LACP system-priority question

[leowls]

Do we use the command only for situation where the 2 switches that are trunking each other is set to 'active' for their lacp configuration? I know a switch can be made a decision maker by making the other passive. So do we use the system-priority command for 2 active switches in order to make the switch of our choosing the decision maker?

[Mark Malone]

Its an optional command but the switch with the lowest priority choses which ports are actively in use in the port-channel at any one time

http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/feature/guide/gigeth.html

Table 2 LACP Parameter Descriptions

Parameter	Description
LACP system priority	A LACP system priority is configured on each router running LACP. The system priority can be configured automatically or through the CLI. LACP uses the system priority with the router MAC address to form the system ID and also during negotiation with other systems. The LACP system ID is the combination of the LACP system priority value and the MAC address of the router.

[Francesco Molino]

Hi

Active and passive means which one will actively bring up the LACP port-channel (active) and which one just replies to requests (passive).

Afterwards, there is a negotiation in terms which ports are bundled into the port-channel and this is where the system-priority has an impact. The lowest value is the one that has the most priority. This has to be configured whenever the LACP is set to active or passive to determine which switch will decide at the end ports bundled into this LACP channel.

Hope that clarify a bit your question.

Thanks.

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[leowls]

Hi Francesco Molino,

I think I understand the purpose of it now. However I have another question, this time regarding port priority. Please take a look at the config below (taken from ccnp switching official guide).

Switch(config)# lacp system-priority 100

Switch(config)# interface range gig 2/1 – 4 , gig 3/1 – 4

Switch(config-if)# channel-protocol lacp

Switch(config-if)# channel-group 1 mode active

Switch(config-if)# lacp port-priority 100

Switch(config-if)# exit

Switch(config)# interface range gig 2/5 – 8 , gig 3/5 – 8

Switch(config-if)# channel-protocol lacp

Switch(config-if)# channel-group 1 mode active

I understand that the 1st 8 links (gig 2/1 – 4 , gig 3/1 – 4) will be used as primary links as they are configured with a lower port priority. What happens when link gig 3/1 fails? Will the switch activate one link from the standby group (gig 2/5 – 8 , gig 3/5 – 8) to replace the failed link? If no, when and how will the standby group links be activated?

[Francesco Molino]

Hi

Port priority is to indicate which ports need to be used as bundled and which one will be in hot standby.

As soon as one active fails, then the next available hot standby ports will be bundled UP based on its priority.

In case you've the question, here are the keys that switch checks to determine which one to take for its decision (negotiation and ports bundle):

• LACP system priority
• System ID (the switch MAC address)
• LACP port priority
• Port number

Thanks

PS: Please don't forget to rate and mark as correct answer if this answered your question