This article explains the impact of one device configured with RSTP while the other devices are configured for VSTP. 

Topology

Whenever the downstream EX4400 is zeroized and put in the network with all uplinks connected, there is a loop and high level of broadcast packets in the network. The factory default configuration on EX4400 has RSTP enabled by default. The QFX5100 devices have VSTP configured on them.

qfx5100-a         Seconds: 0          Time: 04:28:33

Interface  Link Input packets    (pps)   Output packets    (pps)

 xe-0/0/0   Up   5961125487  (11559121)    5968751160  (11541681)

 xe-0/0/1   Up   5968854282  (11541836)    5961674592  (11559141)

 ae5      Up   5961125487  (11559121)    5968751160  (11541681

>> There might be some interoperability issues with VSTP and RSTP based on the topology. 

>> VSTP leverages RSTP as its core spanning tree protocol. VSTP spawns separate STP topology instances for separate VLANS (RSTP for each VLAN) and RSTP (set protocols rstp interface all) spawns a single STP topology which is shared by all VLANS.

>> RSTP calculates its spanning tree topology independently from VSTP.

>> In VSTP, each VLAN has its own root bridge, and the spanning tree topology (including root bridge selection) is determined separately for each VLAN. If one device runs RSTP, it will have its own root bridge for the common spanning tree instance it forms. This means there will be separate root bridges and potentially different spanning tree topologies between the RSTP device and the VSTP network.

>> Since RSTP and VSTP operate independently hence there is a high risk of unintended loops.

>> VSTP will spawn topologies for all VLANS that are defined except for global VLAN i.e VLAN1

Outputs for reference:

EX4400 spanning-tree outputs when it is running RSTP.

root@ex4400# run show spanning-tree bridge

STP bridge parameters

Routing instance name        : GLOBAL

Context ID             : 0

Enabled protocol          : RSTP

 Root ID              : 32768.d0:81:c5:2b:9d:50 <<<< It considers itself the root-bridge in Global instance.

 Hello time            : 2 seconds

 Maximum age            : 20 seconds

 Forward delay           : 15 seconds

 Message age            : 0

 Number of topology changes    : 0

 Local parameters

  Bridge ID            : 32768.d0:81:c5:2b:9d:50

  Extended system ID       : 0

BPDU is sent out and nothing is received which proves that it is the root-bridge and no other device has sent a superior BPDU

lab@ex4400# run show spanning-tree statistics interface

Interface   BPDUs    BPDUs    Next BPDU    TCs    Proposal  Agreement

       Sent    Received   Transmission  Tx/Rx   Tx/Rx    Tx/Rx

xe-0/2/0    57      0       0      0/0     0/0     0/0

xe-0/2/1    57      0       0      0/0     0/0     0/0

Both the interfaces are in forwarding state whereas the ideal scenario would have one of the interface in blocking state. 
 

lab@ex4400# run show spanning-tree interface detail

Spanning tree interface parameters for instance 0

Interface name         : xe-0/2/0

Port identifier        : 128.490

Designated port ID       : 128.490

Port cost           : 2000

Port state           : Forwarding

Designated bridge ID      : 32768.d0:81:c5:2b:9d:50

Port role           : Designated

Link type           : Pt-Pt/EDGE

Boundary port         : NA

Interface name         : xe-0/2/1

Port identifier        : 128.491

Designated port ID       : 128.491

Port cost           : 2000

Port state           : Forwarding

Designated bridge ID      : 32768.d0:81:c5:2b:9d:50

Port role           : Designated

Link type           : Pt-Pt/EDGE

Boundary port         : NA

QFX5100-a device acting as root-bridge however there are broadcast packets still seen because it is receiving unwanted RSTP packets.

lab@qfx5100-a# run show spanning-tree interface 

Spanning tree interface parameters for VLAN 600

Interface         Port ID  Designated     Designated     Port  State Role

                    port ID      bridge ID     Cost

ae5             128:8    128:8  4696.d45a3fe24e02     1000  FWD  DESG 

xe-0/0/1         128:1010   128:1010  4696.d45a3fe24e02     2000  FWD  DESG 

lab@qfx5100-b# run show spanning-tree interface 
Spanning tree interface parameters for VLAN 600

Interface                  Port ID    Designated         Designated         Port    State  Role
                                       port ID           bridge ID          Cost
ae5                          128:8        128:8   4696.d45a3fe24e02         1000    FWD    ROOT 
xe-0/0/1                  128:1010     128:1010   8792.d45a3fedacc2         2000    FWD    DESG 

In summary, while RSTP and VSTP can coexist in a network, having one Juniper device running RSTP while others run VSTP requires careful planning, configuration, and monitoring to prevent potential spanning tree issues such as loops and to ensure optimal network operation. 

However it is always recommended to have consistent flavor of STP running across your network which allows for efficient network management and avert any unwanted issues.

2024-07-13 - Initial Publication
2024-07-29 - Visibility modified