MST Root behaviours between two regions

[D@1984]

Hi,

I have two switches that are configured for region A, and now I need to connect them to some switches that are in region B. there is concern around spanning tree loops and I want to make sure my configuration is right.

so my understanding is that as regions are different, they don't exchange mst instances, but vlans coordinated only via CIST. so they should elect a CIST bridge and then that decides how traffic flow and which redundant ports to get blocked. The trunk link between regions going to be configured for vlan 100 & 200. they are in MST4 in region B and MST1 in region A (I assume this doesn't matter as regions are not the same)

there is a link between my A and B switches and now I'm connecting a1-b1 and a2-b2. a1 has the priority of 0, so am I right that a1 will be collected as CIST root? so B switches see A switches as one virtual switch that is elected as root so one of the links in B region will be forwarding and the other is blocked for vlan 100 that is extended between two regions? 

Is the priority of 0 enough config to make sure A is selected as bridge root? or do I need other considerations? the region A and B switches are from different vendors, do I need any specific config to make this work?  on the port config itself (ports that connect both switches) do I need any bpdu protection like BPDU filter? what are the recommendation config for scenarios like this?

Thanks

[MHM Cisco World]

Check this video 

If it not answer ypur Q please inform us

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MHM

[Enes Simnica]

hello D@1984 and U're right, cause with different MST regions, VLANs 100/200 will follow the CIST path. Since A1 has priority 0, it should win as CIST root, making ur A switches appear as one virtual bridge to region B. The B switches will then block one of the inter-region links (likely the higher-cost path) for these VLANs.

But dont forget that the  Priority 0 should guarantee root election, but confirm all B-region switches have higher priorities. Don't use BPDU filter (it breaks STP), but BPDU guard on edge ports is fine. Keep inter-region trunks clean - just allow VLANs 100/200 and ensure both sides use consistent port costs. Vendor differences matter, they really do.... so please verify all switches use standard 802.1s MSTP (not PVST+) and have matching hello/forward-delay timers. and tell me if u want to LAB SOME MST LABs, we can dive deep, and if u have more question ping me...

• https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_50_se/configuration/guide/scg/swmstp.pdf
• https://www.cisco.com/c/en/us/support/docs/smb/switches/cisco-550x-series-stackable-managed-switches/kmgmt-1363-MSTP-Best-Practices-published.html 

-Enes

[D@1984]

thanks for your reply. I just checked the timers and they match between two pair of switch. also confirm the priority is higher on the other pair of switches. so would below command be sufficient in trunk links:

switchport trunk allowed vlan 
switchport mode trunk

the only other thing in one pain of switches I have : spanning-tree portfast bpduguard default, do I need to disable it on trunk links? 

the only other thing after I connect the devices, what commands should I run to confirm bridge root is the switch with priority 0 and that I didn't break anything? what would be the impact if my config somehow break the connection?if I disconnect the cables what the standard converged time would be port the port to get back to forwarding state.

Thanks

Thanks

[M02@rt37]

Hello D@1984 

Use show spanning-tree mst command to verify root bridge election and port states after connecting !

You don’t need to match instance numbers across regions, but you must ensure proper trunking and BPDU exchange acros the inter-region links ; do not enable bpdu filter or guard on these trunks, as it will block spaning tree operation.

[paul driver]

Hello
Each stp region is interconnected via a CST (common spanning tree) -in your case A-B, They will also have their own IST root(s) (elected by the lowest external path cost towards the the CIST root) and its this convergence IST<>CST that makes the interconnection between the stp regions loop free - CIST common Internal spanning-tree

A CIST root is elected for entire stp estate from the boundary switches in each region running on the CST. (lowest BID preferred)
IST roots are elected by the lowest external path cost towards the the CIST root

So now If you have multiple boundary ports (a1-b1 and a2-b2) , then its those links on the non CIST region end (B1-B2) will have boundary ports that should be blocked to negate loops between the regions just like you would see when connect dual links between two switches.

So based on what you have stated:
A1 -Regional & CIST root
A2- Regional root port should point towards A1 

B1 CIST root port (boundary) points towards A1
B1 is Regional root 
B2 boundary port should be alternate blocking
B2 Regional root port should point towards B1 


Note: -As from manual pruning, MST is a per instance STP so pruning an instance isn't recommended.

[D@1984]

thanks, when you say the whole stp estate, does it mean that I have to consider other switches in my calculation as well if they are in different regions? With B switches I know they are isolated but not sure about A switches and if it has connections with other regions (i.e if they are connected to region C) . but A switches have the priority set to 0, it would make sense that whoever configured them wants them to be the bridge route. 

[paul driver]

Hello
Absolutely - you need to consider all regions and especially non mst regions such as pvst+as then you could incur a pvst simulation scenario and cause an outage 
Do you have such topology ?

[D@1984]

I have to go and check the individual switches as I don't have a layer 2 diagram. what if I have a c switch which is also in region B (not connected to b switches as those are isolated)  but if I connect switch a1 to b1, as a1 connected to c and c is configured as region B, does that cause any issue? 

Thanks

[D@1984]

sorry forgot to mention the other switch (lets call it s) although has the same region and revision config but has priority of 0.

[MHM Cisco World]

You mentioned bdpu filter, 

Can İ know exactly what you try for?

MHM

[MHM Cisco World]

I dont know what you try to do but 

In MST there are two root 

CST root 

MSTI root 

How can I config SW to be root for each one 

For CST (between two MST region ) we use 

Mst 0 priority 

For MSTI (intra root of region) we use 

Mst x priority 

So all config key is mst instance number.

Hope this what you looking for

MHM

[paul driver]

Hello

---

D@1984 wrote:

I have to go and check the individual switches as I don't have a layer 2 diagram. what if I have a c switch which is also in region B (not connected to b switches as those are isolated)  but if I connect switch a1 to b1, as a1 connected to c and c is configured as region B, does that cause any issue? 

sorry forgot to mention the other switch (lets call it s) although has the same region and revision config but has priority of 0.

---

The boundary ports will always be the inter-region links, I would say they will alternate based on the elected CIST root wherever that maybe as the CIST root doesn't have to be a boundary switch, it can be any switch in any region

If you introduce a switch with the same mst priority as the current elected CIST of the estate, into either an existing region or a new one and by the stp election process it is has a lower route BID,route path cost,sending BID etc... it can be elected as the CIST root, thus your stp topology will converge towards that new CIST root switch

So having every switch with the same priority's is like not specifying them and leaving to the switches to elect based on additional stp values( RBID, RPC SBID etc..)

I would want to be deterministic in the root election a spanning-tree estate, so maybe if applicable you could tidy up the regions before you interconnect them?

[D@1984]

I'm thinking the safest option with least risk of interruption, how about changing the config on b switches to be part of region a, in that case, when I join them to region a and they exchange bpdu messages, they will receive superior bpdu from 'a1 switch and accept that as root since a1 switch have priority of 0. then basically I shouldn't be worried about what's connected to a1 and a2 switches already as everything currently seems to be working and I have no issues. is there any document regarding if there is any down time if I change the stp mst config? I'm still abit concern about multi vendor situation as well. how to check that switches are compatible? Thanks

[MHM Cisco World]

Region A - Region B 

Different vendors? No issue since MST is IEEE standard' so all SW can run it

Isolated each region via using bdpu filter? Yes you can do it but after be 1000% sure you have only one link between two region (site) and run bdpu guard as second line protection

Make both site have same region? No good idea' in which if any change in MST of any SW can cause total both sites stp re-election process.

MHM