[c-nsp] MSTP issue. Isolation of core switch

[cnsp at marenda.net]

Hello!

Thanks for you response.

As I know MSTP does not send MSTI's information in separate BPDUs, this
information is piggybacked into the IST's BPDUs using special M-Record
fields.



They are all send UNTAGGED.

They may be filtered (bpdufilter enable) or Carrier-Eqiupment may be
configured not to forward them

(may not be configured to forward them).

To have alle vlans in one instance just don't map them, they will all in
instance 0 .

So, I can have multiple MSTI or one with whole vlan range (1-4096) no
matter. Also we not planned to use some load share mechanism, so i did not
see any sense in multiple instance.

In any case, BPDU will be propagated in MST0 (Internal Spanning Tree) and
will consist of such components as configuration name, revision number and a
hash value calculated over VLANs to MSTI mapping table contents

To form one region, the hash must be same, so the mapping/name/revision must
be identical.


The configuration name and revision parapeters have sense if we used
multiple instase (maybe i'm wrong). But this is not acceptable for us now.



They are mandatory. Did not work on my first 6 switches with MST-config
without name and revision.


I think that this problem may apear due very large L2 segment. So value "Max
Hops"  exhausts itself in some cases.
As a result sw-core receives BPDU 0 and after that  happens  the following
scenario

sw-Core ceases to receive BPDU from all neighbors and and decides that he is
root.
Upstream switches sends superior root bridge information to the sw-Core
bridge but receives the BPDUs with Designated bit set, the upstream switch
concludes that the downstream does not hear its BPDU's. The upstream switch
then blocks the downstream port and marks it as STP dispute link

BUT Why sw-CORE ceases to receive BPDU from ALL neighbors?  - a mystery.



If your sw-core is HP than I would tell you "enable spanning-tree"  it's off
by default.

Filter -Y see above

Wll show spannint-tree tell you that running stp version IS MST ?

(you can have mst config and pvrstp running,

Show mst config will shot it even when pvst is active.)

 

So what Switches do you use?

Ensure identical MST configuration with name and revsion (textual identical
on all switches helps a lot)

And that all your switches have STP/MST running.

Having WAN links with carrier-CPE'S/NT's  ensure sufficient MTU and
forwarding of BPDU's 

(and packet with destination-MAC=local-special-MAC-Address)

Enable UDLD aggressive to find one-way links and disable them.