hi everybody,
i read really a lot of stuff to understand how STP calculate the designated path cost to the root bridge
and i apologize to those who would say "why do create a topic again about a thing that is well known",
but, again sorry, i never understand this thing deep down
So i create my own small lab to see what happens exactly: with GNS3 i emulate a CISCO SW with a router C3745 and the slot NM-16ESW. So Fa1/0 to 1/15 on each SW became working like in a switch
I do this simple lab with 4 SW like figure 1 and figure 2 show it
Figure 1 shows what i found on R4: designated path cost for Fa1/0 (= port 41) is 19
Figure 2 shows what i found on R2:: designated path cost for Fa1/0 (= port 41) is also 19
This is quite not comprehensive for me: why starting from Fa1/0 in R4 the path cost to R1 (SW root) is the same that starting from Fa1/0 in R2 ? This is greatly what i don't understand.
At the same time, i track the BPDUs with Wireshark (see in figure 3)
We see the MAC destination multicast 0180.c200.0000
But we see the source that is (in this exemple) Fa1/0 in R2: why the MAC is c402.1648.f100 instead of the real MAC c402.1648.0000?
Thank you
Regards
