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1. IL-0267 Wikimedia's eqsin cache site went unreachable when both of its transport links to the core data centers were down

   Wikimedia Foundation2023

   Symptoms

   On 10 January 2023 at 16:37 UTC Icinga reported "BGP status on cr3-eqsin is CRITICAL: BGP CRITICAL - AS1299/IPv6: Active - Telia, AS1299/IPv4: Active - Telia", and from 16:45 the hosts in Wikimedia's eqsin site went DOWN with "PING CRITICAL - Packet loss = 100%". Users normally directed to eqsin, mostly in the APAC region, could only read Wikipedia pages already cached in eqsin.

   Network unreachable

   Trigger

   Planned maintenance by the transport provider took down eqsin's second transport link while the first was still down from a long fiber cut.

   Root cause

   Dependency failure

   Component

   Backbone network (WAN)BGP routingCDN and edge network

   Contributing factor

   Single point of failureIgnored early signal

   Detection

   Automated monitoring: Icinga BGP status alerts for cr3-eqsin at 16:37 and cr2-eqsin at 16:44, followed by a flood of host-down alerts and three pages at the same time.

   Remediation

   • Depooled eqsin from DNS
   • Repooled eqsin

   Action items

   • Create a backup GRE tunnel for eqsin (T327265)
   • Ensure the long-down transport link comes back up properly once it is fixed (T322529)
   • Automatically parse maintenance notifications and alert on conflicting maintenance (T230835)

   Lessons

   The site was depooled quickly, and "depool first, investigate later" was the right attitude. A flood of host-down alerts usually means a network problem. eqsin stayed too long on a single working transport link, SREs did not catch that the downtime of the two transport links overlapped, and the provider's planned maintenance was not recorded in a task or reminder.

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