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STP Question - picture included

S

Sauro

Senior member
Please tell me I'm not going crazy here and that this question is simply incorrect. As I understand STP, switch B will be the root, making its f0/0 and f0/1 DP. The ports on the ends of those two segments will be Root Ports. The question in question lists these same results. My trouble comes in when trying to figure out the DP on the bottom segment. I believe Switch C to be the winner of this segment seeing as how its MAC is lower therefore making Switch C's f0/1 the DP of that segment. This question is telling me I'm wrong. Ahy help would be appreciated.

 
I first example switch A has lower bridge ID (since same priority, and same path cost to root, lowest mac wins), therefore it is the designated port for that segment.

On example 2, think about path cost to root. I'm not going to give you the answer. You'll have to remember the default port costs for fast and gig interfaces to calculate it.
 
Regarding the first example - how is 0000.0ace.0000 not lower than 0000.cafe.1111?
Regarding example two - shouldn't the link cost as a whole be considered? By that I mean the link cost being 19 on the SwA-SwB segment since it's only going to be 100meg?
 
Sauro said:
Regarding the first example - how is 0000.0ace.0000 not lower than 0000.cafe.1111?
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It is - switch B will become the root bridge with its lowest MAC. Ports Fa0/0 and Fa0/1 will become designated ports as you already know.

Your last choice is the designated port election between A and C. Switch A (0f00.cafe.1111) is lower than Switch C (0f00.face.0000). So Switch A Fa0/1 is designated port and Switch C Fa0/1 is blocking.

Sauro said:
Regarding example two - shouldn't the link cost as a whole be considered? By that I mean the link cost being 19 on the SwA-SwB segment since it's only going to be 100meg?
Click to expand...

Yes. Switch A > Switch B and Switch C > Switch D would operate at 100Mb and have a cost of 19 due to the lower speed negotiation. To start, identify the root bridge. Then for each non-root bridge, calculate each port cost towards the root bridge.
 
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VirtualLarry said:
Is this kind of knowledge useful in the real world? As long as you are careful not to create bridging loops?
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If you expect to be able to debug spanning tree problems, then yes, you should understand how it works. Being "careful not to create bridging loops" can be easier said than done, especially when you have users that do stupid things. Ever seen an unmanaged switch in someone's office with a cable going between two ports on the same switch? I have. Thank goodness for BPDU guard.
 
jlazzaro said:
Your last choice is the designated port election between A and C. Switch A (0f00.cafe.1111) is lower than Switch C (0f00.face.0000). So Switch A Fa0/1 is designated port and Switch C Fa0/1 is blocking.
Click to expand...

How is 0f00.cafe.1111 lower than 0f00.face.0000?


And in example two - Switch A would be root, correct? I've selected what I believe to be correct in the original images (the check boxes are what I believe to be correct, the coloring is what the quiz believes it be correct/incorrect [red = wrong , green= correct], That being said do you all also believe this quiz is wrong?)


Edit:::After reading this again I'm wondering if by larger/smaller I'm looking at the wrong bits. I have been looking at the last bit first to determine large vs small - should I be starting from the front of the MAC? Man I'm starting to feel dumb. The books I've been reading and the Cisco learning video all used the final two bits to differentiate between MACs - I think this is what made me compare the final bits first - someone please confirm this for me...
 
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yeah its useful. you need to know how to use STP/LACP - sometimes more advanced protocols are not available or feasible. can't run BGP4 on everything 😉

and don't forget san infrastructure it's even more important since devices are less tolerant of timeout when convergence has to occur due to failover.

Moreso build your systems to use active/active to keep the links as close to 100% utilization as possible. mac hashing/ip hashing is evil 😉 can't wait for cheap 10gbe
 
Sauro said:
How is 0f00.cafe.1111 lower than 0f00.face.0000?


And in example two - Switch A would be root, correct? I've selected what I believe to be correct in the original images (the check boxes are what I believe to be correct, the coloring is what the quiz believes it be correct/incorrect [red = wrong , green= correct], That being said do you all also believe this quiz is wrong?)


Edit:::After reading this again I'm wondering if by larger/smaller I'm looking at the wrong bits. I have been looking at the last bit first to determine large vs small - should I be starting from the front of the MAC? Man I'm starting to feel dumb. The books I've been reading and the Cisco learning video all used the final two bits to differentiate between MACs - I think this is what made me compare the final bits first - someone please confirm this for me...
Click to expand...

Yes, always start in the front to determine the smallest MAC address.
 
Jamsan said:
Yes, always start in the front to determine the smallest MAC address.
Click to expand...

Most significant bit/byte, the leftmost one.

On the importance of this in the real world. It is SUPREMELY important in the design and troubleshooting of modern data centers and networks. If you don't know and plan out your layer2 spanning-tree you can have traffic traveling all over the place through switches you wouldn't think would be in the path. With data centers getting more and more "flat" it's becoming even more important. I would consider knowing this one of the most important from a design perspective and even more so with troubleshooting.

spanning-tree is the "routing protocol" at layer2 but without all the control. It's that important. L2 networks in modern data centers can span dozens if not more switches especially those damn blade switches. Thank god we're getting rid of those HP blade enclosures and eliminating all the complexity they present. That's what most of my work now is. Replacing HP blade enclosures with Cisco UCS and simplifying everything.
 
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