Questions for PVST & MSTP interoperability

[ivancheng2002]

Hi,

Here’s output of “show spanning root”:

      PVST+ configured on two C6509 switches:

                                        Root Hello Max Fwd

Vlan                   Root ID          Cost  Time Age Dly  Root Port

---------------- -------------------- ------ ----- --- ---  ----------------

VLAN0001          8192 0024.f905.9801         0    2   20  15                                 

VLAN0100          8192 0024.f905.9864         0    2   20  15                 

             :

VLAN0210          8192 0024.f905.98d2         0    2   20  15                 

VLAN0220          8192 0024.f905.98dc         0    2   20  15                 

VLAN0230         32768 0024.f905.98e6         0    2   20  15                 

VLAN0250         32768 0024.f905.98fa         0    2   20  15                 

VLAN0300          8192 0024.f905.992c         0    2   20  15                 

VLAN0800         32768 0024.f905.9b20         0    2   20  15                 

VLAN0801         32768 0024.f905.9b21         0    2   20  15                 

VLAN0802         32768 0024.f905.9b22         0    2   20  15         

According to above information,, the root bridge of each VLAN is using different ROOT ID while not same as ROOT ID of VLAN 1.  Is it expected?  Can we cconfigrue the switch to use same ROOT ID?

On the MSTP, it just know the ROOT ID of VLAN 1 and don’t know other.  How will the MSTP to verify the topology for other VLANs?

Thanks in advance.

[Mohamed Sobair]

1- Its expected and normal, you can have different STP roots for different vlans. if you want the same root of Vlan 1 to be the root for other vlans , you can also have it.

2- MSTP have special instance which is intance 0 (vlan 1), this instance is the IST and its used to send and recieve BPDUs.

    Each MST instance has its own spanning tree topology, but only instance 0 forward and recive BPDUs. by default all vlans are inserted into this instance if not configured differently.

    Each BPDU in PVST is sent seperately for every Vlan, and Vlan 1 will carry all BPDUs information for all vlans by default, and would therfore start communicating with instance 0 the (IST). In the boundary bridge connecting MST with the PVST+, the BPDUs will all be treated as PVST Bpdus and the timers will be set to PVST timers.

Hope this Helps,

Mohamed

[krahmani323]

Hello,

As you can see different mac addresses for the root are used, although they seem very similar.

I suppose in your switch the "old" original spanning-tree bridge format is used. Is spanning-tree extend system-id present in your config ?

From the Cisco documentation => With the old-style bridge ID format, a switch’s bridge ID for each STP instance (possibly one per VLAN) was identical if the switch used a single MAC address when building the bridge ID. Having multiple STP instances with the same bridge ID was confusing, so vendors such as Cisco used a different Ethernet BIA for each VLAN when creating the old-style bridge IDs. This provided a different bridge ID per VLAN, but it consumed a large number of reserved BIAs in each switch.

The spanning-tree extend system-id global config command will allow to keep the same mac address for the spanning-tree bridge ID. The distinction will then be possible with the new Bridge ID field (system ID Extension) introduced with the command.

Thus the mac address will be preserved (we call it mac address reduction) but you will see the original bridge priority concatenated with the n° of the vlan according to the figure below.

So you have to add the command spanning-tree extend system-id if you want to see the same mac address for the bridge.

But be careful : Enabling or disabling the extended system ID  updates the bridge IDs of all active STP instances, which might change  the spanning tree topology.

**********************

Regarding the second point, can you precise where do you see the bridge is root for only vlan 1 ? Can you post a 'show spanning-tree mst configuration' ?

THanks

Karim

[krahmani323]

Just post 'show spannig-tree mst'.

Thanks . Karim

[ivancheng2002]

Hi Karim,

The MSTP is running on two Procurve Switches and no other MSTP instance configured.

Here's output of "show span" on one of the Procurve switches:

  show spanning-tree

show spanning-tree

Multiple Spanning Tree (MST) Information

  STP Enabled   : Yes

  Force Version : MSTP-operation

  IST Mapped VLANs : 1-4094

  Switch MAC Address : 002561-f5cb00

  Switch Priority    : 61440

  Max Age  : 20

  Max Hops : 20

  Forward Delay : 15

  Topology Change Count  : 8          

  Time Since Last Change : 11 days    

  CST Root MAC Address : 0024f9-059801  <-- This is owned by one of C6509 on VLAN 1

  CST Root Priority    : 8192       

  CST Root Path Cost   : 20000      

  CST Root Port        : Trk4              

  IST Regional Root MAC Address : 002561-f84200  <-- This is owned by other Procurve switch

  IST Regional Root Priority    : 61440      

  IST Regional Root Path Cost   : 2000       

  IST Remaining Hops            : 19         

  Root Guard Ports     :

  TCN Guard Ports      :

  BPDU Protected Ports :                                        

  BPDU Filtered Ports  :                                        

  PVST Protected Ports :                                        

  PVST Filtered Ports  :                                        

                   |           Prio             | Designated    Hello        

  Port   Type      | Cost      rity  State      | Bridge        Time  PtP Edge

  ------ --------- + --------- ----- ---------- + ------------- ----- --- ----

  A1     100/1000T | Auto      128   Disabled   |

    :

              Trk5             | 20000     240   Blocking   | 00d02b-895901 2     Yes No 

  Trk6             | Auto      64    Disabled   |

I have a concern whether the MSTP can learn the topology of each VLAN completely from PVSTP as it just have the ROOT ID from VLAN 1.

Thanks.

[krahmani323 · Accepted solution]

Hello !

On the HP, the reason why you only see information for vlan 1 from Catalyst is due to the fact that the PVST+ Cisco switch sends two different "types" of BPDUs :

1/ VLAN 1 STP BPDUs are sent to the IEEE STP MAC address (0180.c200.0000), untagged.
==> Standardized BPDU undestood and interpreted by HPs.

2/ Non-VLAN 1 (100,210,220 ...) STP BPDUs are sent to the PVST+ MAC address (also called the Shared Spanning Tree Protocol [SSTP] MAC address, 0100.0ccc.cccd), tagged with a corresponding IEEE 802.1Q VLAN tag.

==> This address is not understood by HPs as BPDUs and as a result the HPs (non-Cisco switches in general) forward the BPDUs as a multicast.

Indeed PVST+ is Cisco proprietary and this SSTP multicast address (0100.0ccc.cccd) is interpreted as BPDU only by Cisco.

Otherwise, I invite you to see this very very intersting post which discuss about the same architecture as yours :

https://supportforums.cisco.com/thread/160526

Depending on your network requirments and conditions the best thing, if you want to have a global vlan load-balancing  would be here to migrate your 6509 to MST IEEE Standard compliant version (after 12.2(18)SXF) in the same region as HPs.

Hope it helps.

Regards.
Karim