Routing – How Does a Router Know the MTU of a Network?

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I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is excanged among routers. However, is there another way of finding that information for a router that doesn't use OSPF?

I hope someone can help me. Thank you for your responses.

Best Answer

The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.

Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.

For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.

Related Solutions

IPv6 Routing on EX4200 / EX3300 – Common Questions

If you aren't trying to exchange subnet information with neighboring routers about these /64's, then you'll be fine with the stock EX4200 loadout. However, if you were going to, you'll need routing protocol support (i.e. RIPng, OSPFv3, ISIS, BGP) that only comes with an AFL. Given your design intentions, you don't need the AFL.

Check out Juniper's "Understanding Software Licenses for EX Series Switches" to get more information into what you actually get with the AFL.

Note: All of this functionality exists with/without the AFL, you simply get a warning/syslog message when you don't. They go on more of an honor system.

MTU – How to Verify Current Network MTU Sizes

You can test path MTU discovery across a live network with a tool like tracepath (part of the Linux IPutils package) or mturoute (Windows only). Here's a sample of tracepath output from the lab pictured above, with the MTU of F0/1 reduced to 1400 bytes using the ip mtu command:

Host$ tracepath -n 192.168.1.2
 1:  192.168.0.2       0.097ms pmtu 1500
 1:  192.168.0.1       0.535ms 
 1:  192.168.0.1       0.355ms 
 2:  192.168.0.1       0.430ms pmtu 1400
 2:  192.168.1.2       0.763ms reached
    Resume: pmtu 1400 hops 2 back 254

Best Answer

Related Solutions

IPv6 Routing on EX4200 / EX3300 – Common Questions

MTU – How to Verify Current Network MTU Sizes

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