Spanning- tree priority with VPC

Answered Question
Nov 12th, 2011
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Hi,



I'm deploying a pair of Nexus 3064 switches in a VPC and they will handle all of the Layer 3 Routing and switching for a small data center.  My question is, should I set their spanning tree priority the same if they will be configured as a VPC?


Example:

Primary - 4096

secondary - 4096



Thank you

Correct Answer by rsgamage1 about 5 years 9 months ago

Hi,


Having the same bridge priority is not a requirement to maintain vPC consistency.


Even if you had the highest bridge priority in the secondary, your vPC primary would forward BPDUs in vPC domain.


quoted from "Cisco NX-OS Virtual PortChannel: Fundamental Design Concepts..."


"vPC by default ensures that only the primary switch forwards BPDUs on vPCs. This modification is strictly limited to

vPC member ports. As a result, the BPDUs that may be received by the secondary vPC peer on a vPC port are

forwarded to the primary vPC peer through the peer link for processing.


Note: Non-vPC ports operate like regular spanning-tree ports. The special behavior of the primary vPC member

applies uniquely to ports that are part of a vPC."


{some articles for refe}


http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/design_guide_c07-625857.pdf


http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/C07-572834-00_STDG_NX-OS_vPC_DG.pdf



Having the same (highest) priority would also be no problem because in the non-vPC context (i.e. classic spanning tree topology) there would be a tie-breaker (lowest MAC) to elect the root.


Afterall I suppose it's a choice of design.

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Correct Answer
rsgamage1 Sun, 11/13/2011 - 12:25
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Hi,


Having the same bridge priority is not a requirement to maintain vPC consistency.


Even if you had the highest bridge priority in the secondary, your vPC primary would forward BPDUs in vPC domain.


quoted from "Cisco NX-OS Virtual PortChannel: Fundamental Design Concepts..."


"vPC by default ensures that only the primary switch forwards BPDUs on vPCs. This modification is strictly limited to

vPC member ports. As a result, the BPDUs that may be received by the secondary vPC peer on a vPC port are

forwarded to the primary vPC peer through the peer link for processing.


Note: Non-vPC ports operate like regular spanning-tree ports. The special behavior of the primary vPC member

applies uniquely to ports that are part of a vPC."


{some articles for refe}


http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/design_guide_c07-625857.pdf


http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/C07-572834-00_STDG_NX-OS_vPC_DG.pdf



Having the same (highest) priority would also be no problem because in the non-vPC context (i.e. classic spanning tree topology) there would be a tie-breaker (lowest MAC) to elect the root.


Afterall I suppose it's a choice of design.

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