Incidents/2022-02-01 ulsfo network

==Summary==
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 =
 	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 	
People 	Were the people who responded prepared enough to respond effectively (score 1 for yes, 0 for no) 	1 	
People 	Were more than 5 people paged? (score 0 for yes, 1 for no) 	1 	
People 	Were pages routed to the correct sub-team(s)? (score 1 for yes, 0 for no) 	1 	
People 	Were pages routed to online (business hours) engineers? (score 1 for yes,  0 if people were paged after business hours) 	1 	
Process 	Was the incident status section actively updated during the incident? (score 1 for yes, 0 for no) 	0 	
Process 	Was the public status page updated? (score 1 for yes, 0 for no) 	0 	
Process 	Is there a phabricator task for the incident? (score 1 for yes, 0 for no) 	0 	
Process 	Are the documented action items assigned?  (score 1 for yes, 0 for no) 	0 	
Process 	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 	
Tooling 	Were the people responding able to communicate effectively during the incident with the existing tooling? (score 1 for yes, 0 or no) 	1 	
Tooling 	Did existing monitoring notify the initial responders? (score 1 for yes, 0 for no) 	1 	
Tooling 	Were all engineering tools required available and in service? (score 1 for yes, 0 for no) 	1 	
Tooling 	Was there a runbook for all known issues present? (score 1 for yes, 0 for no) 	1 	
Total score	Total score 	9