Nexus Keepalive failure

[snarayanaraju]

Hello - After reading multiple discussion forms, i am here to ask this question again

What happens when VPC Peer Keepalive link fails whereas Peer-Link is up at that time. Cisco documentation says:

vPC Peer Keepalive Link Failure
The vPC keepalive link carries the heartbeat message between two vPC peer switches. The failure of the vPC keepalive link alone does not impact the vPC operation or data forwarding. Although it has no impact on data forwarding, we recommend that you fix the keepalive as soon as possible to avoid a double failure scenario that could impact the data traffic.

According to the cisco official docs, it seems that nothing is affacted by this failure and the only reaction is that peer-link will act as keepalive link temporarily. So, end users will not be aware of this failure at all, CFS still running in the peer-link.

Q1) I understand "Peer Keepalive" works by Layer 3 whereas Peerlink is on L2. How peerlink can carry Hearbeat messages?

Q2) If peerlink is itself capable of carrying Hear-Beat messages why the need of dedicated Peer Keepalive link?

regards

Sairam

[nazimkha]

Hi Sairam,

Good questions :

As you may know with vPC, two switches appear to work as a single unit. In order for them to appear as a single unit they have to be synchronized. The CFS protocol runs over the peer-link to synchronize the two vPC switches.

Now coming to your questions.

In a good redundant design there is always a 3rd leg. The peer-keepalive is the additional redundancy mechanisms. So even if the heartbeat fails the two peers are still able to synchronize via the peer-link.

So if someone experiences a failure scenario wherein the peer-keepalive goes down followed by peer-link then it will lead to a dual-active scenario

You can check my cisco live session for more details :

https://www.ciscolive.com/global/on-demand-library/?search=vpc#/session/14479207929320017eHp

Nazim

Please do not forget to rate the post

[snarayanaraju]

Thanks Nazim - Do you mean that if the Peer Keepalive fails, the 2 Nexus switches start listening and  learn about each other using PeerLink?

if so, PeerLink is a L2 link whereas PeerKeepalive is L3 Link. How does this happen?

regards,

Sairam

[nazimkha · Accepted solution]

The 2 Nexus are aware of each other through the peer-keepalive as well as the peer-link.

peer-keeaplive : runs heartbeat messages
peer-link : exchange information and know their presence through CFS (Cisco Fabric Services)

peer-keepalive fails : no heartbeat messages but the two vPC switches know about each other through CFS

[snarayanaraju]

Thankyou Nazim for good explanation. I will watch your Cisco Live presentations as well. Thanks again