title: How to determine MTU between my cable modem and ISP?
body: <p>I am trying to understand the "big picture" of MTU. Specifically, many discussions of MTU focus on a single hop (e.g. laptop to router), so the natural question is: how to determine MTU between cable modem and ISP, or more generally, for any given hop of a route.</p>
  
  <p>Now, I can easily see the MTU between my laptop and its: wi-fi router using ifconfig on Mac OS X:</p>
  
  <pre><code>en0: flags=8863&lt;UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST&gt; mtu 1500
  options=2b&lt;RXCSUM,TXCSUM,VLAN_HWTAGGING,TSO4&gt;
  ether 58:b0:35:f0:14:75 
  media: autoselect (none)
  status: inactive    
  en1: flags=8863&lt;UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST&gt; mtu 1500
  ether 58:b0:35:72:64:fa 
  inet6 fe80::5ab0:35ff:fe72:64fa%en1 prefixlen 64 scopeid 0x5 
  inet 192.168.1.100 netmask 0xffffff00 broadcast 192.168.1.255
  media: autoselect
  status: active
  </code></pre>
  
  <p>1500 is the canonical value b/c of the limitation of the wi-fi frame (which I am guessing was designed to match the Ethernet frame; please correct me if that's wrong). </p>
  
  <p>So, the question is: <strong>How to determine the MTU of arbitrary hops in my route?</strong></p>
  
  <p><strong>Answer summary:</strong></p>
  
  <p>Per the below answer, best bet is "tcpdump", "traceroute --mtu" or "tracepath"</p>