Incident documentation/2021-11-18 codfw ipv6 network

document status: in-review

Summary

After preemptively replacing one of codfw row B spine switch (asw-b7-codfw) for signs of disk failure, the new switch was silently discarding IPv6 traffic (through and within the switch).

As this switch was a spine, ~50% traffic toward that row (from cr2) was transiting through it.

Row B being at this time the row hosting the load-balancer in front of upload-lb.codfw, this was the most visible impact.

Monitoring triggered and the interface between asw-b7-codfw and cr2-codfw was disabled, forcing traffic through the cr1<->asw-b2-codfw link. Resolving the upload-lb issue.

Replacing the switch didn't solve the underlying IPv6 issue, showing that it was not a hardware issue. Forcing a virtual-chassis master failover solved what we think was a Junos (switch operating system) bug.

Note that at the time of the issue, our Juniper support contract was expired, preventing us from opening a JTAC case.

Impact: Thanks to Happy Eyeballs there was no visible user impact (or, at worse, a slight latency increase).

For 8 minutes, we had a partial loss of IPv6 connectivity (which affects a subset of Internet providers) in the Codfw cluster (which serves a subset of regions) for upload.wikimedia.org.

Documentation:

• Original maintenance/incident task, T295118

Actionables

• Icinga check for ipv6 host reachability, T163996