document status: in-review

Summary and Metadata

The metadata is aimed at helping provide a quick snapshot of context around what happened during the incident.

Incident ID	2021-11-18 codfw ipv6 network	UTC Start Timestamp:	YYYY-MM-DD hh:mm:ss
Incident Task	https://phabricator.wikimedia.org/T299968	UTC End Timestamp	YYYY-MM-DD hh:mm:ss
People Paged	<amount of people>	Responder Count	<amount of people>
Coordinator(s)	Names - Emails	Relevant Metrics / SLO(s) affected	Relevant metrics % error budget
Summary:	For 8 minutes, the Codfw cluster experienced partial loss of IPv6 connectivity for upload.wikimedia.org. Thanks to Happy Eyeballs there was no visible user impact (or, at worse, a slight latency increase). The Codfw cluster generally serves Mexico and parts of the US and Canada. The upload.wikimedia.org service serves photos and other media/document files, such as displayed in Wikipedia articles.

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: For 8 minutes, the Codfw cluster experienced partial loss of IPv6 connectivity for upload.wikimedia.org. Thanks to Happy Eyeballs there was no visible user impact (or, at worse, a slight latency increase). The Codfw cluster generally serves Mexico and parts of the US and Canada. The upload.wikimedia.org service serves photos and other media/document files, such as displayed in Wikipedia articles.

Documentation:

Original maintenance/incident task, T295118

Scorecard

	Question	Score	Notes
People	Were the people responding to this incident sufficiently different than the previous five incidents? (score 1 for yes, 0 for no)	0	Info not logged
	Were the people who responded prepared enough to respond effectively (score 1 for yes, 0 for no)	1
	Were more than 5 people paged? (score 0 for yes, 1 for no)	0	page routed via batphone
	Were pages routed to the correct sub-team(s)? (score 1 for yes, 0 for no)	0	page routed via batphone
	Were pages routed to online (business hours) engineers? (score 1 for yes, 0 if people were paged after business hours)	0	page routed via batphone
Process	Was the incident status section actively updated during the incident? (score 1 for yes, 0 for no)	0
	Was the public status page updated? (score 1 for yes, 0 for no)	0
	Is there a phabricator task for the incident? (score 1 for yes, 0 for no)	0
	Are the documented action items assigned? (score 1 for yes, 0 for no)	0
	Is this a repeat of an earlier incident (score 0 for yes, 1 for no)	0
Tooling	Was there, before the incident occurred, open tasks that would prevent this incident / make mitigation easier if implemented? (score 0 for yes, 1 for no)	1
	Were the people responding able to communicate effectively during the incident with the existing tooling? (score 1 for yes, 0 or no)	1
	Did existing monitoring notify the initial responders? (score 1 for yes, 0 for no)	1
	Were all engineering tools required available and in service? (score 1 for yes, 0 for no)	1
	Was there a runbook for all known issues present? (score 1 for yes, 0 for no)	0
Total score		5

Actionables

Icinga check for ipv6 host reachability, T163996