L2 Bridging (IRB)

IOS: ISRG2 (1941) sub-int to sub-int.
This will transport BPDUs but will not disrupt an MST instance, the BPDUs coming through will be "down converted" to represent PVST BPDUs and "Inconsitent superior PVST BPDU received" messages will be logged:

bridge irb
bridge 10 protocol ieee

int gi0/0.10
 encaps dot1q 10
 bridge-group 10

int gi0/1.10
 encaps dot1q 10
 bridge-group 10


! The device connected to Gi0/0.10 is the root bridge for MST instance 0 which is VLANs 10 & 20, the device at Gi0/1.10 has err-disabled the port facing Gi0/1

1941-3#show spanning-tree

 Bridge group 10 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, address 442b.0325.6220
  Configured hello time 2, max age 20, forward delay 15
  Current root has priority 4096, address 1cde.a79a.1980
  Root port is 21 (GigabitEthernet0/1.10), cost of root path is 4
  Topology change flag not set, detected flag not set
  Number of topology changes 7 last change occurred 00:00:14 ago
          from GigabitEthernet0/0.10
  Times:  hold 1, topology change 10, notification 1
          hello 1, max age 6, forward delay 4
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 19 (GigabitEthernet0/0.10) of Bridge group 10 is forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.19.
   Designated root has priority 4096, address 1cde.a79a.1980
   Designated bridge has priority 32768, address 442b.0325.6220
   Designated port id is 128.19, designated path cost 4   Hello is pending
   Timers: message age 0, forward del
   Number of transitions to forwarding state: 3
   BPDU: sent 2155, received 2099

 Port 21 (GigabitEthernet0/1.10) of Bridge group 10 is forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.21.
   Designated root has priority 4096, address 1cde.a79a.1980
   Designated bridge has priority 4096, address 1cde.a79a.1980
   Designated port id is 128.13, designated path cost 0
   Timers: message age 1, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   BPDU: sent 9, received 2371

IOS: ISRG2 BVI (Bridge Virtual Interface).
This will allow the bridging router to have a layer 3 interface inside the layer 2 bridge domain:

bridge irb
bridge 10 protocol ieee

int gi0/0.10
 encaps dot1q 10
 bridge-group 10

int gi0/1.10
 encaps dot1q 10
 bridge-group 10

int bvi 10
 ip addr 192.168.0.1 255.255.255.0

IOS ISRG2 port-to-port.
This configuration supports tagged VLANs over the port-to-port bridge and the transporting of BPDUs however the bridge it's self is not participating in spanning-tree. But as above, it is breaking MST, PVST works as expected:

bridge irb
bridge 20 protocol vlan-bridge

int gi0/0
 bridge-group 20

int gi0/1
 bridge-group 20


! The ISRG2 router running the bridge thinkgs it is the spanning-tree root because it is not actually participating in spanning-tree, with the BPDUs passing through. This output is the same when running PVST which works and MST which does not work

1941-3#show spanning-tree

 Bridge group 20 is executing the vlan-bridge compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, address 442b.0325.6220
  Configured hello time 2, max age 30, forward delay 20
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 1 last change occurred 00:22:30 ago
          from GigabitEthernet0/0
  Times:  hold 1, topology change 50, notification 2
          hello 2, max age 30, forward delay 20
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 3 (GigabitEthernet0/0) of Bridge group 20 is forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.3.
   Designated root has priority 32768, address 442b.0325.6220
   Designated bridge has priority 32768, address 442b.0325.6220
   Designated port id is 128.3, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   BPDU: sent 695, received 0

 Port 4 (GigabitEthernet0/1) of Bridge group 20 is forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.4.
   Designated root has priority 32768, address 442b.0325.6220
   Designated bridge has priority 32768, address 442b.0325.6220
   Designated port id is 128.4, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   BPDU: sent 694, received 0