Below is my output. Why is it that MST1 has a root cost of 100000 while MST0 has a cost of 0? The same bridge is the root for both of those instances and I have not modified the cost for any of the ports.
I will have to recreate a similar topology in our lab in the morning but from the output you have posted, it seems that the show spanning-tree root for MST0 shows the External Root Path Cost which is 0 if the CIST root bridge is located in the same region.
Just to reiterate about basic MST principles, you surely know that the MST0 is the only instance that reaches beyond the region boundary and essentially spans the entire switched topology, creating the Common and Internal Spanning Tree - CIST. The CIST is a tree that again has to have its root bridge. This bridge is elected in a normal way - it's basically the bridge in the entire switched domain having the lowest Bridge ID. However, each MST region individually also has its so-called CIST Regional Root Bridge for MST0 which is first elected as the switch that is closest (in means of External Root Path Cost - see below) to the true CIST root bridge, and only in case of tie using the lowest Bridge ID within the region. The distance to the global CIST root bridge is also dealt with in a special way: because each MST region should behave like a single bridge from outside, MST introduces the concept of an External Root Path Cost (ERPC). This ERPC is set to 0 in the entire region that contains the CIST root bridge, and is increased only on links that interconnect different MST regions. Once the ERPC is increased on an inter-region link, it is not increased inside the next region, only at the next inter-region link. The ERPC is thus effectively tunneled across regions. Once again, this is done in order for an MST region to appear like a single bridge from outside.
Now, if you have a single MST region, or if you are located in the region that contains the global CIST root bridge, the ERPC will be set to 0. This appears to corroborate the output for MST0 you have posted:
The Root section describes the global CIST root and the path towards it. The path cost here is set to 0, obviously expressing the External Root Path Cost. Then there is the Regional Root information - notice that it is the same Bridge ID so we're located in the same region that also contains the CIST root - and the cost towards it is 100000.
This makes me believe that the show span root command for MST0 actually displays the ERPC instead of internal RPC. But as I said, I will have to confirm this in a lab - so expect my reaction in 6-8 hours as it's a deep night here
I didn't realize that the 'sh spanning-tree root' command would show the external cost for the CIST root. That would cause a lot of confusion if you didn't map the VLANs to its own instance and just use the IST. I guess its best practice to reserve the IST for the purpose of converging the entire domain of switches.
It would make sense to see zero since we are within the "Root Virtual Bridge".
Studying for the SWITCH exam and was wondering about why this was coming up the way it was within a MST Region. This perfectly ties into INE's explanations on IST_Master and CST_Root examples from the new courses.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...