MST and Layer3 Design

[ctychan]

Above config is fine using PVST/RPVST STP.

For MST, if I put all VLAN2, 3 and 10 to instance 0 only, MST-STP merge to one such that the Layer 3 design does not work.  Any method to make MST work in layer 3 design using instance 0 only?

[lgijssel]

Unfortunately, I was unable to open the attached gif but there is one thing I can say without this info:

MST 0 is used as the CST which interacts with STP outside the MST region.

Combining it with internal vlans is generally not a good design practice because a blocking port on the CST will also block all internal vlans in MST 0 at the same time. Perhaps your layer3 connectivity issue is also related to this property of MST.

regards,

Leo

[Peter Paluch]

Leo,

You wrote:

MST 0 is used as the CST which interacts with STP outside the MST region.
Combining
 it with internal vlans is generally not a good design practice because a
 blocking port on the CST will also block all internal vlans in MST 0 at
 the same time.

Can you elaborate a little further on this statement? It is true that a port in the Discarding state for a particular instance is Discarding for all VLANs mapped onto that instance. However, that is true for just any MST instance, not only for MSTI 0 (the IST).

In addition, a boundary port put into Discarding state is blocked for all instances, and thus for all VLANs.

So - unless I am wrong at some of my reasoning above, I do not see anything wrong with mapping all VLANs onto MSTI 0.

Best regards,

Peter

[lgijssel · Accepted solution]

Hi Peter,

I was refering to the document below:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml#ist_active_ports

The possible issue I wanted to target is described in the section:

IST Instance is Active on All Ports, Whether Trunk or Access

regards,

Leo

[Peter Paluch]

Hi Leo,

Yes, I see your point. That is true. I am trying to pull more information from the OP currently to find out more precisely what is going on.

Best regards,

Peter

[ctychan]

I was refering to the document below:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml#ist_active_ports
The possible issue I wanted to target is described in the section:
IST Instance is Active on All Ports, Whether Trunk or Access

Good doc to see the problem when use MSTP and L3 design

[Peter Paluch]

Hello ctychan,

Can you please provide more information?

1. Are there multiple MSTP regions used in your network, or is there a section of your network running a different protocol than MSTP? If so, where exactly are the boundaries?
2. What are your VLAN-to-MSTI mappings? Are you using only the MSTI 0?
3. What are the roots of the individual MST instances?
4. Did you make sure that the MSTP region contains the root switch for all instances and thus all VLANs?

Best regards,

Peter

[ctychan]

In case the pic cannot open, URL is

http://www.cisco.com/image/gif/paws/10556/16g.gif

Are there multiple MSTP regions used in your network, or is there a section of your network running a different protocol than MSTP? If so, where exactly are the boundaries?
What are your VLAN-to-MSTI mappings? Are you using only the MSTI 0?
What are the roots of the individual MST instances?
Did you make sure that the MSTP region contains the root switch for all instances and thus all VLANs?

1. One region only.  Only MSTP is running.

2. All VLAN mapped to MSTI 0 only.

3. Expect each VLAN had its own root, actual all VLANs have one root only.

4. Will recheck it.

I cannot find any info and config about "MSTP and layer 3 design" doc in cisco.com.  Any URLs?

[Peter Paluch]

Hello,

Thank you for your answers. Let me please ask further:

1. Are you using any switchport trunk allowed vlan command on your trunk links? Does the connectivity issue disappear when you remove these commands? Manually pruning VLANs off trunks has to be done cautiously with MSTP with respect to instances, as also Leo pointed out.
2. Do all your switches have identical MSTP configuration, including both region name, revision and mappings? Please verify using the show spanning-tree mst configuration digest command that on all switches, the MD5 digests match. Note that if the MST configuration is left empty (i.e. in its default form), each switch forms its own region, and Cisco does not implement inter-region MSTP in a particularly workable way, unfortunately.
3. Can you determine if the lost connectivity is caused by any port put into Discarding, Blocking, Broken or Inconsistent state? Use the show spanning-tree and show spanning-tree inconsistent to look for such ports. Try to find out where does the connectivity interruption appear.
4. Do any logging messages appear on any of your switches' consoles or in syslog that may be related to this issue?

I haven't seen any document relating to Layer3 and MST design, and there shall be no special requirements for that.

Thank you!

Best regards,

Peter