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Cisco Bug: CSCvu87973 - Catalyst 9600 cannot further fragment multicast fragments

Last Modified

Aug 29, 2020

Products (118)

  • Cisco Catalyst 9600 Series Switches
  • Cisco Catalyst 9300L-48UXG-4X-A Switch
  • Cisco Catalyst 9300L-24UXG-4X-E Switch
  • Cisco Catalyst IE-3200-8T2S Rugged Switch
  • Cisco Catalyst 9200L-48P-4X Switch
  • Cisco Catalyst 9300L-48P-4X-A Switch
  • Cisco Catalyst C9500-16X-E Switch
  • Cisco Catalyst 9300L-48P-4G-E Switch
  • Cisco Catalyst C9500-32QC Switch
  • Cisco Catalyst 9400 Supervisor Engine-1XL-Y
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Known Affected Releases

16.12.3 17.2.1 Amsterdam-17.2.1

Description (partial)

Symptom:
Any multicast packet that arrives as a fragment (MF=1), and needs to be fragmented further before sending that packet out, will be dropped.

You will see drops recorded in the ASIC at register "MTU_FAIL_DROP_BRIDGED_IP_ROUTED" in "show plat hard fed active fwd-asic drop exception"

*** This will affect any L3 type interface (physical or tunnel) ***

Example scenarios:

1. Fragmented packet (MF bit set) enters the Catalyst 9600 with total size of 1518 bytes (MTU + headers), and egresses a GRE Tunnel that requires the packet to be fragmented (default IP MTU on the GRE tunnel and transport MTU on the GRE tunnel of 1476) there will be 100% loss of that specific packets that need to be fragmented by the 9600. 

2.Fragmented packet (MF bit set) enters the Catalyst 9600 with total size of 1518 bytes (MTU + headers), and egresses a L3 interface with IP MTU 1376, there will be 100% for those packets that need to be fragmented to be sent across the L3 interface.

Conditions:
Any IP packet that ingresses a Catalyst 9600 running IOS XE 17.2.1 with the MF bit set to 1, may experience a complete loss in that packet being forwarded if the packet needs to be fragmented before egressing the Catalyst 9600.
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