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Sim 02: MST Regions and Instances

Blueprint 3.1.c · Difficulty: Medium · Time target: 15 minutes

The campus runs Rapid PVST+ with 30 VLANs and the team wants fewer spanning-tree instances. Migrate the three switches to MST so that the two distribution switches share the load and the access switch sits in the same region.

Download the lab

CML lab file · EVE-NG lab file

4 nodes. The lab boots in its starting configuration. CML: Import a lab, then start it. EVE-NG: import the .unl from the lab list; if your image folder names differ, edit each node's image.

Topology

graph TB
  D1["DIST1"]
  D2["DIST2"]
  A1["ACC1"]
  H1["Host1 VLAN 10"]
  D1 -- "Gi0/0 to Gi0/0 trunk" --- D2
  D1 -- "Gi0/1 to Gi0/1 trunk" --- A1
  D2 -- "Gi0/1 to Gi0/2 trunk" --- A1
  A1 -- "Gi1/0 access VLAN 10" --- H1
Node CML image
DIST1, DIST2, ACC1 IOSvL2
Host1 (optional) Alpine or Ubuntu

Pre-configured state

  • VLANs 10, 20, 30 and 40 exist on all three switches.
  • All inter-switch links are static 802.1Q trunks and are up.
  • DIST1 and DIST2 run Rapid PVST+.
  • A colleague started the migration on ACC1 and already enabled MST there with a region configuration.
  • ACC1 Gi1/0 is an access port in VLAN 10.

Starting configuration

hostname DIST1
spanning-tree mode rapid-pvst
!
vlan 10,20,30,40
!
interface range GigabitEthernet0/0 - 1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
end

hostname DIST2
spanning-tree mode rapid-pvst
!
vlan 10,20,30,40
!
interface range GigabitEthernet0/0 - 1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
end

hostname ACC1
!
vlan 10,20,30,40
!
spanning-tree mst configuration
 name campus
 revision 1
 instance 1 vlan 10, 20
 instance 2 vlan 30, 40
!
spanning-tree mode mst
!
interface range GigabitEthernet0/1 - 2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
!
interface GigabitEthernet1/0
 switchport mode access
 switchport access vlan 10
end

Tasks

  1. Configure all three switches to run MST in a single region named CAMPUS (upper case), revision 1.
  2. Map VLANs 10 and 20 to MST instance 1 and VLANs 30 and 40 to MST instance 2. All other VLANs stay in instance 0 (the IST).
  3. Make DIST1 the root bridge for instances 0 and 1, and the backup root for instance 2.
  4. Make DIST2 the root bridge for instance 2, and the backup root for instances 0 and 1.
  5. Set explicit bridge priorities: 24576 for a root and 28672 for a backup root.
  6. On ACC1, configure Gi1/0 so that it moves straight to forwarding and is err-disabled if it ever receives a BPDU.

Constraints

  • Do not use the spanning-tree mst ... root primary or root secondary macros. Set priorities directly.
  • Do not change any trunk or VLAN configuration.
  • Do not use the global spanning-tree portfast default or bpduguard default commands. Configure ACC1 Gi1/0 only.
Solution

What was wrong on ACC1: the region name was campus. MST region names are case sensitive, so ACC1 would sit in a different region from the distribution switches and its uplinks would be region boundary ports. All three attributes (name, revision and the full VLAN-to-instance map) must match exactly.

spanning-tree mst configuration
 name CAMPUS
 revision 1
 instance 1 vlan 10, 20
 instance 2 vlan 30, 40
!
spanning-tree mode mst
spanning-tree mst 0-1 priority 24576
spanning-tree mst 2 priority 28672
end

spanning-tree mst configuration
 name CAMPUS
 revision 1
 instance 1 vlan 10, 20
 instance 2 vlan 30, 40
!
spanning-tree mode mst
spanning-tree mst 0-1 priority 28672
spanning-tree mst 2 priority 24576
end

spanning-tree mst configuration
 name CAMPUS
!
interface GigabitEthernet1/0
 spanning-tree portfast
 spanning-tree bpduguard enable
end

Changes inside spanning-tree mst configuration are applied when you leave the submode. Configure the region before you change the mode, so the switch does not briefly run MST with an empty region.

Priorities must be multiples of 4096. On an access port, spanning-tree portfast is enough. spanning-tree portfast edge is the same feature with the newer keyword on releases that support it.

Exam trap

The region name is case sensitive and the revision number is not incremented for you. A switch whose name, revision or VLAN map differs by a single character is in its own region, and everything still "works", so the only clue is the boundary port type in show spanning-tree mst.

Verification 

DIST1# show spanning-tree mst configuration

Name      [CAMPUS]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-9,11-19,21-29,31-39,41-4094
1         10,20
2         30,40
Run the same command on DIST2 and ACC1. The name, revision and every line of the VLAN map must be identical. Three instances are reported because the IST (instance 0) counts.

DIST1# show spanning-tree mst 1

##### MST1    vlans mapped:   10,20
Bridge        address ...  priority      24577 (24576 sysid 1)
Root          this switch for MST1
The displayed priority is the configured value plus the instance number (the system ID extension), so 24576 shows as 24577 in MST1 and 24578 in MST2.

DIST2# show spanning-tree mst 2
ACC1# show spanning-tree mst
ACC1# show spanning-tree interface GigabitEthernet1/0 detail
DIST2 reports Root this switch for MST2. On ACC1 the uplink to DIST1 is the root port for MST0 and MST1, and the uplink to DIST2 is the root port for MST2. No port type on ACC1 contains Bound, which proves the switch is inside the region. Gi1/0 shows The port is in the portfast mode (or portfast edge) and Bpdu guard is enabled.