7 - STP - Foundation Per-VLASpanning Tree Concepts 2N 

Switch A is the root switch (the dingy little switch in the cupboard lol) whereas we want

Switch G, as all links/paths are going to find the BEST route to this switch, so it is very IMPORTANT 

that the correct switch is elected the root switch.

So now the flood of traffic that is coming into that distribution switch is now heading to that poking little access switch in the closet!  So the limit of our network is the backplane of that root bridge (which could be like a 1900 switch lol)

So potentially we could be looking at the access switch crashing during peak times of the day and the whole network goes down...until spanning tree re converges and finds a new root path and starts unblocking ports etc

The root switch should be one of our CORE switches,

So looking at the topology with tweaking the priority PER VLAN:

WOW! just think about it, right now im thinking of the spanning tree instances at work....you really need to draw this out to see the big picture and how exactly traffic is flowing .... as we have seeen, that DEFAULT doesn't always equal best.

Lets verify the above, if we look at switch A, we see that to get to the ROOT Bridge/ID it goes out of fa0/11 and we can see the MAC of Switch B,

Bridge ID, it Switch A (see MAC)

100MB links = cost 19

So looking alittle further down the output on Switch A, we can see the port states,

Lets have a look at the first command:

spanning-tree vlan x root primary

this sets the priority to the IEEE recommended standard of 24577

So lets test this out, lets ping from host 10.1.1.10 to host 10.1.1.5, traffic will flow this way

I will pull out the link between SwitchA and SwitchB, causing spanning tree to re-converge:

still waiting .....

RIGHT, Lets check Switch C

fas0/24 is now in a FWD start and is the Root Port!

so we could be looking at 50 seconds total if the port is a blocked port (the backup link) that is NOT ACCEPTABLE in todays standards.





tags:
PVST, spanning-tree