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1. IL-0266 Eqiad routers looped traffic between each other after a BGP MED change and could not reach other sites' public IPs

   Wikimedia Foundation2021

   Symptoms

   On 23 November 2021 at 09:39, two minutes after a change on the eqiad core routers, alerts fired because Icinga in eqiad could not reach the public text-lb address in eqsin (Singapore). For about 12 minutes eqiad could not reach hosts in other data centers, such as the cache PoPs, through their public IP addresses.

   Network unreachable

   Trigger

   At about 09:37 a change on cr1-eqiad and cr2-eqiad, made as part of task T295672 to influence BGP route selection, removed a BGP setting that sets the BGP "MED" attribute to the OSPF cost of reaching the BGP route's next hop.

   Root cause

   Configuration change

   Component

   BGP routing

   Contributing factor

   Ineffective rollback

   Detection

   Icinga health checks against edge caching clusters alerted at 09:39; the first alert was Icinga in eqiad failing to reach the public text-lb address in eqsin.

   Remediation

   • Reverted the configuration change on both eqiad routers
   • Forced a reset of the BGP session between the two eqiad routers

   Action items

   • Updated T295672 with further detail on the incident and a proposal to adjust the BGP sessions to address the "second IXP port" issue and prevent this from happening again

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