Symptom
Input errors are observed in MACSEC enabled interface configured as ISIS point-to-point.
1. NCS-5500-1 tenGigE 0/0/1/9 Sent 40 PSNPs; Received 0.
RP/0/RP0/CPU0:NCS-5500-1#show isis statistics tenGigE 0/0/1/9 | i sent
PTP Hellos (sent/rcvd) : 10/10
Level-2 LSPs (sent/rcvd) : 40/40
Level-2 CSNPs (sent/rcvd) : 1/1
Level-2 PSNPs (sent/rcvd) : 40/0 <<<<<<<< HERE
2. Remote peer NCS-5500-2 interface tenGigE 0/0/0/9 has 40 input errors/runts.
RP/0/RP0/CPU0:NCS-5500-2#show interfaces tenGigE 0/0/0/9
176 packets input, 28177 bytes, 0 total input drops
0 drops for unrecognized upper-level protocol
Received 0 broadcast packets, 172 multicast packets
40 runts, 0 giants, 0 throttles, 0 parity
40 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort <<<<<<<< HERE
3. NCS-5500-2 tenGigE 0/0/0/9 Sent 40 PSNPs; Received 0.
RP/0/RP0/CPU0:NCS-5500-2#show isis statistics tenGigE 0/0/0/9 | i sent
PTP Hellos (sent/rcvd) : 11/9
Level-2 LSPs (sent/rcvd) : 40/40
Level-2 CSNPs (sent/rcvd) : 1/1
Level-2 PSNPs (sent/rcvd) : 40/0 <<<<<<<< HERE
4. Remote peer interface tenGigE 0/0/0/9 has 40 input errors/runts.
RP/0/RP0/CPU0:NCS-5500-1#show interfaces tenGigE 0/0/1/9
173 packets input, 31852 bytes, 0 total input drops
0 drops for unrecognized upper-level protocol
Received 1 broadcast packets, 169 multicast packets
0 runts, 0 giants, 0 throttles, 0 parity
40 input errors, 40 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort <<<<<<<< HERE
Conditions
MACSEC configured and peer nodes using distinct MAC PHYs.
ISIS enabled with peer nodes interfaces configured as ISIS point-to-point interfaces.
Further Problem Description
In described conditions ISIS PSNPs are received at interface level but discarded as input errors.
Any packets with size less than 64 and the associated padding is causing this MAC PHYs inter-op issue due to which both peer nodes are marking a valid packet invalid.
