vPC fast-convergence

noxiosus1 · ‎08-15-2022

Hello all,

I have several questions regarding vPC fast-convergence option.

1) At this moment I found documentation regarding this option only for nexus 9k. Is it supported by nexus7k? If yes, since what release (6.2(16) does not support)? In the config guide for 7k, I didn't find it https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus7000/sw/interfaces/config/cisco_nexus7000_interfaces_config_guide_8x/config-vpcs.html

2) When the option is applied, could it affect current traffic? If yes, then how big the impact could be?

Sergiu.Daniluk · ‎08-15-2022

Hi @noxiosus1

So... just to clarify - "vpc fast convergence" basically means "vPC member ports and orphan ports with vpc orphan-ports suspend command is configured".

In other words, with vpc fast convergence, when vPC peer link goes down, all vpc member ports + vpc orphan ports (so all L2 interfaces that transport vPC vlans) are suspended.

On nexus 7K, by default, when VPC PL goes down, the vpc member ports will be suspended. So what you need to configure additionally to achieve "vpc fast convergence" is to configure "vpc orphan-ports suspend" on all orphan ports.

What I can strongly recommend if you plan on learning about vPC is this document: https://www.cisco.com/c/dam/en/us/td/docs/switches/datacenter/sw/design/vpc_design/vpc_best_practices_design_guide.pdf

Take care,

Sergiu

noxiosus1 · ‎08-16-2022

Hello Sergiu,

thank you for your reply, I agree with what you said, but that does not answer on my questions.

And by the way, it is not stated in documentation, but in several blogs I read, that it is not only suspends orphan ports, but suspends SVI at the same time as physical ports, which is not the case when the option is not applied, which could lead to blackholing traffic for small period of time.

Here:

https://networkdirection.net/articles/virtual-port-channels-vpc/vpc-and-lag-convergence/

and here

https://www.reddit.com/r/Cisco/comments/76un42/vpc_fast_convergence/

And since we made this parallel with "orphan ports suspend" command, afaik, applying this command will lead to flapping the ports. Does it mean, that fast-convergence will flap the ports as well? Only orphan ports?

Sergiu.Daniluk · ‎08-16-2022

is not only suspends orphan ports, but suspends SVI at the same time as physical ports, which is not the case when the option is not applied, which could lead to blackholing traffic for small period of time.

When vPC peer-link fails down and vPC peer-keepalive link is still up, the vPC secondary peer device performs the following operations:

Suspends its vPC member ports
Shuts down the SVI associated to the vPC VLAN

This is the expected behavior for all Nexus platforms and is a mechanism to redirect all traffic to primary peer device. The only interfaces which are not suspended are the orphan ports. In this scenario, the only blackholing of traffic is for orphan ports connected on vPC secondary.

For example, you have a server connected active-stanby to your 2x vpc peers. The active interface is connected on vPC secondary peer. If peer-link goes down, the server's active interface, by default, stays up and traffic is blackholed because vpc member ports are all suspended, including the SVIs for vpc vlans.

The solution to this problem is "vpc orphan-port suspend" command. On Nexus 9000, if we look at the documentation, what is written under fast-convergence command, it says:

The optimization is archived on secondary switch, vPC member ports and orphan ports with vpc orphan-ports suspend command is configured. In case the vPC Peer-Link fails, these ports will be suspended immediately and traffic will be forwarded to primary vPC peer only to improve convergence.

Basically, its normal behavior + orphan-port suspend command.

And by the way, it is not stated in documentation, but in several blogs I read

The behavior described above is mentioned in the document I mentioned in the previous post (vPC design guide). Search in it for "vPC Failure Scenarios" as well for "vPC Systems Behavior When a vPC Peer-Link Goes Down".

And since we made this parallel with "orphan ports suspend" command, afaik, applying this command will lead to flapping the ports. Does it mean, that fast-convergence will flap the ports as well? Only orphan ports?

I am not aware of this interface flap when you configure the "vpc orphan-port suspend". I also don't have a switch to test it out.

Cheers,

Sergiu

noxiosus1 · ‎08-16-2022

Sergiu,

thank you for thorough explanation, but my questions are still valid:

1) Is fast-convergence feature it supported by nexus7k? If yes, since what release (6.2(16) does not support)?

2) When the option is applied, could it affect current traffic? If yes, then how big the impact could be?

Sergiu.Daniluk · ‎08-16-2022

1) Is fast-convergence feature it supported by nexus7k? If yes, since what release (6.2(16) does not support)?

"fast-convergence" command does not exist on N7K, however if you enable "vpc orphan-port suspend" you will get the same behavior as "fast-convergence".

2) When the option is applied, could it affect current traffic? If yes, then how big the impact could be?

As far as I am aware, there should be no impact whatsoever when enabling "vpc orphan-port suspend", and also there is nothing mentioned about an impact in the documentation. However I cannot confirm it as I do not have a lab available.

Cheers,

Sergiu