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MSTP conifguration problem with multiple instances

Christian Reiter
Frequent Advisor

MSTP conifguration problem with multiple instances

Hello!

To ensure that I've understand the very basics of MSTP right, I wanted to configure MSTP on two ProCurve 2910al for two separate VLANs (10 & 20) with a distinct uplink between the two switches for each VLAN.

Therefore I've created two intances of MST (instance 1 and two) and assigned VLAN 10 to instance 1 and VLAN 20 to instance 2. Ports 1-12 belong to VLAN 10 and Ports 13-24 belong to VLAN 20 on both of the switches. I've now interconnected Port 1 of switch 1 with Port 1 of switch 2 and Port 13 of switch 1 with Port 13 of switch 2. What I expected to see was that both uplinks are in "FORWARDING"-state on both switches, but Port 13 went into "BLOCKING"-state on switch 2.

What's wrong with my setup?

Here's the config of switch 1:
=============================================
hostname "ProCurve 2910al-48G Switch"
vlan 1
name "DEFAULT_VLAN"
no untagged 1-48,A1-A2
no ip address
exit
vlan 10
name "testVlan1"
untagged 1-12,25-48,A1-A2
ip address 10.1.21.102 255.255.255.0
exit
vlan 20
name "testVlan2"
untagged 13-24
ip address 10.1.20.1 255.255.255.0
exit
spanning-tree
spanning-tree config-name "MyConfig"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 10
spanning-tree instance 2 vlan 20
spanning-tree priority 0
=============================================


And here's the config of switch 2:
=============================================
hostname "ProCurve 2910al-48G Switch"
vlan 1
name "DEFAULT_VLAN"
ip address dhcp-bootp
no untagged 1-48,A1-A2
exit
vlan 10
name "testVlan1"
untagged 1-12,25-48,A1-A2
ip address 10.1.21.103 255.255.255.0
exit
vlan 20
name "testVlan2"
untagged 13-24
ip address 10.1.20.2 255.255.255.0
exit
spanning-tree
spanning-tree config-name "MyConfig"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 10
spanning-tree instance 2 vlan 20
=============================================


Thanks in advance for your help!

Best Regards,
christian
2 REPLIES
Mohammed Faiz
Honored Contributor

Re: MSTP conifguration problem with multiple instances

Hi,

Something that you should bear in mind with MSTP is that in order prevent your switches/VLANs losing communication with each other in the event of a link failure you should tag every VLAN on every uplink port/trunk.
In the setup you have if, for example, the port 13 uplink failed then VLAN 20 hosts on different switches would not be able to talk to each other (as VLAN 20 only exists on the port 13 uplink).

Back to your question, how are you checking what state the uplinks are in?
In a two switch MSTP setup (an unusual one) you'll want to manually set the per instance port path costs to ensure that the MSTP instances are split across the two uplinks.
It would be more sensible to use a three switch test setup however. In that scenario you'd just change the instance priority on each switch to adjust the load-balancing to match what you wanted.
Richard Brodie_1
Honored Contributor

Re: MSTP conifguration problem with multiple instances

Port 1 has a higher priority by default. You haven't changed the port priorities in either instance, so it will be forwarding in both instance 1 and 2.

Now, you are probably thinking that is a bit hard on poor VLAN 20. The important thing to realise about MSTP is that it doesn't care about your VLANs.

You have to arrange the instance trees to do what you want by adjusting the MSTP parameters. It won't 'route around' VLAN filters.

span instance 2 13 priority 0

I think would do what you want.

http://blog.ine.com/2010/02/22/understanding-mstp/ is the best introduction to MSTP that I've seen.