MST is showing root cost for MST0 as zero?

[vincehgov]

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.

#sh spanning-tree root

                                        Root    Hello Max Fwd

MST Instance           Root ID          Cost    Time  Age Dly  Root Port

---------------- -------------------- --------- ----- --- ---  ------------

MST0             24576 0012.019d.8d80         0    2   20  15  Po1

MST1             24577 0012.019d.8d80    100000    2   20  15  Po1

[Peter Paluch]

Hello Vincent,

Hmmm, this is interesting. What is the switch platform and the IOS version you are running? Also, can you post the output of the show span mst 0 and show span mst 1 commands? Thank you!

Best regards,

Peter

[vincehgov]

Hi Peter!

I'm running c3560-ipservicesk9-mz.122-58.SE2.bin on a WS-C3560-24PS.

Here's the output of those commands:

##### MST0    vlans mapped:   201-4094

Bridge        address 0024.51c0.3f00  priority      32768 (32768 sysid 0)

Root          address 0012.019d.8d80  priority      24576 (24576 sysid 0)

              port    Po1             path cost     0

Regional Root address 0012.019d.8d80  priority      24576 (24576 sysid 0)

                                      internal cost 100000    rem hops 19

Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6

Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1            Desg FWD 200000    128.3    P2p Edge

Fa0/2            Desg FWD 200000    128.4    P2p Edge

Fa0/4            Desg FWD 200000    128.6    P2p Edge

Fa0/5            Desg FWD 200000    128.7    P2p Edge

Fa0/6            Desg FWD 200000    128.8    P2p Edge

Fa0/7            Desg FWD 200000    128.9    P2p Edge

Fa0/8            Desg FWD 200000    128.10   P2p Edge

Fa0/9            Desg FWD 200000    128.11   P2p Edge

Fa0/11           Desg FWD 200000    128.13   P2p Edge

Fa0/12           Desg FWD 2000000   128.14   P2p Edge

Fa0/13           Desg FWD 200000    128.15   P2p Edge

Fa0/14           Desg FWD 200000    128.16   P2p Edge

Fa0/15           Desg FWD 200000    128.17   P2p Edge

Fa0/16           Desg FWD 200000    128.18   P2p Edge

Fa0/17           Desg FWD 200000    128.19   P2p Edge

Fa0/20           Desg FWD 200000    128.22   P2p Edge

Po1              Root FWD 100000    128.64   P2p

##### MST1    vlans mapped:   1-200

Bridge        address 0024.51c0.3f00  priority      32769 (32768 sysid 1)

Root          address 0012.019d.8d80  priority      24577 (24576 sysid 1)

              port    Po1             cost          100000    rem hops 19

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1            Desg FWD 200000    128.3    P2p Edge

Fa0/2            Desg FWD 200000    128.4    P2p Edge

Fa0/4            Desg FWD 200000    128.6    P2p Edge

Fa0/5            Desg FWD 200000    128.7    P2p Edge

Fa0/6            Desg FWD 200000    128.8    P2p Edge

Fa0/7            Desg FWD 200000    128.9    P2p Edge

Fa0/8            Desg FWD 200000    128.10   P2p Edge

Fa0/9            Desg FWD 200000    128.11   P2p Edge

Fa0/11           Desg FWD 200000    128.13   P2p Edge

Fa0/12           Desg FWD 2000000   128.14   P2p Edge

Fa0/13           Desg FWD 200000    128.15   P2p Edge

Fa0/14           Desg FWD 200000    128.16   P2p Edge

Fa0/15           Desg FWD 200000    128.17   P2p Edge

Fa0/16           Desg FWD 200000    128.18   P2p Edge

Fa0/17           Desg FWD 200000    128.19   P2p Edge

Fa0/20           Desg FWD 200000    128.22   P2p Edge

Po1              Root FWD 100000    128.64   P2p

Thanks!

Vince

[Peter Paluch]

Hi Vincent,

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:

##### MST0    vlans mapped:   201-4094

Bridge        address 0024.51c0.3f00  priority      32768 (32768 sysid 0)

Root          address 0012.019d.8d80  priority      24576 (24576 sysid 0)

             port    Po1             path cost     0

Regional Root address 0012.019d.8d80  priority      24576 (24576 sysid 0)

                                      internal cost 100000    rem hops 19

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

Best regards,

Peter

[vincehgov]

Wow, thank you Peter for this explanation.

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".

[shraddha.pajwani]

So if we have all the VLANs in default  MST0 instance, that communicate to another MST0 switch that is Netgear topology, would it cause any design or architect issue ?

[Tudor Adrian Negru]

Perfect explanation!

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. 

The OCG does a poor job at MST.