Solved: extended system id

sarahr202 · ‎09-03-2008

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!

Giuseppe Larosa · ‎09-04-2008

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

View solution in original post

andrew.prince · ‎09-03-2008

Sarah,

Extended system ID's are used for VLAN's numbering 1025 and 4096.

HTH>

vajanuska · ‎09-03-2008

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 ;)

Giuseppe Larosa · ‎09-04-2008

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

sarahr202 · ‎09-04-2008

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!

Giuseppe Larosa · ‎09-04-2008

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

sarahr202 · ‎09-04-2008

Thanks for your help Giuseppe, i really appreciate that!