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Community Member

help with Mac address reduction

Dear All,

as i know that Mac address reduction allow the switch to re allocate priority field in the bridge ID to be 4 bit for priority and 12 bit to vlan id

and without this feature switch will need a seperate mac for each vlan used in spanning tree election.

my question is why in case there is no mac address reduction feature enabled the switch will need diffrent mac for each vlan stp election, why doen`t use only one mac address ?

thanks

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Cisco Employee

help with Mac address reduction

Hello,

my question is why in case there is no mac address reduction feature  enabled the switch will need diffrent mac for each vlan stp election,  why doen`t use only one mac address ?

The reason is that the IEEE 802.1D standard requires each switch to have a unique Bridge ID (BID). If multiple VLANs are used and each VLAN runs its own STP instance, then the switch essentially behaves like a set of virtual switches, one for each VLAN. In order to remain in accordance with the IEEE 802.1D specification, each of these virtual switches needs to have a unique BID, hence the need for a different MAC for each STP instance.

Purely technically, even if a single MAC was used for all per-VLAN STP instances running on a single switch, the STP would work correctly - at least I am not aware of any scenario where using a single MAC could lead to incorrect STP operation. Still, making sure that each STP instance is running with a unique BID is a safe bet.

Best regards,

Peter

2 REPLIES
Cisco Employee

help with Mac address reduction

Hello,

my question is why in case there is no mac address reduction feature  enabled the switch will need diffrent mac for each vlan stp election,  why doen`t use only one mac address ?

The reason is that the IEEE 802.1D standard requires each switch to have a unique Bridge ID (BID). If multiple VLANs are used and each VLAN runs its own STP instance, then the switch essentially behaves like a set of virtual switches, one for each VLAN. In order to remain in accordance with the IEEE 802.1D specification, each of these virtual switches needs to have a unique BID, hence the need for a different MAC for each STP instance.

Purely technically, even if a single MAC was used for all per-VLAN STP instances running on a single switch, the STP would work correctly - at least I am not aware of any scenario where using a single MAC could lead to incorrect STP operation. Still, making sure that each STP instance is running with a unique BID is a safe bet.

Best regards,

Peter

Community Member

help with Mac address reduction

Dear peter,

thanks very much

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