Nexus 9k, packet loss through switch on vlan without SVI

Sun Jul 24 16:32:27 UTC 2016

Running into some weird issues with a Cisco Nexus9k.

We have a Cisco 3750X pair stacked, port channel (2x 1G) to a two different
blades on a Nexus9k. Isolating the links of the port channel , on one link
we can consistently get 800mbps (using iperf), or the other link we
consistently get ~34mbps.

we have seen this across multiple 3750X stacks.

The vlan we were on is just layer2 through the n9k, there are no IP
addresses. We were able to (apparently) resolve this issue by creating an
SVI on the n9k, with an empty config.

Now, even isolating links we can get ~800mbps across the n9k, through the
various 3750X stacks.

I am confused why creating the SVI would have an impact on this, and why it
wouldn't be consistent across both links. If the lack of SVI were at fault,
I would be less surprised if it just flat out didn't work, but this partial
working state feels very odd.

Anyone else seen this? Thoughts? Could traffic be hitting the CPU while
going across modules? This feels like quirky n9k internals...

Thanks!
Jeremy

PS: no CRC errors found on interfaces, all looked clean