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In a Cisco configuration, what is the difference beetween the MTU, ipv4 MTU and the MTU configured into the mpls section of an interface ?
interface X
mtu 9214
ipv4 mtu 9180
[...]
mpls
mtu 9200
!
!
The IGP configured is ISIS. MPLS and LDP are running in the network.
The system MTU is a globally applied setting to all interfaces and protocols (not just IP), as a default MTU unless a different MTU is defined on a particular interface or protocol.
The interface MTU setting is the MTU solely for that interface and affects all protocols used with whatever flows through that interface.
IPv4 MTU is an MTU setting that overrides the global MTU setting for IPv4 traffic only (i.e., it wouldn't apply to IPv6 traffic) and only for the interface it's applied on.
The MPLS MTU is the MTU setting that applies only to MPLS traffic, and this value cannot be higher than the interface MTU used for the MPLS circuit. This setting affects all labeled packets.
As a note, many of Cisco's newer switch models don't allow you to set the MTU on a per-port basis because they're of the mentality that you should not be mixing MTUs within the same switch, and should instead have separate switches for different purposes (such as one for users that would typically be set for the default of 1500 and another for servers and storage that may take advantage of "jumbo frames" (anything above 1500) which would commonly be set for 9000).
