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New Member

spanning-tree extend system-id

Silly question, but what does this command do again? I believe it has something to do with giving each VLAN its own unique ID but I am not certain.

Thank you.


Re: spanning-tree extend system-id

Hi, voiper

You are correct. This command will change the format of Bridge ID and add VLAN ID information to the Bridge ID. Here is the detail:

Traditional BID = priority (16 bits) + MAC (48 bits)

So, the range of the priority is 0~65535. It is same for all VLANs.

BID with extend system-id = priority(4bits) + VLAN ID (12bits) + MAC(48 bits)

Now, it is a little bit trickier to read the priority vlaue. The priority value showed up on the switch is still a 16-bit value. So, if the priority is 1000(in binary), for vlan 1 the priority will be read as

1000 000000000001 in binary


32769 in decimal

for vlan 2, it will be

1000 000000000010 or 32770

for vlan 3 ... ,etc.

But I think this command is enabled by default and I have never tried to disable it before.



New Member

Re: spanning-tree extend system-id

I too believe it is enabled by default and to my knowledge you are unable to disable it.

In regards to my question, I am still unsure why it is required. What purpose does it serve?

Re: spanning-tree extend system-id

Hi, will

I have checked some documents and I find out that the formula I have used is incorrect and have to be revised as follows:

BID = priority(4bits)+extended system-id(12bits)+MAC(48bits)

,where (extended system-id) = (vlan id) in pvst mode or (extended system-id) = (mst instance number) in mst mode.

As for the purpose of extened system-id. Here is my explaination that I try as best as possible.

Suppose a switch is running in pvst mode and there are 4000 vlans configured on this switch. In pvst point of view, there are 4000 logical bridges , and each bridge needs a unique bridge ID. That means you need 4000 unique bridge IDs in ths case.

Let's take a look at the traditional BID fields: priority and MAC. Therefore, you have tow choices to give these 4000 unique BID

1.change the priority filed, the MAC field remains the same

2.the priority field remains the same , change the MAC field

The second method needs 4000 MAC addresses on a single switch. Is it possible to do so? Unlikely.

The first method is what the extended system-id comes in. It require borrowing some bits from the priority field. In this case, you need to borrow 12 bits to make 4096 rooms for the 4096 vlans to use for unique BID and leave 4 bits to remain function of priority. Fortunately, 4 bits is quite enough for us to use for priority.

Hope this will answer your question


New Member

Re: spanning-tree extend system-id

But wouldn't using the MAC address alone make the BID unique seeing as though no two MAC addresses should be the same?

Re: spanning-tree extend system-id

It's enabled by default in MST mode or on some recent switches. The interesting thing is that you only need a single mac address to give a different bridge ID to all you 4K STP instances in PVST mode. Without extended bridge ID, we would need to burn one mac address per vlan, which is extremely expensive.

Sure, there are *lots* of mac addresses in the world, but allocating 4K of them for each switch sold is just not scalable.



New Member

Re: spanning-tree extend system-id

ahh I see what you mean. So if you have 10 switchports in 10 different VLANs, each switchport needs a unique BID which is why using a MAC address on its own is not sufficient. Is this correct?

Re: spanning-tree extend system-id

when you have 10 VLANS and running PVST, you will have 10 STP instances which would have required 10 MAC-addresses.

As francois said, this is not scalable and hence using a extended sysytem ID, we have a single Mac-address for all stp instances and are diffrentiated by the vlan number.

HTH, rate if it does


Re: spanning-tree extend system-id

That's correct, each vlan instance needs a different bridge ID. With extended sysid, they are all built from the same mac address.

Note it's mainly useful for PVST modes, where you can have lots of instance.

However, this is now part of the IEEE standard (802.1t added this functionality) and is used by MST by default.

The default was not change for PVST in Cisco switches, because we did not want to risk the customer topology changing because of a software upgrade.