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09-03-2008 05:49 PM - edited 03-06-2019 01:10 AM
hi every body!
i just want to know if we have to use extended system id in a network, is it necessary all switches should have extended system id enabled ?
Can we use extended system id on one switch and traditional system id on others?
thanks a lot!
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09-04-2008 10:16 AM
Hello Sarah,
>>that means for root bridge selection, priority is considered as priority+vlan id for switches using extended system id in contrast to just priority for switches using traditional system id.
that's absolutely correct.
pri = base_pri + Vlan-id and a single MAC
(modern switches)
pri = base_pri but increment MAC address as Vlan-id in lower bytes (traditional PVST)
Hope to help
Giuseppe

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09-03-2008 11:30 PM
Sarah,
Extended system ID's are used for VLAN's numbering 1025 and 4096.
HTH>
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09-03-2008 11:43 PM
Sarah,
extended system-id as mentioned is used in PVST as VLAN ID, because in PVST every VLAN is diferent instance therefore a separate virtual bridge with diferent Bridge ID.
It alters the original 802.1D Priority field in this way :
Priotity in increment of 4096 + Vlan id.
This is crucial for root bridge election, so if you use this extend-id stuff make sure you set your root bridge priority manualy :)
Also you can disable this by issuing "no" command :)
Interoperability remains as I understand.
The best solution is tu use MSTP ;)
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09-04-2008 12:33 AM
Hello Sarah,
it is not necessary but I would suggest to do so.
I've seen sometimes a campus network with a mix of devices some using system-id and some not using it.
Devices not using it have more chances to become root bridge because their Bridge priority doesn't increase with the vlan-id but be prepared because they use a different MAC for each Vlan.
Actually, system-id is also called MAC address reduction for this reason: it allows to use a single MAC address in all vlans
Hope to help
Giuseppe
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09-04-2008 07:45 AM
Thanks Giuseppe for your reply! From your reply, i get this impression in mix enviroment
where some switches are using extended system i and others are using traditional system id,priority is considered as priority+ vlan id for switches using extended system id. That means for root bridge selection, priority is considered as priority+vlan id for switches using extended system id in contrast to just priority for switches using traditional system id. Am i right?
thanks a lot!
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09-04-2008 10:16 AM
Hello Sarah,
>>that means for root bridge selection, priority is considered as priority+vlan id for switches using extended system id in contrast to just priority for switches using traditional system id.
that's absolutely correct.
pri = base_pri + Vlan-id and a single MAC
(modern switches)
pri = base_pri but increment MAC address as Vlan-id in lower bytes (traditional PVST)
Hope to help
Giuseppe
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09-04-2008 10:26 AM
Thanks for your help Giuseppe, i really appreciate that!
