Description

This article explains the unintended spanning-tree convergence behavior, due to non-compliant IEEE BPDUs being processed by RSTP on the EX Switch. 

When the EX Switch receives BPDU's from multiple PVST+ instances, which have different priorities, it processes the PVST+ BPDU's and reports priority changes between different Cisco PVST instances and the EX Switch RSTP instance.

Symptoms

RSTP might process non-IEEE BPDU's, which could lead to unintended spanning-tree convergence.

Topology:

[EX8200 ae0]---|trunk all vlans allowed|---[po1 cisco 6500]  

          RSTP                                        PVST
    Bridge priority = 12K                        BP = 8K(Vlan 1,2),
                                                 All other Vlans = default (32K)


In this setup, the EX Switch should not be selected as a root bridge; but you can observe the following root bridge flaps in the network:
lab@sw2> show spanning-tree bridge | match "Root ID" Root ID : 12288.84:18:88:98:48:00
lab@sw2> show spanning-tree bridge | match "Root ID" Root ID : 8193.00:09:44:ad:74:c0
lab@sw2> show spanning-tree bridge | match "Root ID" Root ID : 8193.00:09:44:ad:74:c0
lab@sw2> show spanning-tree bridge | match "Root ID" Root ID : 12288.84:18:88:98:48:00
lab@sw2> show spanning-tree bridge | match "Root ID" Root ID : 8193.00:09:44:ad:74:c0



Solution

  • With the EX Switch configured for RSTP and it receiving BPDU's from multiple PVST instances with different priorities, there is an EX Switch interlope issue with PVST (or VSTP).
  • This combination leads to the processing of PVST BPDU's and you may see priority changes between different CISCO PVST BPDU's. RSTP on the EX Switch will keep receiving BPDU's from multiple PVST instances with priorities lesser and greater, re-converge, and will not decipher the PVST instance. 
  • According to the standard, RSTP should ignore BPDUs, if it receives tagged BPDUs; but the EX Switch processes all BPDU's (untagged, tagged IEEE, tagged non IEEE).

This issue is fixed as per PR683829 . Contact Juniper Technical Assistance Center (JTAC), if you have further questions about this issue.

  • Synopsis: The root bridge is flapping between Juniper EX and Cisco; even though EX8200 is configured for RSTP and Cisco is configured for PVST.
  • Release Note: RSTP might process BPDUs, which do not comply with the IEEE standard; this might lead to unintended spanning-tree convergence behavior.
  • Resolved In: 10.4R7; 11.1R6; 11.2R3; 11.3R2.

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