nexus and stp

Answered Question
Feb 26th, 2010

hi all,

simple question...

                     _________

                    | switch c_  |

                      /            \

    _________ /              \ _________

   | switch a_  | -----------   | switch b_  |

           |         \             /          |    

           |            \       /             |

           |               \ /                |

______ |_______/      \  ______ |_____

|NEXUS 7KA     |         | NEXUS7KB   |

so i've got vPC1from switch a and n7K A&B and vPC2 from switch b and N7K A&B wher5e Nexus A is master for vPC.

switch a is connectd also to switch b and switch c and b is connected to swsitch c and switch a.

Nexus A has lower STP priority (so is root of stp) than Nexus B.

question is....how stp choose root, designated and block port? of corse i mean from switch a-b-c point of view.

tnx

Das

I have this problem too.
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Correct Answer by Lei Tian about 6 years 9 months ago

Hi Das,

You are correct. Without VPC, normal stp behavior occurs. A looped triangle topology will have one uplink in fwd state one in blk state.

When you use VPC with SPT, then the switches within same VPC domain will follow the rules I said in last post.

HTH,

Lei Tian

Correct Answer by Lei Tian about 6 years 9 months ago

Hi Das,

Just tread the VPC as single switch, follow the same SPT rule.

So on Switch a and Switch b, the port-channel to VPC will be root port; ports between Switch a and Switch b will as Designate and BLK; ports to Switch c will both be Designate. On Switch c, one port will root port, the other will be BLK.

HTH,

Lei Tian

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Correct Answer
Lei Tian Fri, 02/26/2010 - 14:21

Hi Das,

Just tread the VPC as single switch, follow the same SPT rule.

So on Switch a and Switch b, the port-channel to VPC will be root port; ports between Switch a and Switch b will as Designate and BLK; ports to Switch c will both be Designate. On Switch c, one port will root port, the other will be BLK.

HTH,

Lei Tian

danilodicesare Sat, 02/27/2010 - 00:13

hi Lei,

so i'll follow normail stp rule, bridge id and tiebreak like that. so no difference than having one switch as neuh if vpc is implemented.

tnx

das

Lei Tian Sat, 02/27/2010 - 04:55

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Hi Das,

That is correct. From none VPC switches point of view, there is no difference as regular SPT. They look 2 VPC switches in same domain as single identity.

The difference is on VPC switches; there are 2 main SPT changes

1, VPC peer-link will never in BLK state

2, VPC primary generates and process bpdu.

HTH,

Lei Tian

danilodicesare Sat, 02/27/2010 - 18:48

ok Lei.

but if i've got a switches connected in a 'normal way' i mean square topology or triangle is always master switch that process BPDU?

i thought that if i connect switches without vPC a normal stp behaviour occours....am i wrong?

tnx a lot

Dan

Correct Answer
Lei Tian Sat, 02/27/2010 - 19:50

Hi Das,

You are correct. Without VPC, normal stp behavior occurs. A looped triangle topology will have one uplink in fwd state one in blk state.

When you use VPC with SPT, then the switches within same VPC domain will follow the rules I said in last post.

HTH,

Lei Tian

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