Maximum spanning tree instances (PVST) on 6500

Answered Question
Mar 22nd, 2008

What would be the maximum recommended spanning tree instances on a 6500 with SUP720's running native IOS? Is there a certain threshold or best practice as the maximum number of VLANS running on a 6500...before things get too chatty?

Thanks

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Correct Answer by Istvan_Rabai about 8 years 8 months ago

Hi Brandon,

You have to configure each switch in the MST region to use the multiple spanning-tree protocol. Currently no method exists to distribute configuration to several switches, as far as I know.

I do not recommend to migrate your production network to MST on the fly, as there may be problems or network outages while you configure each switch.

Instead, I would make a sample configuration in a lab a would play around with it until I am familiar enough with its configuration and operation.

Then I would migrate the network out of working hours, with the possibility of rollback to the old configuration and possibility of thorough tests of the new configuration.

The configuration you have to do on each switch is the following:

conf t

spanning-tree mode mst

spanning-tree mst configuration

name xxxxxxxxxxx - this is the mst region name, this should be identical on each switch

revision X (X= 0 - 65535, this should be identical on each switch)

instance 1 vlan 1 - 30 ( this will assign vlan 1 to 30 to instance 1)

instance 2 vlan 31 - 60 ( this will assign vlan 31 to 60 to instance 2)

exit

Remember, that this is just a sample configuration. You need to adapt this to your network.

The MST configuration commands are very similar to the traditional spanning-tree commands. For example to change the priority of a switch on intance 1 and 2:

spanning-tree mst 1 priority xxxx

spanning-tree mst 2 priority yyyy

With these commands you will be able to select the root switches for each instance.

This link will give you a good overview of MST basics:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html

Cheers:

Istvan

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Istvan_Rabai Sat, 03/22/2008 - 11:28

Hi Brandon,

If you have many vlans but only a limited number of possible spanning tree topologies, then I would recommend using MST.

In MST you can configure 15 different instances altogether (instance 0 being the Internal Spanning Tree instance).

So, if you have not more than 15 possible and reasonable topologies for load balancing or separating traffic, then you can use MST.

This would reduce the chattiness you are referring to.

Cheers:

Istvan

mbroberson1 Sun, 03/23/2008 - 07:03

We have about 200 or so L2 switches for host devices on our network. A combination of 3550, 3560, & 3750's. All switches have redundant paths back to the core...for example in the closets the top of stack switch connects to the core and the bottom of stack switch connects to the core. With this topology wouldn't you say that is many STP instances? We have 60 or so VLANS. Each VLAN is a /24. I am not really sure what is a high number of VLANS on a 6500? Would this rule out using MST?

HTH

Istvan_Rabai Sun, 03/23/2008 - 08:28

Hi Brandon,

60 vlans is not so a big number. This means you have 60 STP instances in pvst+ spanning-tree mode, and it should not overwhelm the 6500 switches with an average utilization.

However, if you have a topology and vlan configurations where you can significantly decrease the number of STP instances, it is always better to do it, as it will decrease the cpu utilization. This way the switch cpu will have more resources for other tasks, such as dhcp server or relay functions, arp requests, management traffic and so on.

The accent here is on "significantly decrease". If you decrease the number of STP instances by 10%, you probably will not achieve the desired result, as MST itself is a bit more processor intensive.

But if you can decrease the STP instances with MST by let's say 90%, this can give you much better results.

Cheers:

Istvan

mbroberson1 Mon, 03/24/2008 - 05:16

Istvan,

Thanks for your response. I am trying to better understand the stance on MST. If I am understanding correctly the type of topology where I could significantly descrease the number of STP instances is a topology where switches don't have redundant paths...basically switches with only one uplink back to the core. Does this sound accurate?

HTH,

Brandon

Istvan_Rabai Mon, 03/24/2008 - 05:44

Hi Brandon,

When you have only one uplink to the core (the root switch) from each other switch, you basically don't have to use spanning-tree, except for security purposes, maybe.

You have to use spanning-tree when multiple uplinks or paths exist within the topology to prevent loops.

If you have 2 or more possible topologies because of the existence of alternative uplinks, then there is a possibility for loops to form, and also a possibility arises to have several instances of spanning-tree to load balance traffic to the root switch (and possibly to the secondary root switch).

This is when spanning-tree has a role here.

If you have let's say 2 or 3 different possible topologies and 60 vlans, then your gain using MST may be significant, as you will have 2 or 3 MST instances compared to 60 for PVST+.

But you need to examine your topology to see where the possibilities for multiple uplinks exist and how many tree structures are possible.

Make a drawing to see the different possible tree structures for the given root switch (and secondary root switch if one exists).

If you need help in this, just post the drawing here and I will help you.

Cheers:

Istvan

mbroberson1 Mon, 03/24/2008 - 06:02

Istvan,

Thanks. Every switch on our LAN 200 or so points to both the root for primary and secondary root for backup. So I suppose with 60 or so VLANS running pvst+ we have 2 STP topologies for each VLAN since each switch has two redundant uplinks to our dual core setup (6513's)? If I am understanding correctly this would come to 120 STP instances for 60 VLANS. An I understanding correctly?

Thanks

Istvan_Rabai Mon, 03/24/2008 - 06:27

Hi Brandon,

No you will have only 1 spanning-tree instance per vlan, that is 60 instances.

To configure your network for load balancing, you need to do the following (just for example):

For vlans 1 - 30 you may configure Switch1 as the primary root switch, Switch2 as secondary root switch.

For vlans 31 - 60 you may configure Switch2 as primary root switch and Switch1 as the secondary root switch.

So traffic for vlans 1 - 30 will go through Switch1, the access switch ports pointing to Switch2 will be blocking.

Traffic for vlans 31 - 60 will go through Switch2, the access switch ports pointing to Switch1 will be blocking.

But as I mentioned earlier, you will have 30 + 30 instances, that is 60 instances.

The problem with this is that 30 instances have the same topology for vlans 1 - 30, and another 30 instances have the same topology for vlans 31 - 60.

This is unnecessary.

You can then have 2 MST instances instead and you will save many processor cycles in your switches (not only in the root switches, but in the access switches as well).

Cheers:

Istvan

mbroberson1 Mon, 03/24/2008 - 08:32

Configuring MST...You configure it on the root and it pushes it out to the clients (L2 siwtches) correct? Is migrating from pvst+ to mst tyoically a daunting task?

Correct Answer
Istvan_Rabai Mon, 03/24/2008 - 10:49

Hi Brandon,

You have to configure each switch in the MST region to use the multiple spanning-tree protocol. Currently no method exists to distribute configuration to several switches, as far as I know.

I do not recommend to migrate your production network to MST on the fly, as there may be problems or network outages while you configure each switch.

Instead, I would make a sample configuration in a lab a would play around with it until I am familiar enough with its configuration and operation.

Then I would migrate the network out of working hours, with the possibility of rollback to the old configuration and possibility of thorough tests of the new configuration.

The configuration you have to do on each switch is the following:

conf t

spanning-tree mode mst

spanning-tree mst configuration

name xxxxxxxxxxx - this is the mst region name, this should be identical on each switch

revision X (X= 0 - 65535, this should be identical on each switch)

instance 1 vlan 1 - 30 ( this will assign vlan 1 to 30 to instance 1)

instance 2 vlan 31 - 60 ( this will assign vlan 31 to 60 to instance 2)

exit

Remember, that this is just a sample configuration. You need to adapt this to your network.

The MST configuration commands are very similar to the traditional spanning-tree commands. For example to change the priority of a switch on intance 1 and 2:

spanning-tree mst 1 priority xxxx

spanning-tree mst 2 priority yyyy

With these commands you will be able to select the root switches for each instance.

This link will give you a good overview of MST basics:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html

Cheers:

Istvan

mbroberson1 Mon, 03/24/2008 - 10:55

Istvan,

Thanks for the reply. I have 4 or so 3550's I can lab up and play around with the config. The best way for me to adapt this to my network will be to map it out first. 4 switches should be enough for to build a lab to test this don't you think?

Thanks,

Brandon

Istvan_Rabai Mon, 03/24/2008 - 10:59

Yes, I think so.

Good luck with your new configuration!

Please let me know how you proceed and I will try to help you in the meantime, if you drop me a message.

Though I'm not always behind my computer, once a day or in 2 days I check my e-mails.

Best regards:

Istvan

gaurav_dixit06 Thu, 05/21/2009 - 04:50

Hi,

i do have also one query.

i do have flat L2 Network ..

Take a example ...I do have tow Virtual firwall host in HA mode (high avilabily).

it is connecting to 3750 switches ...with Ether channel (between 3750)...

i had migrate from PVST to MST ..then i can't see any ARP in Virtal Firewall host .

then i had reverted back to PVST..I do have question why it is not working .

i followed these below mentioend steps.

conf t

spanning-tree mst configuration

revision 1

instance 1 vlan 1500-1800

exit

spanning-tree mode mst

no spanning-tree backbonefast

end

wr mem

i am going to PVST to MST ..due to it will support ..as i believe it will support only 128 Vlan(correct me if i am wrong)

Awaiting from you a Answer ..

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