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Cisco Bug: CSCvv20275 - UFI : ECMP FEC being used for DH multi homing PIC prefix on core bundle shut

Last Modified

Oct 09, 2020

Products (1)

  • Cisco ASR 9000 Series Aggregation Services Routers

Known Affected Releases

7.2.1.BASE

Description (partial)

Release-note

Symptom:
This error may be seen,

LC/0/0/CPU0:Jul 29 10:47:12.023 PDT: fib_mgr[225]: %PLATFORM-PLAT_FIB-4-OOR_PROT_START_WARNING : Request for ECMP_FEC exceeded the maximum allowed. OOR protection started

Conditions:
1 - Large number of BGP prefixes (32K), in per-prefix label mode
2 - The BGP prefixes have protection, ie, backup paths. However, the same underlying BGP LU is shared.
3 - The BGP paths are BGP LU (collapsed mode), and also protected. Primary and backup use their own separate interfaces.

When the core interface is shut, unexpected events happened.

A - Received FRR interface down event from PFI infra, which triggers switch over to use backup interface at BGP LU level
B - Both interfaces stay up. This is not consistent with the interface down event from PFI
C - Adjacency routes got deleted for both interfaces
D - primary LSD attached route is not deleted, but the backup LSD attached route is deleted

The above sequence caused miscalculation at BGP PIC. Thus primary, and backup paths become ECMP.
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