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<Warn:HAL.FDB.L2SlotTblFull> MSM-A: FDB entry not added on slot 1. Hardware Table full.

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Title<Warn:HAL.FDB.L2SlotTblFull> MSM-A: FDB entry not added on slot 1. Hardware Table full.
Symptoms
The below log messages could appear when the Hardware table is full or when there is a hash collision.

<Warn:HAL.FDB.L2SlotTblFull> MSM-A: FDB entry not added on slot 8. Hardware Table full Process bcmRX will be high. 

 
Environment
  • BD8810
  • Hardware Table
  • EXOS ALL

 
Cause
Below are two reasons why table full messages are showing up.
  • Hardware table is actually full
  • Hash collision table full

How does a hash collision happen?

A hashing algorithm determines what bucket a MAC address should be programmed.  Each bucket in hardware can only hold only 8 entries.  Hash collisions occur when the Algorithm chooses to add a MAC address in a bucket which has 8 entries already.  Hash collisions can be considered table full as the bucket that the MAC is going to be programed to is full.  Because of this we see the Hardware Table full message even before reaching the theoretical limit of FDB table size.  This is an expected behavior.

Resolution
There are ways to change how the hashing algorithm adds entries to HW but it does not always work.

1) Could try to change the hash algorithm to CRC 16 from CRC 32 (which is default).
  • configure forwarding hash-algorithm crc16 (reboot needed)
2) Change the hardware from C series to XL blades, as the XL blades use the TCAM table and not the hash table.
3) Need to move these MAC addresses which are not added to the Hardware Table away from the switch, or segregate the network to aggregation switches, and break the l2 boundaries to the aggregation switches.
Additional notes

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