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Purpose and Usage of Virtual Port Channel (vPC) system-priority command in NX-OS (Nexus

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Introduction

What is the purpose and usage of the system priority command under vpc configuration 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". 

Solution

System priority should be the same between the two vpc peer switches for vpc to come up. If you don't configure it, it will default to 32667 and vpc will still come up in that case.

In the LACP case we 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, we guarantee that our 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.

Question 1

In case we have less than 8 links, vpc system priority is not really of much use. Is that correct?

Answer: Yes that is correct, vpc system priority is not really of much use.

Question 2

Is 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 we are using LACP in vpc environment, then there is no need to configure lacp system priority. Configure the vpc system priority will do. 

Answer: Yes, normally we do not tweak the lacp system priority in vpc environment. just stay with vpc system priority.

More Information

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.

vPC Concepts

 

system-priority priority

 

Example:

 

switch(config-vpc-domain)# system-priority 4000

 

switch(config-vpc-domain)#

 

(Optional) Enter the system priority that you want for the specified vPC domain. The range of values is 1 to 65535. The default value is 32667.

 

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. When you manually configure the system priority, make sure that you configure the same priority value on both vPC peer devices. If these values do not match, vPC will not be activated.

show vpc role

Displays the peer status, role of the local device, vPC system MAC address and system priority, and MAC address and priority for the local vPC device

 

System-Priority Command Details

To manually configure a system priority for the virtual port channel (vPC) domain, use the system-priority command. To restore the default system priority, use the no form of this command.

system-priority priority_value

no system-priority priority_value

Syntax Description

priority_value
System priority that you want for the specified vPC domain. The range is from 1 to 65535, and the default value is 32667.

 

Command Default

The default for the system priority is 32667.

Command Modes

vPC domain configuration mode

Command History

Release
Modification
4.2(1)N1(1)
This command was introduced.

 

Usage Guidelines

We recommend that you manually configure the vPC system priority when you are running Link Aggregation Control Protocol (LACP) to ensure that the vPC peer devices are the primary devices on LACP. When you manually configure the system priority, ensure that you configure the same priority value on both vPC peer devices. If these values do not match, vPC will not come up.

Examples

This example shows how to configure the system priority for the vPC domain:

switch(config-vpc-domain)# system-priority 3000
switch(config-vpc-domain)#

Related Commands

Command
Description
copy running-config startup-config
Copies the running configuration to the startup configuration.
show running-config vpc
Displays the running configuration information for vPCs.
show vpc role
Displays the vPC system priority.

 

Manually Configuring the System Priority

When you create a vPC domain, the system automatically creates a vPC system priority. However, you can also manually configure a system priority for the vPC domain.

Note We recommend that you manually configure the vPC system priority when you are running LACP to ensure that the vPC peer devices are the primary devices on LACP. When you manually configure the system priority, ensure that you configure the same priority value on both vPC peer devices. If these values do not match, vPC will not come up.
BEFORE YOU BEGIN

Ensure that you have enabled the vPC feature.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

  1. configure terminal
  2. vpc domain domain-id
  3. system-priority priority
  4. exit
  5. (Optional) show vpc role
  6. (Optional) copy running-config startup-config

DETAILED STEPS

 

 
Command
Purpose

Step 1

configure terminal

 

Example:

switch# configure terminal

switch(config)#

Enters global configuration mode.

Step 2

vpc domain domain-id

 

Example:

switch(config)# vpc domain 5

switch(config-vpc-domain)#

Enters the vPC domain number that you want to configure. The system enters the vpc-domain configuration mode.

Step 3

system-priority priority

 

Example:

switch(config-vpc-domain)# system-priority 4000

switch(config-vpc-domain)#

Enters the system priority that you want for the specified vPC domain. The range of values is from 1 to 65535. The default value is 32667.

Step 4

exit

 

Example:

switch(config-vpc-domain)# exit

switch(config)#

Exits the vpc-domain configuration mode.

Step 5

show vpc role

 

Example:

switch# show vpc role

(Optional) Displays the vPC system priority.

Step 6

copy running-config startup-config

 

Example:

switch# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

 

This example shows how to manually configure the vPC domain system priority:

switch# configure terminal
switch(config)# vpc domain 5
switch(config-vpc-domain)# system-priority 4000

Reference

Comments

Hai vinay,

 

Thanks for your article.

Im still confused what exactly function vpc system priority.

Like you said,
1) "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". "System priority should be the same between the two vpc peer switches for vpc to come up. If you don't configure it, it will default to 32667 and vpc will still come up in that case".

Question:

How to vPC peer device elect primary device on LACP environtment in the same system priority?

 

2)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

Question:

How to determine which port are bundled into channel and which port are pun in hot-standby mode in the same switch system priority?

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