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VPC from 6509 to Nexus 5010 triggeting Spanning Tree loops


Hello Experts,

I am facing Spanning Tree loop issue after i connect two of my 6509 core-switches with Nexus 5010 .

Below are the things which i have covered while configuring however still STP loop appears randmly in FEW VLAN but not in all vlans.

1) Core1-switch Spanning tree root bridge with prioroy of 4096 for all my vlans

2) Core2-switch Spanning tree root bridge with prioroy of 8192 for all my vlans

3) Core1-swich is configured VTP server mode /pwd authentication and & Core2 is VTP client & pwd auth.

4) Nexus switches in Data Centre * HA site have been configued with VTP Transparent mode with only required vlans.

Server required ( Vlan2,11,15,222-227) on Nexus 5010 .

5) HSRP config : Core1 switch : All  Vlans priority - 110

Core2 switch : All vlans priority 100 (default)

HA core3 switch : priority 90

6) nexus 5010 is layer2 switch

attaching the diagram for reference....  

plz let me know if any of you..have experienced the same problem......experts : plz comment....your recommendations...

Actual Problem : If only either one of Core1 or Core2 is lets assume Core2 forming VPC with N5k switches  and Core 1 is not connected to N5k

for VPC connectvitiy then i dont face any STP loop/HSRP L3 interface flapping issue , Howerver as soon as i connect Core1 to N5k using VPC method then after couple of hours the vlans which are allowe to pass through trunk of N5k ( vlan2,11,15,222-227) are Flapping in HSRP in Core1 switch.

6509 core1 ------allowed valn 2,11,15,222-227 in portchannel2 --- Nexus side trunk same vlans allowed

6509 core2 ------allowed valn 2,11,15,222-227 in portchannel3 --- Nexus side trunk same vlans allowed

In HA site we have single N5010 switch and hence no VPC only ether-channel is being used , but I am sure this issue not triggering from HA site.

Root cause : ???



1 Reply 1

Reza Sharifi
Hall of Fame Master Hall of Fame Master
Hall of Fame Master


Are you using MST?  The Nexus default STP is RSTP.  so, make sure if you are configuring MST on the 6500,apply it to the 5ks as well.

What is the output of "sh spanni root"? from the 5ks?


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