vpc system priority

[e-chuah]

Hi,  Can anyone explain what is the purpose and usage of the system priority command under vpc config mode? The vpc quick config guide mentioned :   "You should manually configure the vPC system priority when you are running Link Aggregation Control Protocol (LACP) to help ensure that the vPC peer devices are the primary devices on LACP". But I don't understand what does this mean...  Thanks Eng Wee

[vdsudame]

System priority should be the same between the two vpc peer switches for vpc to come up. If you dont configure it, it will default to 32667 and vpc will still come up in that case. In the LACP case you can have up to 16 Ethernet  ports of the same type. Up to eight ports can be active, and up to eight ports can be in standby mode. When there are more than eight ports in an LACP channel-group, the switch on the controlling end of the link uses port priorities to determine which ports are bundled into the channel and which ports are put in hot-standby mode. By lowering the system priority, you guarantee that your peer partner (i.e. access switch or other vPC pair) would not be able to take any decision in term of the aggregation capabilities/advertisement.

Thanks, Vinayak

[e-chuah]

Hi Vinayak, Thanks for the explanation. I have two more questions. (Question1) - If i have less than 8 links, vpc system priority is not really of much use. Is that correct? (Question2) - I read further documentation on CCO and it seems that vpc system priority is very similar in function to lacp system priority. It's just that vpc system priority is used when in vpc environment. If i am using LACP in vpc environment, then there is no need to configure lacp system priority. Configure the vpc system priority will do. THanks Eng Wee

[vdsudame · Accepted solution]

q1) yes you are correct

q2) yes normally we dont tweak the lacp system priority in vpc environment. just stay with vpc system priority

Thanks, Vinayak

[danrya]

To add clarity, the system-priority is used as the LACP priority in the LACP negotiation.  The system with the lower LACP priority will become master and the other side will be slave.  The LACP master will dictate things like which interfaces are in standby (as previously stated) but it will also dictate the LACP rate, such as FAST or SLOW timers.  With a Nexus switch, if a device negotiates with fast timers, the switch will not be able to complete an ISSU, and all upgrades will be impact-full.

Also, as mentioned, the system-priority is only for vPC links.  If you have other non-VPC links (such as L3 links to a core router), you should also use the lacp system priority command.

[navaidmallick2009]

Please correct me if i am wrong.

q1-if we want to configure our primary switch as master to dictate things and secondary switch as slave we have to lower down the system priority (LACP priority) of primary switch but according to cisco guide system priority must be same at both end when we configured it manually.

[Pavinder Singh]

Hi Navaid,

I think you mixed local priority (vpc) with system priority.

:)

[Vinay Sharma]

Thank you Vinayak for sharing your expertise and being a consistent top contributor on community 

5+ :-).

This discussion has been converted into Document.

Many Regards,

Vinay Sharma,
Community Manager

[Yan Dong Zhang]

very helpful!