Doubts about STP id priority

ciscolover · ‎12-01-2010

Hi all¡¡¡

I have some doubts about the prioriti traditional or extended in STP. I don't know why its necessary using one or other. Can anyone explain me in easy words the differences between this 2 forms to obtain the ID priority???

Thanksss

jorge.calvo · ‎12-02-2010

Hello,

The extended STP system-id is used to provide more granularity. It is used too to allow having only one MAC address per switch making it easier to troubleshoot STP issues.

Please see this example to see the differences:

With standard system-id we have, for instance, a priority of 32768 which is the bridge priority.

With the extended system-id, we would have that priority, 32768, plus the vlan id. For example:

For VLAN 100 --> 32768 + 100 = 32868

For VLAN 200 --> 32768 + 100 = 32968

And so on...

In newer IOS versions the extended id is enabled by default. One situation where it is recommended is a switch with many SVIs configured (STP instances). With the standard system-id, every SVI would have one different MAC address.

Since a MAC address is assigned to every SVI and hence to every VLAN, you can have the same MAC address on every SVI and differenciate them on STP instances level by adding the VLAN id to its STP priority by using the extended system-id.

Hope this helps.

Peter Paluch · ‎12-02-2010

Hello Jorge,

Allow me to add some comments to your reply.

The extended STP system-id is used to provide more granularity.

I am not sure about this one. Granularity means that you can do something in small, fine steps. The extended system ID does not provide you with an enhanced granularity in any way in my opinion. What it solves is the consumption of MAC addresses per switch backplane. The 802.1D requests that each switch shall have a unique BID. Therefore, for PVST/PVST+, a switch behaves as many switches as how many VLANs are defined. Each of these virtual switches therefore must have a unique BID. You can solve that only via two approaches: either your switch will be equipped with a large number of MAC addresses per backplane to be used for individual STP instances, or you will use the VLAN ID itself to create different unique BIDs. The second approach, originally specified in 802.1t, ultimately won.

In newer IOS versions the extended id is enabled by default.

In fact, in newer IOS versions, it is enabled and cannot be deactivated.

One situation where it is recommended is a switch with many SVIs 
configured (STP instances). With the standard system-id, every SVI would
 have one different MAC address. Since a MAC address is assigned to every SVI and hence to every VLAN, 
you can have the same MAC address on every SVI and differenciate them on
 STP instances level by adding the VLAN id to its STP priority by using 
the extended system-id.

I have a feeling this is incorrect. You are confusing the MAC addresses of SVI interfaces for VLANs, and the STP BIDs for these VLANs. Consider this output:

Switch#show int vlan 138 | i bia
Hardware is EtherSVI, address is 001b.8f8f.de4b (bia 001b.8f8f.de4b)
Switch#show int vlan 139 | i bia
Hardware is EtherSVI, address is 001b.8f8f.de4c (bia 001b.8f8f.de4c)
Switch#show span vlan 138

VLAN0138
[cut]
Bridge ID Priority    32906 (priority 32768 sys-id-ext 138)
             Address     001b.8f8f.de00
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

Switch#show span vlan 139

VLAN0139
[cut]
Bridge ID Priority    24715 (priority 24576 sys-id-ext 139)
             Address     001b.8f8f.de00
             Hello Time   2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

This switch, although using the extended-system-ID, is using the same BID for both VLAN138 and VLAN139, although the MAC address of SVIs for these VLANs is different and distinct. As I indicated, the extended-system-ID is an STP feature - it is not related to MAC address consumption for SVIs.

Best regards,

Peter

jorge.calvo · ‎12-02-2010

Hello Peter,

Thanks for this! I actually mixed the SVI MACs with the Bridge ID.

And with granularity I was refering only to the fact that the priority fields is divided in 4 bits for the bridge priority and 12 bits for the extended system-id instead of having the 16 bits dedicated to the bridge priority.

Thanks again!

Cheers.

Mohamed Sobair · ‎12-02-2010

Hi Peter,

I agree with you that Extended system id is not related to the consumption of the vlan mac address, however STP uses the vlan id as extended system Id as denoted in the documentation.

the extended system-id is actually a vlan fraom range 1025 - 4096 in older IOS releases, and in newer IOS releases its a 12 bit value added to the bridge priority.

the difference between standard system-id and extended-system-id is therfore related to the bridge-id. as we already know , a bridge-id is characterised as follows:

1- a bridge priority of 2 byte + a bridge mac address of 6 bytes ,So its 8 byte in total.

The bridge with the lower priority is always prefered and chosen as an STP root bridge, if there is a tie, then a bridge with lower MAC Address is prefered.

Now with the extended system id, the bridge-id becomes 8 bytes + 12 bits.

HTH

Mohamed

Peter Paluch · ‎12-02-2010

Hi Mohamed,

I am afraid I do not agree with some of what you wrote. Let me explain.

I agree with you that Extended system id is not related to the consumption of the vlan mac address, however STP uses the vlan id as extended system Id as denoted in the documentation.

Yes, I know that and I indicated that when I wrote: "You can solve that only via two approaches: either your switch will be equipped with a large number of MAC addresses per backplane to be used for individual STP instances, or you will use the VLAN ID itself to create different unique BIDs." By referencing the VLAN IDs, I had exactly the extended system ID in mind - combining the MAC address of the switch with the VLAN ID.

the extended system-id is actually a vlan fraom range 1025 - 4096 in 
older IOS releases, and in newer IOS releases its a 12 bit value added 
to the bridge priority.

I am not sure about this one. A VLAN from range 1025-4095 (not 4096) as the system ID? Are you perhaps confusing this with the extended VLAN ID range?

The extended system ID length is not (and can not be) dependent on the VLAN number. The extended system ID is |VLAN ID (12 bits)|MAC (48 bits)|, i.e. a 60-bit long field formed by concatenating the VLAN ID with the MAC address of the switch, but no one cares whether it is created for VLAN 1 or VLAN 4095. It is there for all VLANs.

Now with the extended system id, the bridge-id becomes 8 bytes + 12 bits.

I don't agree. The bridge ID always has been and still is 8 bytes long, not 8 bytes+12 bits. It is just split differently: upper four bits are for priority, middle 12 bits are for VLAN ID, remaining 48 bits are for MAC address of the switch. That is actually the reason why you can configure the STP priority only in the increments of 4096 because the lower bits are already reserved for the VLAN ID.

Best regards,

Peter

Mohamed Sobair · ‎12-02-2010

Peter,

From Cat 4500 Config guide,

Bridge Priority Value

The bridge priority value determines whether a given redundant link will be given priority and considered part of a given span in a spanning tree. Preference is given to lower values, and if you want to manually configure a preference, assign a lower bridge priority value to a link than to its redundant possibility. With releases prior to 12.1(12c)EW, the bridge priority is a 16-bit value (see Table 10-1).With Release 12.1(12c)EW and later releases, the bridge priority is a 4-bit value when the extended system ID is enabled (see Table 10-2). See the "Configuring the Bridge Priority of a VLAN" section.

Extended System ID

Extended system IDs are VLAN IDs between 1025 and 4096. Releases 12.1(12c)EW and later releases support a 12-bit extended system ID field as part of the bridge ID (see Table 10-2). Chassis that support only 64 MAC addresses always use the 12-bit extended system ID. On chassis that support 1024 MAC addresses, you can enable use of the extended system ID. STP uses the VLAN ID as the extended system ID. See the "Enabling the Extended System ID" section.

Full documentation reference here:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/12ew/configuration/guide/spantree.html

Please see the sentences highlited with a bottom lines.

Regards,

Mohamed

Peter Paluch · ‎12-02-2010

Mohamed,

This may sound to you that I am finding excuses to prove my version right but I really do not want that, I am merely seeking what is correct here And I have a strong feeling that the documentation you are referencing is itself incorrect.

First, there cannot be a VLAN ID 4096. That would require a 13-bit number although 802.1Q VLANs use only 12-bit numbering. 802.1Q VLANs are only in the range of 0-4095 (0x0 - 0xFFF). That is the first sign that the documentation is flawed.

Second, an extended bridge ID by definition cannot be a VLAN ID number in a particular range. If the extended bridge ID was a VLAN ID in the range of 1025-4095, what would be the ID related to the VLAN 1000? Or VLAN 1? It just does not make sense! A VLAN ID is not an extended bridge ID. A VLAN ID is a part of extended bridge ID, and that must hold for all VLANs, not just for VLANs in a particular range.

Third, the documentation for a number of other switches and available IOSes puts it quite differently:

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which controls the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+ and rapid PVST+, the same switch must have a different bridge IDs for each configured VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most-significant bytes are used for the switch priority, and the remaining 6 bytes are derived from the switch MAC address

The switch supports the IEEE 802.1t spanning-tree extensions, and some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 26-1, the 2 bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.

See:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_55_se/configuration/guide/swstp.html#wp1236904

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/configuration/guide/swstp.html#wp1236904

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/spantree.html#wp1036624

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2ZY/configuration/guide/spantree.html#wp1096536

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html#wp1096536

Best regards,

Peter

ciscolover · ‎12-02-2010

Hi all¡¡¡

I go to try to resume this ...

The bridge id its composed with the bridge id + Mac addrese of the bridge.This is the normal form to calculate this id.

With extended bridge id you use the id of the bridge + a unique mac address for each vlan of the bridge???

Please, this its correct?

Thanks''¡¡¡¡

Mohamed Sobair · ‎12-02-2010

Hi,

1- The bridge - id is consist of two element ( Bridge priority of 2 bytes + Bridge Mac address of 6 Byte ) , and the total becomes 8 Byts.

2- Now , this what got me confused and I will need to check the documentation and do some extra consultation on it. The Documentation I refered to claims that (When extended system Id) is used , it adds 12 bit and STP uses the extended vlan range 1025 - 4096 or 4095 as extended System id , thats why I conclude that when its used , it adds these values.

However , Peter disagreed with me on that and he claimed the documentation is incorrect , and I will have to check it once more.

Regards,

Mohamed

Peter Paluch · ‎12-03-2010

Hello Mohamed,

Have you had any time to look into this issue?

Best regards,

Peter

Mohamed Sobair · ‎12-03-2010

Hello Peter,

I am Actually Away from office for short period vacation, I will be back with an update on the coming few days.

Please let us work together on this , If you also come with a new update , it will be an added value.

Regards,

Mohamed

Mohamed Sobair · ‎12-03-2010

Peter,

I found this thread is very useful, awhile looking at it also from the 6500 Config guide, I think it also supports my previous understanding that, STP extended System id is used for the bridge id election.

please look into it and let me know your feedback,

https://supportforums.cisco.com/thread/1003007

HTH

Mohamed

Peter Paluch · ‎12-03-2010

Hello Mohamed,

I have had a look at that thread and I agree with everything said there. I believe it is perfectly aligned to everything I have written here so far.

I think it also supports my previous understanding that, STP extended System id is used for the bridge id election.

I have never said otherwise. What I have disagreed with was your definition of what is the extended system ID and what is its structure. Let me restate my standpoint:

A bridge ID (BID) is an 8-byte long value.
Originally, the BID was split into two parts: priority (2 bytes) and system ID (6 bytes). The concatenation of the priority and the system ID constitutes the bridge ID. The priority was freely configurable in the range of 0-65535, the system ID was the MAC address of the switch and was not configurable.
With the need of having unique BIDs for each VLAN, the lowest 12 bits of the priority field are redefined to hold the VLAN ID, however, no additional bits are added to the BID - it is still 8 bytes long. The definition of the BID is therefore: priority (4 bits), VLAN ID (12 bits), system ID (6 bytes). The concatenation of the VLAN ID and the system ID is called the extended system ID, and thus, the BID could - from this viewpoint - also be defined as consisting of priority (4 bits) and extended system ID (60 bits), with the extended system ID consisting of VLAN ID (12 bits) and system ID (6 bytes).
As a result of the redefinition of the priority field, only highest 4 bits are available for priority configurable value. This is the reason why the priority can be expressed only in the multiples of 4096 because all bits necessary to express number lower than 4096 are already reserved to hold the VLAN ID value.

Any ideas or disagreements about this?

Best regards,

Peter

Mohamed Sobair · ‎12-03-2010

Peter,

Nice explanation , So the 12 bit was taken from the originaly 2 byte priority and not added to to the BID 8 Byte As I was originally thought. I missed this point , thanks for pointing it out.

Now, what is the correlation between a Unique BID for every vlan and the extended VLAN range from 1025 - 4096

when the Extended System ID is used.

BTW:

I have rated you with full points.

Regards,

Mohamed