STP Designated/Non-designated Port Election Confusion

Unanswered Question
Feb 12th, 2009

My BS in Computer Science is very old (used punch cards in college), I lost my job, and am trying to upgrade my skills. I am enrolled in the Cisco CCNA Exploration curriculum and am currently using the LAN Switching & Wireless text and study guide (SG). The example in my attachment is from the SG. All the textbook examples in the chapter on STP use 3 switches, which is a no-brainer. SG has problems that use 4 switches. My confusion from the book, my instructor, and the Internet is on exactly what cost path is used for designated/non-designated port election. The attachment shows my root bridge and the root ports. The instructions I have seen for doing designated port election seem to be contradictory. I have been led to understand also that any port opposite a root bridge port (which are all designated) is blocked unless it is a root port.

Consider the SWB-SWC segment. First thing to look at is the root cost path. Is this the cost of traffic going out of SWB-GI0/2 to the root bridge (4+19, SWB-SWC-SWA) or (4+4+4, SWB-SWC-SWD-SWA) or the cost of traffic coming into SWB-GI0/2 and going out the root port of SWB (cost 4, or is the cost of the SWB-SWC segment included, too, which would be 4+4?)?

Looking at the opposite port on the same segment (SWB-SWC), the values I would get using the same logic would be going out of SWC-GI0/2 to the root bridge (4+4, SWC-SWB-SWA) or (4+19+4, SWC-SWB-SWD-SWA) or the cost of traffic coming into SWC-GI0/2 and going out the root port of SWC (cost 4+4, or is the cost of the SWB-SWC segment included, too, which would be 4+4+4?)?

Depending on the answer to this, would you ever use in the calculation a path through a port that has already been configured to be blocked?

I have read where the BID is used if the cost is equal but also read that the port with the lowest ID is used if the cost is equal.

I may be making this harder than it is, but I want to be able to understand how it works. Thank you in advance for any light someone can shed on this.

I have this problem too.
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ullasupendran Thu, 02/12/2009 - 13:00


1.The root cost path is the lowest cost path to the root bridge.The interface or port that provides that least cost path for the swicth to the root bridge is the root port of the bridge. In the case of SWB the path SWB-SWA is of cost 4 which is out GI0/ its the root port.

DEsignated port is identified in regards to the segment. By defnition "desinated port is a port that provides the lowest cost path for a segment(segmen SWB-SWC) to reach the root bridge.In the case of SWB the port GI0/1 provides the lowest cost path for segments between SWB-SWC and SWB-SWD to the root bridge SWA.In this case the root port is also the designated port too.The designated ports and root ports will be forwarding .so the ports GI0/2 and FA0/1 will be blocking and prevents the possibility of loop.

Hope my brief explanation helps.

Rate it if it helps.



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