Wired Intelligent Edge (Campus Switching and Routing)

Problem:

You see a high amount ARP broadcasts to the default gateway Aruba core switch. From a wireshark trace, you will see several ARP requests that have not been acknowledged.  Thus a broadcast storm is present. MSTP has been enabled. 

If running MSTP with multiple instances, you must make sure that the MSTP VLAN instance assignments are consistent across the network environment. If that is not done correctly it will cause network instability because the MST Configuration digest algorithm is not the same for every switch. Switches with different digests will operate independently, and thus cause network instability. 

For example, below you will see the output from a cli "show spanning-tree mst-config" from 2 core switches and 2 distribution switches. You will notice that the IST Mapped VLANs and the MST Configuration Digest are not the same between the Core switches and distribution switches. 

You will also no longer see MST Config Error BPDUs incrementing. 

CORE 1

show spanning-tree mst-config

MST Configuration Identifier Information

  MST Configuration Name : CORE1
  MST Configuration Revision : 1
  MST Configuration Digest : 0x17209E9E475DF9A706C3A3B2FBA6D42E

  IST Mapped VLANs :

  Instance ID Mapped VLANs
  ----------- ---------------------------------------------------------
  1           1-191,193-755
  2           756-4094
  3           192

show spanning-tree debug-counters

Status and Counters - MSTP Bridge Common Debug Counters Information

  Counter Name                      Aggregated Value  Collected From
  ---------------------------------        ----------------             --------------
  Invalid BPDUs                             0                             CIST
  Errant BPDUs                              0                             CIST
  MST Config Error BPDUs       2666552                      CIST

CORE 2

show spanning-tree mst-config

MST Configuration Identifier Information

  MST Configuration Name : CORE 2
  MST Configuration Revision : 1
  MST Configuration Digest : 0x17209E9E475DF9A706C3A3B2FBA6D42E

  IST Mapped VLANs :

  Instance ID Mapped VLANs
  ----------- ---------------------------------------------------------
  1           1-191,193-755
  2           756-4094
  3           192

how spanning-tree debug-counters

Status and Counters - MSTP Bridge Common Debug Counters Information

  Counter Name                      Aggregated Value Collected From
  ---------------------------------             ----------------          --------------
  Invalid BPDUs                                        0                CIST
  Errant BPDUs                                         0                CIST
  MST Config Error BPDUs                 2669443          CIST

show spanning-tree mst-config

MST Configuration Identifier Information

  MST Configuration Name : Distribution Switch 1
  MST Configuration Revision : 1
  MST Configuration Digest : 0x73FD0C5AE3C841183F6A002617BB545A

  IST Mapped VLANs :

  Instance ID Mapped VLANs
  ----------- ---------------------------------------------------------
  1           1-755
  2           756-4094

show spanning-tree mst-config

MST Configuration Identifier Information

  MST Configuration Name : Distribution Switch 2
  MST Configuration Revision : 1
  MST Configuration Digest : 0x73FD0C5AE3C841183F6A002617BB545A

  IST Mapped VLANs :

  Instance ID Mapped VLANs
  ----------- ---------------------------------------------------------
  1           1-755
  2           756-4094
 

From the example given, Instance ID 3/VLAN 192 assignment should either be added to the distribution switches or removed from the Core switches. The MST Configuration Digest will then be the same.