port channel-stp hypothetical

[k. cheng]

i'm sorry if this question sounds silly but say at a small branch location I connect port 47 on an access switch to port 1 on a core switch and then repeat with ports 48 & 1 but for some reason each connection is assigned to a different port channel. In this hypothetical, the outcome is STP stepping in to block one of the POs right? 

[Richard Pidcock · Accepted solution]

I would expect in that scenario that yes, STP would end up blocking one of the port channels.  Why would you connect in such a way that allows for two separate port channels with only a single interface?  Why don't you include both ports 47 and 48 into the same channel-group and thus have only one port channel and no STP blocks?

[balaji.bandi]

yes if this is simple trunk configuration, if you have controller VLAN allowed in each trunk then that wont be the case.

is this for testing or why you looking to do this ? what is the use case ? (rather make bundle the interfaces ?)

[MHM Cisco World]

How you connect 47 and 48 to port 1?

Can you elaborate 

MHM

[Joseph W. Doherty]

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@k. cheng wrote:

i'm sorry if this question sounds silly but say at a small branch location I connect port 47 on an access switch to port 1 on a core switch and then repeat with ports 48 & 1 but for some reason each connection is assigned to a different port channel. In this hypothetical, the outcome is STP stepping in to block one of the POs right? 

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You're connecting branch port 47 (PC x) to core port 1 and branch port 48 (PC y) to core port 1?

Perhaps you meant connecting branch port 47 (PC x) to core port 1 and branch port 48 (PC y) to core port 2?  If so, PC links are treated like any other logical link.  So, yes, if STP is active, it should block one of the paths.  I'm assuming each core port is not in a PC bundle with the other.

If you meant the two ports on core are in the same PC, well not quite sure what will happen, as I believe that's not a valid setup.  STP hopefully should block one of the branch ports, but unsure how it would consider the logical core PC port.  With a branch port block, you no longer have a loop.

[k. cheng]

hi MHM,

joseph is right, that 2nd "1" was a typo. 

thx for confirming everyone. long story short the change window for my core switch migration was almost up and then i find a physical problem with one of the fiber pairs between the 1fl idf switch and the new core on the 2fl. my company heavily emphasizes uptime and change windows are hard to come by (i know, take a number and get on line!), so i have to dream up weird hacks. like this weird dual-active-po idea and not have it blow up in my face.

[balaji.bandi]

sure agreed some time engineers are under stresh to fixing things, which not given ideal time for planning. and not proper maintenance window .