Why did we really need this "spanning-tree extend system-id" command?

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Jul 27th, 2013
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On the Spanning tree protocol I understood how does this spanning-tree extend system-id command work.

But I have not understood why it is in place? or why do we really need it?


Nikhil Kulkarni.

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Overall Rating: 5 (8 ratings)
Peter Paluch Sat, 07/27/2013 - 00:32
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Hi Nikhil,

The STP and RSTP standard specifications mandate that each switch running STP/RSTP must have a unique Bridge ID (BID). Because Cisco runs STP or RSTP in each VLAN separately (called PVST and RPVST or PVRST), in each VLAN, the switch behaves like a standalone (albeit virtual) switch and thus, each STP/RSTP instance is required to have a unique BID to comply with the standard. Simply, having X VLANs means having X separate STP/RSTP instances and X unique BIDs.

The question now is how to make sure the BIDs of STP/RSTP instances run on the same switch in different VLANs are truly unique. Older switches actually had a large reserve of MAC addresses. As new VLANs were created, these switches allocated a new MAC address for each new STP/RSTP instance in a new VLAN (recall that the BID originally consisted of the priority and the MAC address), making the BIDs unique.

However, the consumption of MAC addresses this way was simply too large and ineffective. At the same time, having 65536 different values for priority in the BID was largely useless. So IEEE came with the idea of Extended System ID in which they reused a part of the priority field for a unique instance identifier. In Cisco's implementation, this field is populated with the VLAN number the STP/RSTP instance runs in. This easily and effectively makes the BID unique - even with the same priority for all VLANs on a single switch, and a single switch MAC address, multiple STP/RSTP instances running on this same switch with the same priority have different BIDs thanks to different VLAN numbers embedded into the BID.

Some switch platforms actually allowed you to deactivate the Extended System ID and revert to the older style of assigning unique MAC addresses to individual STP/RSTP instance BIDs. That is why the command spanning-tree extend system-id exists in the first place. However, removing this command is only possible on those switching platforms which are equipped with 1024 MAC addresses for their disposal. Most new switching platforms have only 64 MAC addresses for their internal use, and while the spanning-tree extend system-id command is present in their configuration, you can not remove it. It is simply there to inform you that the Extended System ID is being used but you can not really deactivate it.

Read more here:


Best regards,


Rolf Fischer Sat, 07/27/2013 - 01:29
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Hi Nikhil,

I think Peter gave a fantastic explanation and there's nothing important to add.

But if you want to memorize a single term: "MAC-Address  reduction feature"

I always liked that CatOS term, it's pretty much self-explaining.

Best regards


Peter Paluch Sat, 07/27/2013 - 01:38
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Hi Rolf,

I am honored, thank you! - and oh, yes, the MAC Adress Reduction Feature - that's the term I forgot!

Best regards,


Jon Marshall Sat, 02/22/2014 - 03:02
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I am bumping this post up again because a question was asked about why a switch needs to have a unique BID per vlan. I gave a rather bad answer so i then started to look into it and still cannot work exactly why.

I understand the specifications mandate a unique BID per STP instance. So when Cisco introduced PVST to comply with the standards they needed to have a unqiue BID per vlan. This was achieved either by using a unique mac address per vlan or utilising the extended system id.

But is that the only reason ?

I ask because, as pointed out by the OP in a recent thread there is nothing in the BID that identifies the vlan itself. So the switch must keep a record of which BPDUs belong to which vlan. So why can't the switch simply have one BID for all vlans ie.

if the BID is not used to derive the vlan then a shared mac address would still uniquely identifty the sending switch. So if you needed to change the priority for a specific vlan then you could still do that ie. 

1) the receiving switch would receive a BPDU per vlan.

2) the BPDUs would all have the same mac address but it doesn't matter because they are per vlan

3) so for that specific vlan the switch would know the switch that transmitted the BPDU was x prioritry for that vlan

I initally thought that because the BID was not unique that the switch would not be able to work out which vlan each BPDU belonged to.

But it can't do that from the BID even if it is unique.

In both cases it is the vlan ID in the frame tag that tells the switch which vlan this BDPU is for.

I know i am missing something here. But i can't see it.  I am hoping someone a lot more intelligent than me eg. Peter, comes along to explain it all.

I have to admit i am seriously considering relinquishing my HoF status (and probably VIP as well) as if i cannot work this out i'm not really sure i deserve them


Ahmet Mustafa Mungan Thu, 11/03/2016 - 07:14
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Hi to all,

I know that there has been 3 years after this thread. However I have the same question with Jon. Does anybody has the answer?

Kind Regards,

Ahmet Mustafa Mungan

JohnTylerPearce Sat, 02/22/2014 - 03:16
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It's ok, there are enough Johns to go around

I must say this has been an excellent thread, and I"m glad I found it. It makes waking up at 3am worth it, at least for right now....


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