When ACX7k is in the transit and the incoming interface MTU is higher than the outgoing interface IPv4/IPv6 traffic we should see ICMP error code which is not seen. to intra-area links.
In the topology below, MX1 and ACX7k has a higher mtu configured and ACX7k to MX2 has lower mtu.
ICMP with higher packet sizes are not returned when transiting ACX7k for both ipv4 and ipv6
MX1(xe-0/3/0)-HigherMTU-->(et-0/0/4) ACX7K(et-0/0/5)-LowerMTU--> (xe-0/1/7) MX2
labroot@ACX7k> show version Hostname: ACX7kModel: ACX7024Junos: 23.4R1.1-EVOYocto: 3.0.2Linux Kernel: 5.2.60-yocto-standard-g6b641dfJUNOS-EVO OS 64-bit [junos-evo-install-acx-f-x86-64-23.4R1.1-EVO]If the traffic is in the main instance, packet-too-big is returned.<--- ipv4 testlabroot@MX1-re0> ping 192.168.0.100 source 192.168.0.1 routing-instance VR1 count 3 size 100 PING 192.168.0.100 (192.168.0.100): 100 data bytes108 bytes from 192.168.0.100: icmp_seq=0 ttl=63 time=1.056 ms108 bytes from 192.168.0.100: icmp_seq=1 ttl=63 time=0.808 ms108 bytes from 192.168.0.100: icmp_seq=2 ttl=63 time=0.860 ms--- 192.168.0.100 ping statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max/stddev = 0.808/0.908/1.056/0.107 mslabroot@MX1-re0> ping 192.168.0.100 source 192.168.0.1 routing-instance VR1 count 3 size 1000 PING 192.168.0.100 (192.168.0.100): 1000 data bytes1008 bytes from 192.168.0.100: icmp_seq=0 ttl=63 time=1.194 ms1008 bytes from 192.168.0.100: icmp_seq=1 ttl=63 time=1.128 ms1008 bytes from 192.168.0.100: icmp_seq=2 ttl=63 time=1.013 ms--- 192.168.0.100 ping statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max/stddev = 1.013/1.112/1.194/0.075 ms<-- no packet too big received with ipv4labroot@MX1-re0> ping 192.168.0.100 source 192.168.0.1 routing-instance VR1 count 3 size 1500 PING 192.168.0.100 (192.168.0.100): 1500 data bytes--- 192.168.0.100 ping statistics ---3 packets transmitted, 0 packets received, 100% packet losslabroot@MX1-re0> ping 192.168.0.100 source 192.168.0.1 routing-instance VR1 count 3 size 1500 PING 192.168.0.100 (192.168.0.100): 1500 data bytes--- 192.168.0.100 ping statistics ---3 packets transmitted, 0 packets received, 100% packet loss<-- nothing on the wirelabroot@MX1-re0> monitor traffic interface ae100.1 matching ip 01:01:26.597079 Out IP truncated-ip - 1478 bytes missing! 192.168.0.1 > 192.168.0.100: ICMP echo request, id 39197, seq 0, length 150801:01:27.598162 Out IP truncated-ip - 1478 bytes missing! 192.168.0.1 > 192.168.0.100: ICMP echo request, id 39197, seq 1, length 150801:01:28.599236 Out IP truncated-ip - 1478 bytes missing! 192.168.0.1 > 192.168.0.100: ICMP echo request, id 39197, seq 2, length 150801:01:28.679470 In IP 10.0.1.2 > ospf-all.mcast.net: OSPFv2, Hello, length 6001:01:28.939255 Out IP truncated-ip - 30 bytes missing! 10.0.1.1 > ospf-all.mcast.net: OSPFv2, Hello, length 6001:01:38.523942 Out IP truncated-ip - 30 bytes missing! 10.0.1.1 > ospf-all.mcast.net: OSPFv2, Hello, length 6001:01:38.678791 In IP 10.0.1.2 > ospf-all.mcast.net: OSPFv2, Hello, length 60^C429 packets received by filter0 packets dropped by kernel<--- ipv6 test<-- no packet too big received with ipv6labroot@MX1-re0> ping 3ffe::192:168:0:100 source 3ffe::192:168:0:1 routing-instance VR1 count 3 size 100PING6(148=40+8+100 bytes) 3ffe::192:168:0:1 --> 3ffe::192:168:0:100108 bytes from 3ffe::192:168:0:100, icmp_seq=0 hlim=63 time=12.308 ms108 bytes from 3ffe::192:168:0:100, icmp_seq=1 hlim=63 time=11.942 ms108 bytes from 3ffe::192:168:0:100, icmp_seq=2 hlim=63 time=1.218 ms--- 3ffe::192:168:0:100 ping6 statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max/std-dev = 1.218/8.489/12.308/5.144 mslabroot@MX1-re0> ping 3ffe::192:168:0:100 source 3ffe::192:168:0:1 routing-instance VR1 count 3 size 1000 PING6(1048=40+8+1000 bytes) 3ffe::192:168:0:1 --> 3ffe::192:168:0:1001008 bytes from 3ffe::192:168:0:100, icmp_seq=0 hlim=63 time=1.342 ms1008 bytes from 3ffe::192:168:0:100, icmp_seq=1 hlim=63 time=0.990 ms1008 bytes from 3ffe::192:168:0:100, icmp_seq=2 hlim=63 time=2.984 ms--- 3ffe::192:168:0:100 ping6 statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max/std-dev = 0.990/1.772/2.984/0.869 mslabroot@MX1-re0> ping 3ffe::192:168:0:100 source 3ffe::192:168:0:1 routing-instance VR1 count 3 size 1500 PING6(1548=40+8+1500 bytes) 3ffe::192:168:0:1 --> 3ffe::192:168:0:100--- 3ffe::192:168:0:100 ping6 statistics ---3 packets transmitted, 0 packets received, 100% packet loss<-- nothing on the wirelabroot@MX1-re0> monitor traffic interface ae100.1 matching icmp6 01:04:10.645887 Out IP6 truncated-ip6 - 1498 bytes missing!3ffe::192:168:0:1 > 3ffe::192:168:0:100: ICMP6, echo request, seq 0, length 150801:04:10.646850 Out IP6 truncated-ip6 - 1498 bytes missing!3ffe::192:168:0:1 > 3ffe::192:168:0:100: ICMP6, echo request, seq 1, length 150801:04:11.645951 Out IP6 truncated-ip6 - 1498 bytes missing!3ffe::192:168:0:1 > 3ffe::192:168:0:100: ICMP6, echo request, seq 2, length 150801:04:15.339326 In IP6 fe80::3e08:cd00:188:ded6 > fe80::66c3:d600:1b8:c784: ICMP6, neighbor solicitation, who has fe80::66c3:d600:1b8:c784, length 3201:04:15.339430 Out IP6 truncated-ip6 - 14 bytes missing!fe80::66c3:d600:1b8:c784 > fe80::3e08:cd00:188:ded6: ICMP6, neighbor advertisement[|icmp6]^C312 packets received by filter0 packets dropped by kernel
This is a day one behavior in ACX7k and we need to take the fix of PR1793775 to make this work.