[c-nsp] general questions regarding MTU

[Mikael Abrahamsson]

On Tue, 5 Apr 2011, Martin T wrote:

> 1) as I understand, frames larger than MTU are automatically
> fragmented by switch.

No, generally this is only done by routers, not on L2-switches.
Usually the packet is dropped.

> 2) when we talk about MTU(standard Ethernet v2 frame 1500B, jumbo frames 
> 1500B - 9000B etc), then we don't count in L2 header? So for example 
> 802.1qinq frame including multiple VLAN tags, srcMAC, dstMAC, EtherType 
> and CRC with payload less 1500B is still counted as regular Ethernet v2 
> MTU?

It depends on platform and OS.

> 3) on the image, all the switches have "system mtu 1552". Am I correct, 
> that "system mtu" setting only affects 10BASE-T and 100BASE-TX ports? 
> And this 1552B does not include the L2 header, right?

This affects all ports and for IOS, doesn't include L2 header.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se