Incidents/2022-02-01 ulsfo network

document status: in-review

Summary

Incident metadata (see Incident Scorecard)

Incident ID	2022-02-01 ulsfo network	Start	2022-02-01 16:05
Task		End	2022-02-01 16:08
People paged		Responder count	1
Coordinators		Affected metrics/SLOs
Impact	For 3 minutes, clients served by the ulsfo POP were not able to contribute or display un-cached pages. 8000 errors per minute (HTTP 5xx).

A firewall change was pushed to ulsfo routers, which caused ulsfo to lose connectivity to the other POPs and core sites for 3min.

Timeline

• 16:03 - apply configuration change on cr3-ulsfo
• 16:05 - apply configuration change on cr4-ulsfo - outage starts
• 16:06 - Icinga notifies about connectivity issues to ulsfo - paging
• 16:08 - change rolled back - outage ends

Documentation:

• https://grafana.wikimedia.org/d/000000503/varnish-http-errors?from=1643645069528&to=1643645324222

Root cause:

The change incorrectly restricts BFD to BGP peers:

+      term allow_bfd {
+          from {
+              source-prefix-list {
+                  bgp-sessions;
+              }
+              protocol udp;
+              port 3784-3785;
+          }
+          then accept;
+      }

While BFD is also used by OSPF sessions, which caused them to be tear down. One surprising point is that the issue didn't show up in the verification commands (show ospf interfaces, show ospf neighbors), all neighbors are present.

show ospf interface   

Interface           State   Area            DR ID           BDR ID          Nbrs

ae0.2               PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1

et-0/0/1.401        PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1

xe-0/1/1.0          PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1

show ospf neighbor    

Address          Interface              State     ID               Pri  Dead

198.35.26.197    ae0.2                  Full      198.35.26.193    128    35

198.35.26.199    et-0/0/1.401           Full      198.35.26.194    128    33

198.35.26.209    xe-0/1/1.0             Full      208.80.154.198   128    34

While it was effectively down:

rpd[16292]: RPD_OSPF_NBRDOWN: OSPF neighbor 198.35.26.209 (realm ospf-v2 xe-0/1/1.0 area 0.0.0.0) state changed from Full to Down due to InActiveTimer (event reason: BFD session timed out and neighbor was declared dead)

Conclusion

• More progressive roll out (longer wait time between each routers, as well as fewer changes at a time) could have reduced the risk of the issue happening
• Monitoring properly caught the issue

Scorecard

Incident Engagement™  ScoreCard		Score
People	Were the people responding to this incident sufficiently different than the previous N incidents? (0/5pt)	N/A
	Were the people who responded prepared enough to respond effectively (0/5pt)	5
	Did fewer than 5 people get paged (0/5pt)?
	Were pages routed to the correct sub-team(s)?	1
	Were pages routed to online (working hours) engineers (0/5pt)? (score 0 if people were paged after-hours)
Process	Was the incident status section actively updated during the incident? (0/1pt)	N/A
	If this was a major outage noticed by the community, was the public status page updated? If the issue was internal, was the rest of the organization updated with relevant incident statuses? (0/1pt)	N/A
	Is there a phabricator task for the incident? (0/1pt)	N/A
	Are the documented action items assigned?  (0/1pt)	N/A
	Is this a repeat of an earlier incident (-1 per prev occurrence)	0
	Is there an open task that would prevent this incident / make mitigation easier if implemented? (0/-1p per task)	0
Tooling	Did the people responding have trouble communicating effectively during the incident due to the existing or lack of tooling? (0/5pt)	5
	Did existing monitoring notify the initial responders? (1pt)	1
	Were all engineering tools required available and in service? (0/5pt)	5
	Was there a runbook for all known issues present? (0/5pt)	5
Total Score