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I need help regarding the following spanning-tree issue.

I have a network core of 2 HP5412 (procurve) connected together using VRRP over a L2 tagged link. I have a Cisco 3560 connected to each of the 5412 with L2 links. For loop avoidance, there is spanning-tree running on all switches involved.

HP5412_F is Root Primary Bridge (priority 4096) running forced RSTP HP5412_A is Root Secondary Bridge (priority 8192) running forced RSTP Marbot_batF4 is an access switch which is running PVST.

Now, when I look at the 3560 logs, I can see something wrong :

Marbot_batF4_1#sh spanning-tree root

                                        Root    Hello Max Fwd
Vlan                   Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---          ------------
VLAN0001          4096 001b.3fbd.6100     20000    2   20  15  Gi0/50
VLAN0002          4098 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0003          4099 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0004          4100 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0005          4101 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0010          4106 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0011          4107 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0012          4108 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0013          4109 0017.597d.d600     20000    2   20  15  Gi0/50
VLAN0014          4110 0017.597d.d600     20000    2   20  15  Gi0/50

The Root ID for vlan 1 is not the same as for all other vlans! The Root Bridge MAC Address matches the Root ID for VLAN 1, but the other Root ID pictured above is not known on my network.

HP5412_BatF_1# sh spanning-tree

 Multiple Spanning Tree (MST) Information

  STP Enabled   : Yes
  Force Version : RSTP-operation
  IST Mapped VLANs : 1-4094
  Switch MAC Address : 001b3f-bd6100
  Switch Priority    : 4096
  Max Age  : 20
  Max Hops : 20
  Forward Delay : 15

  Topology Change Count  : 3971
  Time Since Last Change : 6 days

  CST Root MAC Address : 001b3f-bd6100
  CST Root Priority    : 4096
  CST Root Path Cost   : 0
  CST Root Port        : This switch is root

  IST Regional Root MAC Address : 001b3f-bd6100
  IST Regional Root Priority    : 4096
  IST Regional Root Path Cost   : 0
  IST Remaining Hops            : 20

I'm guessing that the Root ID of 0017.597d.d600 also belongs to HP5412_F, but I can't prove it.

Is there a way to relate this unknown Root ID to my Root Bridge ? Also, the switch says that I am running MST (Multiple Spanning-Tree) but I don't have any instance configured, as I am forcing RSTP operation (just the one instance for all vlans) - is this maybe the source of the behavior seen above ? Mixing RSTP on ProCurves with PVST on Cisco ?

Thank you for your inputs.

  • Are you running MST on the Cisco switches? It's a really bad idea to mix the IEEE STP with Cisco's PVST or Rapid-PVST. – Ron Maupin Dec 15 '15 at 17:22
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    Ron : I am not running MST on the Cisco access layer. This was not my design - it is indeed a really bad decision made 10 years ago. The easiest way would be to just run MST on all switches, but that can't be done right now. – Jeremy G. Dec 15 '15 at 18:09
  • Then you are going to have some anomalies, especially with VLAN 1 and any VLAN set to native. I did an OUI lookup for that MAC address, and the result is: 00:1B:3F ProCurve Networking by HP. The OUI lookup of the other root MAC address results in: 00:17:59 Cisco Systems, Inc. – Ron Maupin Dec 15 '15 at 18:16
  • You can get a lot of information by searching on cisco mstp pvst+ interoperability. This may help you: ifconfig.it/wordpress/2012/10/pvst-interoperability – Ron Maupin Dec 15 '15 at 18:25
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Cisco PVST uses a non-standard destination MAC for BPDUs on all VLANs, so HP will not understand it. To the Cisco switch, it is the only switch on VLANs 2-14, which is why it thinks it is the root.

Cisco sends BPDUs with the standard MAC (01:80:C2:00:00:00) untagged on VLAN 1, even if VLAN 1 is not allowed on the trunk.

So HP will only see BPDUs for VLAN 1. It will ignore all other VLAN BPDUs from Cisco.

  • Ron : the only problem I have is that the Cisco switch does not think himself as the root, as I have shown above. It just has a different Root ID on all vlans except vlan #1. It seems that the BPDUs get somehow processed by the HP5412. – Jeremy G. Dec 15 '15 at 20:34
  • That's not what your output shows. The MAC address 0017:59xx:xxx is a Cisco mac address. VLAN 1 shows the HP switch as the root. – Ron Trunk Dec 15 '15 at 20:37
  • "untagged" and "VLAN 1" don't mix... if it's untagged, then it belongs to whatever "native vlan" that port is set for. This can cause all manner of oddities with Cisco's STP, if CDP is off so it cannot detect the native vlan and/or any mismatches. – Ricky Beam Dec 15 '15 at 21:02
  • Right. Got it. I am still amazed that I can't find this Cisco MAC address anywhere on my network though. It just HAS to be something Cisco creates to "simulate" a Root Bridge for other vlans. – Jeremy G. Dec 15 '15 at 21:24
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The only standards compliant STP method common to all switches is MST. You don't have to go nuts with it, 'tho -- running everything from the CIST is fine. (I do just that between half a dozen vendors.)

End your headache and switch them all to MST. Yes, there will be a small disruption while things are in transition, but It Has To Be Done.

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