port and system priority

[vishalpatil86]

Hi guys,

What is the difference between port priority and system priority in LACp?? are they same??

And why exactly it is used?

Thanks inadvance                  

[flokki123 · Accepted solution]

hi vishal,

a set of up to 16 potential links(ports) can be defined for each Etherchannel.

The switch with the lowest system priority can choose the ports with the lowest port priority to become active within the Etherchannel. The other links will be placed in a standby mode, in case one link experiences a failure.

If that should be the case the port with the next lowest priority will become active.

So here a low priority is actually good.

Hope this helps.

[vishalpatil86]

means if there are two switches A and B. Switch A is having low system priority. SO, A will choose A's and B's port with lowest port ID? it will decide the port for itself or neighboring port also? can you explain it with the example?

[flokki123]

the way i see it is that actually all 16 links which can be in an etherchannel have to be physically connected in order to be a redundant path and if one link fails the next can be up right away.

switch A, the switch with the lower priority, would choose on itself the 8(lets assume we have 16 ports in the channel) ports with the lowest priority, if they should have all the default priority(32768), the port number is the tie-breaker.

these 8 ports and the corresponding ports on switch B would become the active ports in the channel.

now if one link of these 8 fails, switch A would choose the port with the next lowest port-priority, or port number and make it active thus making the corresponding port on switch B active as well.

so i would say the "master" switch chooses only ports on itself.

thats how i see it.

the FLG and OCG and not very specific about the selection process so i can only tell you what i think.

[vishalpatil86]

ohk!!!

thanks