spanning tree instance

can anybodu descrive about "what is spanning tree instance".ACtully i need some idea about spanning tree instance .i know what spanning tree works.but i am confused about spanning tree instance .why we r using spanning tree instance during configuring spanning tree on switch?

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Bl8ckr0uter

shelly117 wrote: »

can anybodu descrive about "what is spanning tree instance".ACtully i need some idea about spanning tree instance .i know what spanning tree works.but i am confused about spanning tree instance .why we r using spanning tree instance during configuring spanning tree on switch?

I think this is because cisco uses per vlan spanning tree

look at this Spanning tree protocol - Wikipedia, the free encyclopedia

and this Per VLAN Spanning Tree (PVST) - Cisco Systems

I could be wrong but maybe this will help?

adam-b

A few months ago the following syslog was generated on one of our core 6509 switches

"Dec 12 13:52:08 nameofswitch 1981: Dec 12 13:52:06.167 EST: %PM-SP-4-LIMITS: The number of vlan-port instances on module 1 exceeded the recommended limit of 1800 "

What was interesting about this message was that we do not have 1800 vlans configured on the switch...so it makes me wonder if the number of "vlan instances" equates with the number of vlans as configured on the switch it is runing Per VLan STP? Could it be that an STP instance represents the amount of ports that STP is running on?

Bl8ckr0uter

adam-b wrote: »

A few months ago the following syslog was generated on one of our core 6509 switches

"Dec 12 13:52:08 nameofswitch 1981: Dec 12 13:52:06.167 EST: %PM-SP-4-LIMITS: The number of vlan-port instances on module 1 exceeded the recommended limit of 1800 "

What was interesting about this message was that we do not have 1800 vlans configured on the switch...so it makes me wonder if the number of "vlan instances" equates with the number of vlans as configured on the switch it is runing Per VLan STP? Could it be that an STP instance represents the amount of ports that STP is running on?

Was VTP prunning turned on for the switches? Perhaps do you have 1800 vlans in your vtp domain...

Idk Im still uncertified

so take my thoughts with a grain of salt..

edit: http://puck.nether.net/pipermail/cisco-nsp/2006-January/027798.htmlI also found this

Cyanic

From everything I have read so far, it seems that an instance in this case is just a process, e.g., you can have two instances of firefox open and doing different things. If you close one the other is up and running and unaffected.

That error has "vlan-port instances" which may mean something different.

networker050184

A VLAN port instance is created every time a VLAN is carried over a trunk. So if you have a single VLAN that is carried over two trunks on that module you will have two VLAN port instances. These instances are recommended to be below 1800 per slot and I believe its around 10,000 per chassis.

We ran into an issue with this with a FWSM not long ago. The recommended action from Cisco was to go to layer 3, prune VLANs or switch to MST.

miller811

adam-b wrote: »

A few months ago the following syslog was generated on one of our core 6509 switches

"Dec 12 13:52:08 nameofswitch 1981: Dec 12 13:52:06.167 EST: %PM-SP-4-LIMITS: The number of vlan-port instances on module 1 exceeded the recommended limit of 1800 "

What was interesting about this message was that we do not have 1800 vlans configured on the switch...so it makes me wonder if the number of "vlan instances" equates with the number of vlans as configured on the switch it is runing Per VLan STP? Could it be that an STP instance represents the amount of ports that STP is running on?

From my CCDP ARCH exam manual

Cisco STP Scaling Recommendations

In a Layer 2 looped topology design, spanning-tree processing instances are created on each interface for each active VLAN. These logical instances are used to process the spanning-tree-related packets for each VLAN. These instances are referred to as active logical ports and virtual ports.

STP Logical Interfaces
Active logical ports are a systemwide value that reflects the total number of spanning-tree processing instances used in the whole system. Total active logical interfaces can be calculated by a formula:
Sum [(Each trunk on the switch) * (Active VLANs on each trunk)] + (Number of nontrunking interfaces on the switch)

so if you have 30 trunks that are pushing 60 vlans across each, you reach the 1800 number.

Bl8ckr0uter

Wow. Ok so I guess I was wrong. Sorry about that

networker050184

knwminus wrote: »

Wow. Ok so I guess I was wrong. Sorry about that

Well, you were kind of right. Pruning could have possibly helped the situation.

Bl8ckr0uter

networker050184 wrote: »

Well, you were kind of right. Pruning could have possibly helped the situation.

My thoughts on a vlan instance were wrong. That is what I was refering to. But hey we live and we learn. What Im curious to know is why 1800? There are 1000s assignable vlans ids, why stop at 1800?

networker050184

I'm guessing it would have to be the result of some testing that had issue over 1800. So that is the recommended limit. The switch will not fall over and die if you put more, but I guess it would eventually cause issues eating up resources.

Bl8ckr0uter

networker050184 wrote: »

I'm guessing it would have to be the result of some testing that had issue over 1800. So that is the recommended limit. The switch will not fall over and die if you put more, but I guess it would eventually cause issues eating up resources.

From Website:

the short answer is that there is a limit on how many spanning tree
port-instances can exist on a single slot. If you have lots of VLANs and
need to trunk them to lots of ports, you probably should consider switching
to MST. Depending on your configuration, you might be able to work around
the problem (at least for a while) by ensuring that connections carrying
large numbers of VLANs are spread out and mixed with single(or low) VLAN
ports on the same cards.

I am not sure what MST is yet but seems like I will have something new to study..

jason_lunde

You should be able to verify how many instances you are running by doing a:
show spanning-tree summary

The number in the bottom-most right hand corner should be your total number of instances.

Just found this out doing some labbing. Pretty cool factoid I did not know about STP instances. Thanks guys!

adam-b

miller811 wrote: »

From my CCDP ARCH exam manual

Cisco STP Scaling Recommendations

In a Layer 2 looped topology design, spanning-tree processing instances are created on each interface for each active VLAN. These logical instances are used to process the spanning-tree-related packets for each VLAN. These instances are referred to as active logical ports and virtual ports.

STP Logical Interfaces
Active logical ports are a systemwide value that reflects the total number of spanning-tree processing instances used in the whole system. Total active logical interfaces can be calculated by a formula:
Sum [(Each trunk on the switch) * (Active VLANs on each trunk)] + (Number of nontrunking interfaces on the switch)

so if you have 30 trunks that are pushing 60 vlans across each, you reach the 1800 number.

Nice! This is a great explanation.

@Jason_L Good to know!