2.1.a (iii) L2 MTU

This section discusses L2 MTU as it relates to Cisco and 2.1.a (iii) L2 MTU.

Recall in the parent section that we showed the total size of an Ethernet frame (and in 1.1.d we even discused it more). Well the L2 MTU is is general this:

IP packet (1500)
Ethernet src+dst+ethertype+fcs+(optional .1q) = 18-22 bytes

Thus L2 MTU of a frame that holds an IPv4 packet by default can be up to 1518-1522.

In Cisco land when we configure MTU and IP MTU, recall that Cisco just uses the perspective of the IP MTU. Hence even L2 MTU is configured via the perspective of NO ethh eaders.

To increase the ethernet mtu for 1Gbps and 10Gbps interfaces on a Catalyst L3 switch we use the command “system mtu jumbo xxxx”.
To increase it for 10 and 100Mbps interfaces use “system mtu xxxx
To increase the IP MTU for routing, sourcing and replying to traffic, use “system mtu routing xxxx
How would you verify MTU on a catalyst L3 switch? “show system mtu”.


2.1.a (iii) L2 MTU