Tech Tip: Determining MTU via MacOSX Ping oh yeah and Windows

The command:
ping -D -s 1472

What the command does
ping = Obvious
-D = Don’t fragment
-s <value> = the ping size.

Why did I start with 1472? That is the total packet size plus 28 bytes, which equals a 1500 byte packet.

Example Output:

Justins-MacBook-Pro:~ j2sw$ ping -D -s 1472
PING ( 1472 data bytes
1480 bytes from icmp_seq=0 ttl=57 time=426.164 ms
1480 bytes from icmp_seq=1 ttl=57 time=110.762 ms

--- ping statistics ---
2 packets transmitted, 2 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 110.762/268.463/426.164/157.701 ms

Justins-MacBook-Pro:~ j2sw$ ping -D -s 1473

PING ( 1473 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 0

If you want to learn how to do this on windows:

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ethernet MTU and overhead

One of the most common questions is how much overhead do I need to account for on my transport network? I have put together a quick list to help when you are calculating your overhead.

-GRE (IP Protocol 47) (RFC 2784): 24 bytes (20 byte IPv4 header, 4 byte GRE header)
-6in4 encapsulation (IP Protocol 41, RFC 4213): 20 bytes
-4in6 encapsulation (e.g. DS-Lite RFC 6333): 40 bytes
Addition IPv4 header:20 bytes
-IPsec encryption:
73 bytes for ESP-AES-256 and ESP-SHA-HMAC overhead (overhead depends on transport or tunnel mode and the encryption/authentication algorithm and HMAC)
-MPLS: 4 bytes for each label in the stack
-802.1Q tag: 4 bytes
Q-in-Q: 8 bytes
-VXLAN: 50 bytes
-OTV: 42 bytes

Some rules of thumb when setting MTUs. You won’t get fragmentation if your L2 MTU is higher than your L3 MTU. This is just not the setting, but the actual overhead in use. Just setting it to a number doesn’t necessarily make it right. The above list will help you calculate the minimum MTU you may need. I try to get gear that supports a 1548 MTU and set everything to that. Makes it simple. I still want to know how much MTU I am utilizing because it helps me validate my designs.
The most important rule of thumb is you won’t get fragmentation if your l3 MTU is less than your L2 MTU.

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