Does changing the Spanning-tree root to preferred core SW cause any impact?

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Nov 8th, 2008
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Hi all,

2Core Switches, port channeled. Only 2 Vlans in the LAN. Each Access switches with 2 dot1q uplinks connects to Core Switches -most comon design. It is observed that one of the access switch acting as spanning-tree root (hence not blocking second uplink to core SW2). I want to make Core SW1: ROOT and Core SW2: Secondary root, bu lowering their priorities. Does that cause any impact on the other access switches connectivity ?(all other Access switches are connected and one port is blocking).


Thank you in advance


MS

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Jon Marshall Sat, 11/08/2008 - 12:55
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MS


Changing the STP root for those vlans will cause an STP reconvergence. Assuming your uplinks are trunks running all vlans then there will be an outage.


You should definitely do this during a scheduled outage or at the very least out of key hours.


Jon


mvsheik123 Sat, 11/08/2008 - 14:43
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Thanks Jon. I just gave a try by lowering priority for one of the Vlans but do not see any change int the roles..


Command used on Core SW1:

spanning-tree vlan 5 priority 8192


Still the access switch is the root.

All the switches running with default ieee priority : 32769


Also the pvst mode is enabled by default on switches

!

spanning-tree mode pvst

spanning-tree extend system-id

!

Thank you

MS

Jon Marshall Sat, 11/08/2008 - 15:25
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Can you post ouput of "sh spanning-tree vlan 5" off core SW1 and the access-layer switch which is still root.


I'm assuming that vlan 5 is allowed on the link that connects the access-layer switch to the core switch ?


Jon

mvsheik123 Sat, 11/08/2008 - 15:52
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I will post tonight (I need to login to n/w).but yes.. its a simple Dot1Q trunk between core & access Switch and allows all vlans. May be a stupid q..but does Spanning tree (ieee) and trunk (dot1Q) diff has anything to do with it..?


Thanks you in advance

MS

mvsheik123 Sat, 11/08/2008 - 20:28
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Hi,

did a little more investigation, and I was missing the command...


spanning-tree vlan <..> root primary/Secondary


with the commands in place, adding the below took effect..


spanning-tree vlan (list) priority 8192


Thank you

MS

JORGE RODRIGUEZ Sat, 11/08/2008 - 20:30
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MS, completely agree with Jon with this types of changes will trigger stp reconvergance, so you want to try this

during non-production hours.


you have not specified root keyword for root bridge assigments for SW1 become primary root bridge for the vlans.


IN SW1 root you can do for your two vlans, say 5 and 6


CORE SW1

spanning-tree vlan 5,6 root primary


CORE SW2

spanning-tree vlan 5,6 root secondary


You do not need to enter priority when using root primary as it will automcatically set the priority to 8192 given

that your current access switch have the default 32768 priority value , SW1 will become the primary root bridge

for vlan 5 and 6.


have a look here, this is for 6500 platform but applies to any IOS based switch.

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/spantree.html#wp1037815



Once core sw1 is configured as primary , check your access switch issue show spanning-tree detail

take notes it should show the current root bridge mac address which is the mac address if CORE SW1 designated root.

You may also confirm designated root in SW1 by using same show spaning-tree detail.



Rgds

Jorge


PS: Jon how are you, I think I may need to travel to London next year after all, branch office is expanding there, so if

I go I may pay you a visit in Bristol so I read you are moving into :) Rgds





mvsheik123 Sun, 11/09/2008 - 06:43
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Good info Jorge. Thank you. But how the switch act with the command:


spanning-tree vlan 5,6 root secondary


Does the SW still have 32768 as priority and will become root incase of Primary failure or

Priority need to set manully to greater than Root but less than default..?


Also, from the access switch point, when ALTN link takeover (during DESG up link failure)- after the DESG link comes back, it is taking over from the ALTN link. That, as you know cause brief disconnect. Is there any way to avoid it or it will be avoided now with the root designation..?


Thank you

MS

JORGE RODRIGUEZ Sun, 11/09/2008 - 23:02
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Does the SW still have 32768 as priority and will become root incase of Primary failure or Priority need to set manully to greater than Root but less than default..?



Before you enter in SW2 spanning-tree vlan 5,6 root secondary , is goog to take notes of some info , issue in CORE SW2 "show spanning-tree detail" and it should show the root bridge MAC address of sw1 for vlan 5,6 as being the primary root bridge for these vlans

which you have already define it manually , when you set it in SW2 spanning-tree vlan 5,6 root secondary without specifying priority manually I believe it will set the priority automatically higher posibly to 16384. You may hard code the priority on SW2 as well to to be grater than root.

If the Primary Root fails the secondary root becomes the primary root but when primary comes back online it will reclaim primary role because you have configured it with keyword "root primary and set the priority hard coded to 8192 ".


Also as you create more SVIs and VLANS you need to remember to tell CORE SW1 primary root as well as SW2 which is primary and secondary roots for new vlans.


say you create another vlan 7 in future you need to tell SW1 spanning-tree vlan 7 root primary and same in SW2 for secondary.


Have a look again at configuring root bridge and secondary root bridge section in the link

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/spantree.html#wp1037815




Also, from the access switch point, when ALTN link takeover (during DESG up link failure)- after the DESG link comes back,

it is taking over from the ALTN link. That, as you know cause brief disconnect. Is there any way to avoid it or it will

be avoided now with the root designation..?


I don't think this cannot be avoided, you have two links one is blocking and other forwarding, if forwading link fails there will be TCN ( Topology change notofication ) and st recalc have to occor , go over this link for more info on default ST timers

http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml#stp


Rgds

Jorge

cbeswick Mon, 11/10/2008 - 00:24
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Hi,


Just a thought, but is there any chance you could migrate to rapid-pvst before changing your root bridge placements ?


This would improve convergence considerably and hence any potential impact.

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