrect. Backup ports require the switch (SW3) to have >1 port connected to the same collision domain, and this kind of topology does not exist on SW3.
Regarding #4 and #6 …
#4 Is this True because SW2 G0/1 is not in blocking state due to winning the tie breaker with SW3? Resulting in SW3 G0/2 to block on that port? Basically the reason why this is an alternate port is due to the port is actively forwarding traffic? I don’t know if I am making much sense..
#6 I think I just remembered after making the wrong choice. Has to do with the only way to get a backup port is to be using a hub with multiple connections going to SW2, which is not shown here. Correct?
Hi Gabriel. An alternate port is any port that receives hellos that identify the same root switch that receives the Root port, but also, alternate ports (and backup ports also) always stay in discarding state, until the root port fails and then it goes to forwarding state instantly, without even passing for a learning state since this is why you define an alternate port, to be the second best root. Then, in RSTP, the only ports that apply to have an alternate role, are those which after defining the main roles (root and designated ports) remain in discarding state. Then if you see the question, if SW3 Gi0/2 wins to competition for the designated role, the SW2 Gi0/1 stays in discarding state and then it becomes the alternate port for the SW2. Ports that get a root or designated role, can´t be considered as alternate since they already have another role.
Hi – regarding RSTP, SW2, G0/1, Alternate Port.
The textbook says the criteria for an alt port are both both the RP and the alternate port must receive Hellos that identify the same root switch.
I’m not sure what this has to do with SW3 being a DP on the link between S3 and S2.
Thanks for your reply, Christopher. The need to have >1 port for a port to be a backup in RSTP was confusing because R3’s G0/1 and G0/2 seemed to me to be >1. I assumed that the S2-S3 link would be blocking, but your explanation was helpful: RP and DP can’t be alternates.