Arista EOS MTU Behavior (3) - L3 Routed Ethernet Ports, IPv4

Now we switched to L3 routed ethernet (IPv4) ports:

• MTU enforcement is for egress traffic
• Hardware programs the MTU value (default value is 1500, in this case, 2000) and checks against forwarding packet length (in this case, it is ipv4 packet). 
• For IPv4 packets, if the length is more than MTU, the packets will be fragmented by CPU. 
• And the number of fragmented and dropped packets are reflected by CoPPEgressTrap counter in the output of "show cpu counters queue summary | nz"

Arista EOS MTU Behavior (2) - L2 Switched Ports

Below is a quick lab using 7280CR which is Jericho-based system, 

• Use EOS tool - Ethxmit on the generator to send packets
• --ip-dst pointing to the destination address on the receiver device
• --size specifies the packet size. Please the real ethernet packet size is this number minus 4 (I believe it is ethernet trailer). So 1504 means 1500-byte packet is received on sender ingress port
• Run tcpdump + mirror on R3/receiver to check MTU behavior

Conclusion:

• For switched traffic, no MTU enforcement, ingress nor egress

Some other observations:

• For using ethxmit:
• --size parameter for ethxmit includes the 4-byte CRC/ethernet trailer
• If egress is access port, the ethernet frame size = L3 MTU + 18-byte including 14-byte ethernet header and 4-byte CRC. 
• If egress port is trunk, ethernet frame size = L3 MTU + 22-byte including 14-byte ethernet, 4-byte vlan and 4-byte CRC
• No matter the mtu size under L2 interfaces, the forwarding MTU is set to maximum value 10178 (even show interface says Ethernet MTU, but it is not actual ethernet frame size, but forwarding MTU before ethernet header). Look at the ip header total length value in tcpdump output. 
• Trident-based system would use 9214. 

R2#sh int e47/1 | grep -i mtu
  Ethernet MTU 10178 bytes , BW 10000000 kbit
R2#sh run int e47/1
interface Ethernet47/1
   mtu 100
   switchport access vlan 100

Arista EOS MTU Behavior (1)

Summary:

In this series of blogs, I will cover the MTU (Maximum Transmission Unit) behavior on Arista Jericho-based routers. Here is the summary:

• 1. MTU enforcement happens on the egress interfaces. 
• So MTU is not MRU (maximum receive unit), large size packets are NOT dropped at ingress interfaces
• 2. Arista MTU is layer-3 MTU on Jericho chipset:
• 2.1 The MTU enforcement is only for routed packets, no action on switched traffic
• 2.2 The actual packet size is MTU size plus L2 header size, which is 14-byte for L3 routed interface or 18-byte for the trunk interface
• 3. IPv4 packets are fragmented by router CPU and capped by CoPP

Reference:

• Arista EOS Manual, Section 19.4 MTU
• Arista 4.23.1F TOI - Support for L3 MTU on 7280R3/7500R3/7800R3