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Cisco Bug: CSCvu17177 - N9K - "ttag" is not configured under an interface in running-config but still enabled in hardware.

Last Modified

Jun 16, 2020

Products (65)

  • Cisco Nexus 9000 Series Switches
  • Cisco Nexus 9516 Switch
  • Cisco Nexus 3548-X Switch
  • Cisco Nexus 3548 Switch
  • Cisco Nexus 93600CD-GX Switch
  • Cisco Nexus 3636C-R Switch
  • Cisco Nexus 92348GC-X Switch
  • Cisco Nexus 31108TC-V Switch
  • Cisco Nexus 92160YC-X Switch
  • Cisco Nexus 3548-XL Switch
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Known Affected Releases


Description (partial)

TTAG header is added to packets, where we can see there is an extra 8 bytes added with the first 2 bytes representing the IP protocol header as 8905, which indicates the TTAG header is added to a packet when running-config says it should be.

Trigger has been seen to be a reboot.
For interfaces where "ttag" is not configured but TTAG header is still being added on ingress we can check in hardware to see if "ttag" is actual enabled or not despite it not being configured in running-config.

1a) First check the interface mapping, get the "SrcId" for Eth1/19 for example (Eth1/19 maps to SrcId 128): 
1b) Also record the "Unit" and "Slice" column and the number is "0" and "0" and use these in the next command (Unit is the same as ASIC in this case).

le-266-n10-1-ent# show int hardware-mappings | i i 1/19|Name
       HName - Hardware port name. None means N/A
Name       Ifindex  Smod Unit HPort FPort NPort VPort Slice SPort SrcId MacId MacSP VIF  Block BlkSrcID
Eth1/19    1a002400 1    0    64    255   72    -1    0     64    128   16    0     73   1     56         <--- SRC_ID 128

2) Copy and paste the same command, make sure to put the correct ASIC and slice number as seen from the last command, then make sure to copy the SrcId, in this case 128:

le-266-n10-1-ent# attach module 1
module-1# debug hardware internal hea dump asic 0 slice 0 table tah_hea_rwx_rwsrcidtable 128 1
asic instance is 0
asic slice is 0
tbl name is tah_hea_rwx_rwsrcidtable
start entry is 128
entry count is 1
Block base address: 0x02000000 
1st table entry address: 0x02481400 
ENTRY[128] = { ihdr_if=0x00000000 inner_ttag=0x00000000 outer_ttag=0x00000002 spare=0x00000000 ecc=0x00000000  }     <--- outer_ttag set to 0x2... TTAG encap is enabled even though "ttag" not configured on Eth1/19.

3) In the output above we can see outer_ttag=0x2, which means "ttag" is enabled for Eth1/19. If we see outer_ttag=0x0 then ttag is disabled. For us to be hitting the defect the running-config will not match what is configured in hardware, for example, if we see in running-config "ttag" is configured for Eth1/19, the hardware configured should show "outer_ttag=0x00000000", if it shows 0x2 like in the output above then running-config and hardware configuration is inconsistent and this bug is hit.
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