Unexpected MSTI bridge ID?

txfz · February 17, 2026, 3:38pm

Hi,

Don’t have much experience with MSTP. Seeing some strange output.

Switch 1 looks fine.

> interface/bridge/print  
Flags: D - dynamic; X - disabled, R - running 
 0  R name="bridge" mtu=auto actual-mtu=1500 l2mtu=1584 arp=enabled arp-timeout=auto 
      mac-address=78:9A:18:3A:04:CB protocol-mode=mstp fast-forward=yes igmp-snooping=no auto-mac=no 
      admin-mac=78:9A:18:3A:04:CB ageing-time=5m priority=0x8000 max-message-age=20s forward-delay=15s 
      transmit-hold-count=6 region-name="" region-revision=0 max-hops=20 vlan-filtering=yes 
      ether-type=0x8100 pvid=1 frame-types=admit-all ingress-filtering=yes dhcp-snooping=no 
      port-cost-mode=long mvrp=no max-learned-entries=auto

> interface/bridge/msti/monitor 1
                    state: enabled                 
               identifier: 2                       
      current-mac-address: 78:9A:18:3A:04:CB       
                bridge-id: 0x8000.78:9A:18:3A:04:CB
              root-bridge: yes                     
  regional-root-bridge-id: 0x8002.78:9A:18:3A:04:CB
           root-path-cost: 0                       
                root-port: none                    
               port-count: 25                      
    designated-port-count: 7

Why does switch 2 get the same MAC address/bridge ID? Consequently, they both think they are the root bridge?

> interface/bridge/print 
Flags: D - dynamic; X - disabled, R - running 
 0  R name="bridge" mtu=auto actual-mtu=1500 l2mtu=1584 arp=enabled arp-timeout=auto 
      mac-address=78:9A:18:12:C8:70 protocol-mode=mstp fast-forward=yes igmp-snooping=no auto-mac=no 
      admin-mac=78:9A:18:12:C8:70 ageing-time=5m priority=0x8000 max-message-age=20s forward-delay=15s 
      transmit-hold-count=6 region-name="" region-revision=0 max-hops=20 vlan-filtering=yes 
      ether-type=0x8100 pvid=1 frame-types=admit-all ingress-filtering=yes dhcp-snooping=no 
      port-cost-mode=long mvrp=no max-learned-entries=auto

> interface/bridge/msti/monitor 1
                    state: enabled                 
               identifier: 2                       
      current-mac-address: 78:9A:18:3A:04:CB       
                bridge-id: 0x8000.78:9A:18:3A:04:CB
              root-bridge: yes                     
  regional-root-bridge-id: 0x8002.78:9A:18:3A:04:CB
           root-path-cost: 0                       
                root-port: none                    
               port-count: 25                      
    designated-port-count: 6

erlinden · February 17, 2026, 3:40pm

Did you copy the config of switch 1 to switch 2?
Can you share both configs?

/export file=anynameyoulike

Remove serial and any other private info, post as Preformatted text by using the </> button.

txfz · February 17, 2026, 3:51pm

Sure.

S1

# 2026-02-17 16:46:50 by RouterOS 7.20.8
#
# model = CRS326-24S+2Q+
/interface bridge add admin-mac=78:9A:18:3A:04:CB auto-mac=no name=bridge protocol-mode=mstp vlan-filtering=yes
/interface ethernet set [ find default-name=sfp-sfpplus1 ] comment=HV1
/interface ethernet set [ find default-name=sfp-sfpplus2 ] comment=HV2
/interface ethernet set [ find default-name=sfp-sfpplus3 ] comment=HV3
/interface ethernet set [ find default-name=sfp-sfpplus4 ] comment=HV4
/interface ethernet set [ find default-name=sfp-sfpplus5 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus6 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus7 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus8 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus9 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus10 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus11 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus12 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus13 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus14 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus15 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus16 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus17 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus18 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus19 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus21 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus22 ] disabled=yes
/interface vlan add interface=ether1 name=MGMT vlan-id=5
/interface bonding add lacp-rate=1sec mlag-id=10 mode=802.3ad name=mlag-hv1 slaves=sfp-sfpplus1
/interface bonding add lacp-rate=1sec mlag-id=20 mode=802.3ad name=mlag-hv2 slaves=sfp-sfpplus2
/interface bonding add lacp-rate=1sec mlag-id=30 mode=802.3ad name=mlag-hv3 slaves=sfp-sfpplus3
/interface bonding add lacp-rate=1sec mlag-id=40 mode=802.3ad name=mlag-hv4 slaves=sfp-sfpplus4
/interface bonding add lacp-rate=1sec mlag-id=1 mode=802.3ad name=mlag-rt11 slaves=sfp-sfpplus23
/interface bonding add lacp-rate=1sec mlag-id=2 mode=802.3ad name=mlag-rt12 slaves=sfp-sfpplus24
/interface bonding add mode=802.3ad name=qsfp slaves=qsfpplus1-1,qsfpplus1-2,qsfpplus1-3,qsfpplus1-4,qsfpplus2-1,qsfpplus2-2,qsfpplus2-3,qsfpplus2-4
/interface list add name=NEIGHBOR_DISCOVERY
/interface list add name=OOB
/interface list add name=K2
/port set 0 name=serial0
/user group add name=backup policy=ssh,ftp,read,write,policy,test,sniff,sensitive,api,!local,!telnet,!reboot,!winbox,!password,!web,!romon,!rest-api
/interface bridge mlag set bridge=bridge peer-port=qsfp priority=64
/interface bridge msti add bridge=bridge identifier=2 vlan-mapping=19
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus1 pvid=999
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus2 pvid=999
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus3 pvid=999
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus4 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus5 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus6 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus7 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus8 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus9 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus10 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus11 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus12 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus13 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus14 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus15 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus16 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus17 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus18 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus19 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus20 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus21 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=sfp-sfpplus22 pvid=999
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus23 pvid=999
/interface bridge port add bridge=bridge disabled=yes frame-types=admit-only-vlan-tagged interface=sfp-sfpplus24 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=qsfp pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-hv4 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-hv1 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-hv2 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-hv3 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-rt11 pvid=999
/interface bridge port add bridge=bridge frame-types=admit-only-vlan-tagged interface=mlag-rt12 pvid=999
/ip neighbor discovery-settings set discover-interface-list=NEIGHBOR_DISCOVERY protocol=lldp
/interface bridge vlan add bridge=bridge tagged=mlag-rt11,mlag-rt12,qsfp,K2 vlan-ids=20-29
/interface bridge vlan add bridge=bridge tagged=sfp-sfpplus20,mlag-rt11,mlag-rt12,qsfp,K2 vlan-ids=9
/interface bridge vlan add bridge=bridge tagged=mlag-rt11,mlag-rt12,qsfp,K2 vlan-ids=19
/interface list member add interface=bridge list=NEIGHBOR_DISCOVERY
/interface list member add interface=mlag-hv4 list=K2
/interface list member add interface=mlag-hv1 list=K2
/interface list member add interface=mlag-hv2 list=K2
/interface list member add interface=mlag-hv3 list=K2
/ip address add address=10.7.5.111/24 interface=MGMT network=10.7.5.0
/ip cloud set update-time=no
/ip dns set servers=8.8.8.8,1.1.1.1
/ip route add gateway=10.7.5.101
/ip service set ftp disabled=yes
/ip service set telnet disabled=yes
/ip service set www disabled=yes
/ip service set api-ssl disabled=yes
/ip ssh set strong-crypto=yes
/snmp set enabled=yes
/system clock set time-zone-autodetect=no time-zone-name=Europe/Stockholm
/system logging add disabled=yes topics=stp
/system ntp client set enabled=yes
/system ntp client servers add address=10.7.5.101
/system package update set channel=long-term
/system routerboard settings set auto-upgrade=yes enter-setup-on=delete-key
/tool mac-server set allowed-interface-list=none
/tool mac-server mac-winbox set allowed-interface-list=OOB
/tool mac-server ping set enabled=no

S2

# 2026-02-17 16:46:54 by RouterOS 7.20.8
#
# model = CRS326-24S+2Q+
/interface bridge add admin-mac=78:9A:18:12:C8:70 auto-mac=no name=bridge protocol-mode=mstp vlan-filtering=yes
/interface ethernet set [ find default-name=sfp-sfpplus1 ] comment=HV1
/interface ethernet set [ find default-name=sfp-sfpplus2 ] comment=HV2
/interface ethernet set [ find default-name=sfp-sfpplus3 ] comment=HV3
/interface ethernet set [ find default-name=sfp-sfpplus4 ] comment=HV4
/interface ethernet set [ find default-name=sfp-sfpplus5 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus6 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus7 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus8 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus9 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus10 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus11 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus12 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus13 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus14 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus15 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus16 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus17 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus18 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus19 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus20 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus21 ] disabled=yes
/interface ethernet set [ find default-name=sfp-sfpplus22 ] disabled=yes
/interface vlan add interface=ether1 name=MGMT vlan-id=5
/interface vlan add interface=bridge name=VM vlan-id=23
/interface bonding add lacp-rate=1sec mlag-id=10 mode=802.3ad name=mlag-hv1 slaves=sfp-sfpplus1
/interface bonding add lacp-rate=1sec mlag-id=20 mode=802.3ad name=mlag-hv2 slaves=sfp-sfpplus2
/interface bonding add lacp-rate=1sec mlag-id=30 mode=802.3ad name=mlag-hv3 slaves=sfp-sfpplus3
/interface bonding add lacp-rate=1sec mlag-id=40 mode=802.3ad name=mlag-hv4 slaves=sfp-sfpplus4
/interface bonding add lacp-rate=1sec mlag-id=1 mode=802.3ad name=mlag-rt11 slaves=sfp-sfpplus23
/interface bonding add lacp-rate=1sec mlag-id=2 mode=802.3ad name=mlag-rt12 slaves=sfp-sfpplus24
/interface bonding add mode=802.3ad name=qsfp slaves=qsfpplus1-1,qsfpplus1-2,qsfpplus1-3,qsfpplus1-4,qsfpplus2-1,qsfpplus2-2,qsfpplus2-3,qsfpplus2-4
/interface list add name=NEIGHBOR_DISCOVERY
/interface list add name=OOB
/interface list add name=K2
/port set 0 name=serial0
/user group add name=backup policy=ssh,ftp,read,write,policy,test,sniff,sensitive,api,!local,!telnet,!reboot,!winbox,!password,!web,!romon,!rest-api
/interface bridge mlag set bridge=bridge peer-port=qsfp
/interface bridge msti add bridge=bridge identifier=2 vlan-mapping=19
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus1
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus2
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus3
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus4
/interface bridge port add bridge=bridge interface=sfp-sfpplus5
/interface bridge port add bridge=bridge interface=sfp-sfpplus6
/interface bridge port add bridge=bridge interface=sfp-sfpplus7
/interface bridge port add bridge=bridge interface=sfp-sfpplus8
/interface bridge port add bridge=bridge interface=sfp-sfpplus9
/interface bridge port add bridge=bridge interface=sfp-sfpplus10
/interface bridge port add bridge=bridge interface=sfp-sfpplus11
/interface bridge port add bridge=bridge interface=sfp-sfpplus12
/interface bridge port add bridge=bridge interface=sfp-sfpplus13
/interface bridge port add bridge=bridge interface=sfp-sfpplus14
/interface bridge port add bridge=bridge interface=sfp-sfpplus15
/interface bridge port add bridge=bridge interface=sfp-sfpplus16
/interface bridge port add bridge=bridge interface=sfp-sfpplus17
/interface bridge port add bridge=bridge interface=sfp-sfpplus18
/interface bridge port add bridge=bridge interface=sfp-sfpplus19
/interface bridge port add bridge=bridge interface=sfp-sfpplus20
/interface bridge port add bridge=bridge interface=sfp-sfpplus21
/interface bridge port add bridge=bridge interface=sfp-sfpplus22
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus23
/interface bridge port add bridge=bridge disabled=yes interface=sfp-sfpplus24
/interface bridge port add bridge=bridge interface=qsfp
/interface bridge port add bridge=bridge interface=mlag-hv4
/interface bridge port add bridge=bridge interface=mlag-hv1
/interface bridge port add bridge=bridge interface=mlag-hv2
/interface bridge port add bridge=bridge interface=mlag-hv3
/interface bridge port add bridge=bridge interface=mlag-rt11
/interface bridge port add bridge=bridge interface=mlag-rt12
/ip neighbor discovery-settings set discover-interface-list=NEIGHBOR_DISCOVERY protocol=lldp
/interface bridge vlan add bridge=bridge tagged=mlag-rt11,mlag-rt12,qsfp,K2 vlan-ids=9,11-29
/interface list member add interface=bridge list=NEIGHBOR_DISCOVERY
/interface list member add interface=mlag-hv4 list=K2
/interface list member add interface=mlag-hv1 list=K2
/interface list member add interface=mlag-hv2 list=K2
/interface list member add interface=mlag-hv3 list=K2
/ip address add address=10.7.5.112/24 interface=MGMT network=10.7.5.0
/ip address add address=10.7.23.5/24 interface=VM network=10.7.23.0
/ip cloud set update-time=no
/ip dns set servers=8.8.8.8,1.1.1.1
/ip route add gateway=10.7.5.101
/ip service set ftp disabled=yes
/ip service set telnet disabled=yes
/ip service set www disabled=yes
/ip service set api-ssl disabled=yes
/ip ssh set strong-crypto=yes
/snmp set enabled=yes
/system clock set time-zone-autodetect=no time-zone-name=Europe/Stockholm
/system ntp client set enabled=yes
/system ntp client servers add address=10.7.5.101
/system package update set channel=long-term
/system routerboard settings set auto-upgrade=yes enter-setup-on=delete-key
/tool mac-server set allowed-interface-list=none
/tool mac-server mac-winbox set allowed-interface-list=OOB
/tool mac-server ping set enabled=no

txfz · February 17, 2026, 4:07pm

There are a couple more devices participating in the spanning tree. They both get the same root bridge ID, but their own, expected bridge ID.

Edit: I see that all the MSTP bridge IDs are shown as 0x8000. Isn’t that supposed to be 0x8002?

txfz · February 18, 2026, 7:18am

The unexpected bridge ID is probably due to MLAG, come to think of it.