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Cisco Bug: CSCuj72464 - IOS-XR BGP Umbrella SMU#5 for 4.2.1

Last Modified

May 15, 2018

Products (20)

  • Cisco Carrier Routing System
  • Cisco CRS-1 Line Card Chassis (Dual)
  • Cisco CRS-1 16-Slot Line Card Chassis
  • Cisco CRS-1 Line Card Chassis (Multi)
  • Cisco IOS XR Software
  • Cisco CRS-1 4-Slot Single-Shelf System
  • Cisco ASR 9010 Router
  • Cisco CRS-3 16-Slot Single-Shelf System
  • Cisco XR 12410 Router
  • Cisco CRS-1 Fabric Card Chassis
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Known Affected Releases


Description (partial)

CSCuh02932    BGP Stale routes seen when a peer runs into TCP Oper-Down back-2-back
- Stale prefix remains in table instead of properly aging out.
- The messages in the InQ for the affected peer are not processed.
- Back-to-back IOS messages (within 5 min) indicating Neighbor aborting NSR can be treated as a potential trigger. Sample IOS message is given below.

RP/0/RP0/CPU0:Sep 10 10:40:13.083 EDT: bgp[1045]: %ROUTING-BGP-3-NBR_NSR_DISABLED : NSR disabled on neighbor due to TCP retransmissions

The occurrence of this defect is rare and the following conditions need to aligned to be able to run into this issue.
- A NSR ready neighbor runs into too many TCP retransmissions due to data-path issue. TCP disables NSR for the neighbor and announces OPER_DOWN condition.
- After a cool-down period (2 min), BGP triggers scoped -sync for that neighbor in an effort to restore the NSR state.
- Scoped-sync is a light-weight mechanism to sync information specific to a single neighbor. It is done in two phases:
 (a) TCP Session Synchronization (replication of the TCP session information), (b) BGP Database Sync (replication of the BGP neighbor information)
- BGP on the active RP throttles reading incoming BGP updates during the TCP sync so that both active and standby BGP can synchronize processing updates after the standby BGP session comes up
- In this particular situation, a second TCP OPER_DOWN condition was encountered when the TCP sync was in progress to recover from the first OPER_DOWN condition
- The problem related to stale route results from the fact that BGP fails to reset the neighbors NSR state to OPER-DOWN. As a result, instead of another attempt to start the scoped-sync and restore normal update processing,  BGP fails to resume reading from the socket resulting into permanently throttled Rx side.
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