Help Understanding Jumbo Frames

[dave cooper]

I am trying to understand jumbo frames and want to understand what happens in the following scenario. I have two VLANs configured on a switch.

VLAN 10 has mtu 9216 enabled on the SVI interface. The other VLAN is 11 and it has a mtu of 1518 on the SVI interface. if a host in VLAN 10 sends a jumbo packet of 9216 to a host in VLAN 11 which only supports a mtu of 1518 what happens on the switch. Will the switch fragment the 9216 packet into 1518 byte packets?

Thanks

[johnd2310]

Hi,

Yes, the switch will fragment the packets to the mtu of the outgoing interface.

Thanks

John

[Steve Fuller]

This will depend upon the setting of the Dont Fragment (DF) bit in the IP header. If the host OS has set the DF bit then the router will drop the packet and send back an Internet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message. If the host supports Path MTU Discovery then it should adjust the size of the packets it sends for the destination to that specified within the ICMP response.

Path MTU Discovery is covered in RFC 1191, with an explanation at the Path MTU Discovery blog over at Packet Pushers and at WikiPedia.

As you'll see from the age of the blog post, and the even older RFC, Path MTU Discovery has been around a long time and I think all modern operating systems support by default.

Regards