This post represents the solution and explanation for quiz-19.
Have a look at it to understand the problem.

Quiz Review

This quiz talks about making configuration changes to the MSTP by modifying the vlan to instance mapping. There are 4 switches (Dist-1, Dist-2, Acc-1 and Acc-2), all configured to run MSTP with one region:

MSTP with one region

  • Dist-1 is primary root for MST0 and MST1 and secondary root for MST2
  • Dist-2 is primary root for MST2 and secondary root for MST0 and MST1

At this moment, the network engineer creates vlan 200 and then he maps it to instance/MST 2 on all switches, in this order: Acc-1 --> Acc-2 --> Dist-1 --> Dist-2 which causes some sensitive applications (connected to Acc-1 and Acc-2) to experience short network cuts, alerting the server team.

MSTP Review

Before explaining why this happens, let's review some of the characteristics of MSTP:

  • MSTP uses the concept of regions = a collection of switches that share the same MSTP configuration
  • the following must match for two switches to consider themselves in the same region:
    • configuration name
    • revision number
    • vlan to instance mapping

If any of the above (configuration name, revision number or vlan-to-instance mapping) is different, the switches will be in separate regions !
  • switches do not exchange the vlan-to-instance mapping but instead, they compute a hash of this mapping and exchange it between them
  • vlans that are not mapped to a specific instance will be automatically in IST / MST 0

Usually you configure a single MSTP Region for your network. Of course, there might be cases when more regions would make more sense, but these are corner cases.

In case that multiple MSTP Regions exist, you have to remember the following rules:

  • the switch with the lowest Bridge ID among all regions will be selected as CIST Root (Common and Internal Spanning Tree)
  • the links between the regions are known as boundaries
  • switches that contain boundary links are known as boundary switches
  • each region will elect a Regional Root based on the lowest external cost toward the CIST Root - only boundary switches are eligible for this election !

One result of the above rules is: between two regions, only one boundary port will be in FWD state - the rest of boundary ports will be in BLOCKING state.

Quiz Explanation

As soon as the junion engineer changes the vlan-to-instance mapping (by moving vlan 200 to instance 2), he creates un-intentionally more MSTP regions because the hash of the vlan-to-instance mapping will be different. This will trigger some links to become boundary ports and to transition from FWD to BLK or vice-versa.
The next diagram shows how the ports' state will change for vlans in instance 2 / MST 2, as the engineer made the configuration touching the switching in this order Acc-1 --> Acc-2 --> Dist-1 --> Dist-2:

mstp-states-for-mst-2 How Can MSTP Configuration Changes Impact Your Network

A continuous ping between two hosts in vlan 150 (mapped to MST2), connected to Acc-1 and Acc-2 respectively, shows that connectivity is lost during re-convergence. The time of impact could be different depending on whether the portfast/edge feature is configured or not on the ports connected to the hosts:

- portfast not enabled - note the 30 sec outage (2x forward delay):

host-2#ping timeout 1 repeat 10000000

Type escape sequence to abort.
Sending 10000000, 100-byte ICMP Echos to, timeout is 1 seconds:

- portfast enabled - note the 2 sec outage:



For the first time since writing quizzes on this website, this quiz does not have any solution !! It is not possible to change the vlan-to-instance mapping on all 4 switches without causing at least a short network cut !
The only way to do such configuration changes to the MSTP in production environments is by scheduling a maintenance window approved by all teams impacted.
Of course, not all MST instances are impacted the same: for example, after reconvergence some interfaces could maintain the same status (BLK or FWD) for some instances/vlans but not for other ones.

My point of view is: in networks like the one in diagram with MSTP between Access Layer and Aggregation Layer, you can count how many possible ways/paths exist between the Access Layer and Aggregation Layer (usually equal to the number of uplinks) - and this case (and most of the cases) this number is 2: from access switches to aggregation there are only two uplinks / paths. In this case, with two MST instances you cover all situations. As a result, you map half of the vlans to instance 1 and the other half to instance 2, so that you will never have to change the mapping again.
Remember that it is always a good practice with MSTP to avoid leaving vlans in IST / MST 0 instance (more about this in future quizzes/posts.

In the end of this article, I'm attaching another diagram with the states for vlan 200 (the one that is newly added into production). During the process, this vlan 200 will be mapped to IST / MST 0 in some regions and to MST 2 in others. As you can see from this diagram, the impact on vlan 200 is much less than for vlans 100-199 (MST 2) shown above - this is because most of the times, the same interfaces remain blocked:

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