This article provides more information about the default MRU value setting in ACX Series devices and confirms with an example that MRU = MTU on all ACX platform devices.
In ACX Series devices, the MRU value is not displayed in the output of show interface <interface> extensive and customer would like to confirm if it is the same as in MX Series devices, that is whether MRU = MTU + 8.
show interface <interface> extensive
The following lab test explains the default MRU setting on an ACX device:
ACX2200------------------------MX2020
ACX
labroot@jtac-acx2200> show interfaces terse extensive xe-0/3/1 Interface Admin Link Proto Local Remote xe-0/3/1 up up xe-0/3/1.0 up up inet 10.1.1.2/24 multiservice
labroot@jtac-acx2200> show interfaces extensive xe-0/3/1 | match MTU Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, Carrier transitions: 3, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0 Protocol inet, MTU: 1500 Protocol multiservice, MTU: Unlimited, Generation: 175, Route table: 0
MX
labroot@jtac-mx2010-re0> show interfaces terse xe-7/1/0 Interface Admin Link Proto Local Remote xe-7/1/0 up up xe-7/1/0.0 up up inet 10.1.1.3/24 multiservice labroot@jtac-mx2010-re0> show interfaces extensive xe-7/1/0 | match MTU Link-level type: Ethernet, MTU: 9014, MRU: 9022, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None, Loop Detect PDU Error: None, MAC-REWRITE Error: None, Carrier transitions: 1, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0 Protocol inet, MTU: 9000 Protocol multiservice, MTU: Unlimited, Generation: 179, Route table: 0
In the above output, the MTU value on the MX device is changed to 9014 ethernet MTU and on the ACX device, the default value of 1514 is used.
When we ping from the MX device to the ACX device with an ICMP payload of 1472, it would accept the packet and punt it to the Routing Engine.
labroot@jtac-mx2010-re0> ping 10.1.1.2 count 2 size 1472 do-not-fragment PING 10.1.1.2 (10.1.1.2): 1472 data bytes 1480 bytes from 10.1.1.2: icmp_seq=0 ttl=64 time=1.291 ms 1480 bytes from 10.1.1.2: icmp_seq=1 ttl=64 time=1.370 ms --- 10.1.1.2 ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/stddev = 1.291/1.331/1.370/0.040 ms
labroot@jtac-mx2010-re0> monitor traffic interface xe-7/1/0 no-resolve verbose output suppressed, use <detail> or <extensive> for full protocol decode Address resolution is OFF. Listening on xe-7/1/0, capture size 96 bytes 08:38:34.536987 Out IP truncated-ip - 1440 bytes missing! 10.1.1.3 > 10.1.1.2: ICMP echo request, id 18569, seq 0, length 1480 08:38:34.538220 In IP 10.1.1.2 > 10.1.1.3: ICMP echo reply, id 18569, seq 0, length 1480 08:38:35.538297 Out IP truncated-ip - 1440 bytes missing! 10.1.1.3 > 10.1.1.2: ICMP echo request, id 18569, seq 1, length 1480 08:38:35.539611 In IP 10.1.1.2 > 10.1.1.3: ICMP echo reply, id 18569, seq 1, length 1480
labroot@jtac-acx2200> monitor traffic interface xe-0/3/1 no-resolve verbose output suppressed, use <detail> or <extensive> for full protocol decode Address resolution is OFF. Listening on xe-0/3/1, capture size 96 bytes 08:38:34.529782 In IP 10.1.1.3 > 10.1.1.2: ICMP echo request, id 18569, seq 0, length 1480 08:38:34.529840 Out IP truncated-ip - 1440 bytes missing! 10.1.1.2 > 10.1.1.3: ICMP echo reply, id 18569, seq 0, length 1480 08:38:35.531074 In IP 10.1.1.3 > 10.1.1.2: ICMP echo request, id 18569, seq 1, length 1480 08:38:35.531138 Out IP truncated-ip - 1440 bytes missing! 10.1.1.2 > 10.1.1.3: ICMP echo reply, id 18569, seq 1, length 1480
If we initiate a ping with a payload of 1473, it would not punt the packet to the RE:
labroot@jtac-mx2010-re0> ping 10.1.1.2 count 2 size 1473 do-not-fragment PING 10.1.1.2 (10.1.1.2): 1473 data bytes --- 10.1.1.2 ping statistics --- 2 packets transmitted, 0 packets received, 100% packet loss
labroot@jtac-mx2010-re0> monitor traffic interface xe-7/1/0 no-resolve verbose output suppressed, use <detail> or <extensive> for full protocol decode Address resolution is OFF. Listening on xe-7/1/0, capture size 96 bytes 08:39:51.134500 Out IP truncated-ip - 1441 bytes missing! 10.1.1.3 > 10.1.1.2: ICMP echo request, id 28809, seq 0, length 1481 08:39:52.138302 Out IP truncated-ip - 1441 bytes missing! 10.1.1.3 > 10.1.1.2: ICMP echo request, id 28809, seq 1, length 1481
labroot@jtac-acx2200> monitor traffic interface xe-0/3/1 no-resolve verbose output suppressed, use <detail> or <extensive> for full protocol decode Address resolution is OFF. Listening on xe-0/3/1, capture size 96 bytes ^C 0 packets received by filter 0 packets dropped by kernel
Whereas if we change the MTU value as 9014 on the ACX device and as 1514 on the MX device, the MX device would be able to accept the ICMP payload till 1480 since the MRU is 1522, that is, 1480B ICMP payload + 8B ICMP header + 20B IP header + 14B Ethernet Header + extra 8B =1522.
From the above test, it confirms that ACX devices will accept a packet/punt the traffic to the RE if the received packet size is a maximum size of 1472 ICMP payload + 8B ICMP header + 20B IP header + 14B Ethernet Header = 1514.
Thus MRU = MTU is for all ACX platforms.