MST pre-standard and links down

[anasaizgarc]

Hello everyone

We have experienced a network outage due to a port-channel going down. Checking the logs we have seen the following messages:

%SPANTREE-3-PRESTD_NEIGH: pre-standard MST interaction not configured (Port-channel3). Please, configure: 'spanning-tree mst pre-standard' on ports connected to MST pre-standard switches.

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/1, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet2/0/1, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel3, changed state to down

%LINK-3-UPDOWN: Interface Port-channel3, changed state to down

%LINK-3-UPDOWN: Interface GigabitEthernet1/0/1, changed state to down

%LINK-3-UPDOWN: Interface GigabitEthernet2/0/1, changed state to down

When this happened, the interface that was actually blocked by STP didn't come to forwarding state, and as a result this switch became isolated.

This switch is connected to a switch that runs standard MST, and to some switches that run PVST but have bpdufilter enabled as we don't want them to participate on the STP. It is a 3750 switch with software version 12.2(55)SE5.

As far as I knew, when a MST pre-standard BPDU was received, the interface was able to receive more pre-standard BPDUs, but I didn't know it would turn the link down.

I would like to ask if anyone knows why the links went down, and if there is any way we can configure those links to accept only MST standard BPDUs.

I would also like to know if there is any chance a switch running PVST can send a MST pre-standard BPDU if it has a software version 12.2(25)SEE3.

Thanks in advance

[Arumugam Muthaiah]

Hi anasaizgarc,

%SPANTREE-3-PRESTD_NEIGH: pre-standard MST interaction not configured (Port-channel3). Please, configure: 'spanning-tree mst pre-standard' on ports connected to MST pre-standard switches.

Description:

%SPANTREE-3-PRESTD_NEIGH: pre-standard MST interaction not configured ([chars]). Please, configure: 'spanning-tree mst pre-standard' on ports connected to MST pre-standard switches.

The switch has received a prestandard multiple spanning-tree (MST) BPDU on a port that is not configured for prestandard MST BPDU transmission. The switch will automatically adjust its mode of operation on this port and will start sending prestandard BPDUs. However, this autodetection of prestandard neighbors is not 100 percent accurate and we recommended that you explicitly configure the port for prestandard MST BPDU transmission. This warning message will be displayed only once.

Recommended Action:
Configure the #spanning-tree mst pre-standard command on all the ports that are connected to switches running the Cisco prestandard version of MST. We recommended that you migrate all the switches in the network to the IEEE standard MST version when it is possible.

• This doesnt impact any link down issue
  
• Need to verify the neigbhor switch logs as well to understand the link down issue
  

Even with the default configuration, the port can receive both prestandard and standard BPDUs. A standard switch only sends pre-standard bpdus on an interface:

1. if it has received a pre-standard bpdu on the interface
   
2. or if it is explicitly configured to send pre-standard bpdu on this interface.
   

Prestandard BPDUs are based on the Cisco IOS MST implementation that was created before the IEEE standard was finalized. Standard BPDUs are based on the finalized IEEE standard

Please refer the below link which has more detail,

http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst3550/software/release/12.2_25_see/configuration/guide/swmstp.html#wp1160010

Regards,

Aru

[anasaizgarc]

Hi Aru, and thanks for your help

However, this isn't helpful because what I need is just the opposite, I want the whole environment to run just standard MSTP, I don't want any pre-standard BPDU going through any link.

Regards,

Ana

[rizwanr74]

Hi Anas,

I assume the switch became isolated is running PVST, in which case you must have its vlans cannot be the root when you have connected to MST domain.

So, increase the priority for the given vlans on the isloated switch and bounce the ports (shut and no shut).

Thanks

Rizwan Rafeek

[anasaizgarc]

Hi Rizwan

Thanks for your help, but no, this is not the issue. The isolated switch is running MSTP and it has the right priority in order not to become root.

Thanks, and Regards,

Ana

[Peter Paluch]

Dear friends,

Please allow me to join.

What Aru has explained is correct - one of your switches is using an older version of MSTP and because this version may have issues interpreting the new MST BPDUs, newer switches insist that all links towards pre-standard switches must be configured with spanning-tree mst pre-standard command. Quite correctly, the dynamic detection of pre-standard switches is not foolproof - if the pre-standard switch sends no BPDUs (like having a root port towards the standard switch), the standard switch will have no means to detect that the opposite switch speaks an older MST version.

Ana, if you want to run standard MSTP without any reverting to pre-standard MST, your switches must run at least 12.2(25)SED or newer. MST implementations in IOSes older than 12.2(25)SED are pre-standard.

Best regards,

Peter