How to clear Don't Fragment (DF) bit

I can't see page 2 either FWIW...

 

 

However you need to look at both the ethernet standards (Which define Max MTU and minimum packet size. Yes there is one, it's about 60Bytes IIRC which is calculated from the time it takes the pre-amble of an ethernet packet on coax to make it all the way from one end of a repeater network to the other. Anything less is a runt and should be dropped), and the IPv4 standards as well (Which define such things as all hosts on a given subnet MUST have the same MTU). The MTU (MAX size) DOES change with faster speeds. Up to 100Mbps it's defined as a hard limit of 1500 Bytes. Gb and above have about 9kB (Jumbo Frames).

 

 

OK. So in the early days IP stacks used to accept packets up to the size of the configured MTU. Which meant if you changed the MTU to something lower on ONE host, you'd get strange issues with hosts on the same VLAN being unable to talk to each other properly. On a modern OS that's (Hopefully) unlikely to happen. The incoming packet is USUALLY not just truncated.

 

 

BUT... That doesn't hold for routers. Drop the MTU on a router and you'll affect almost everything. For a start things will need to fragment... So if you drop the MTU on one interface of the BigIP where it's acting as a router, you'll have to drop it on all... So anything passing across this interface you've just altered using a FastL4 profile and network VS's (e.g. configured as a router) is now going to have to fragment... Add in a few more over zealous firewall admins, and you've just broken the traffic across your own BigIP.

 

 

But back to the original discussion regarding dropping the MTU for ONE host on a network... For reference checkout http://tools.ietf.org/html/rfc1042 Page 4. Bottom of the discussion about Maximum Transmission Unit. Note the word MUST. It's not a SHOULD which means it's a good idea. It's a MUST. That's a current RFC BTW, even if it is probably older than most of the audience (Circa 1988)

 

 

Maximum Transmission Unit

 

 

The Maximum Transmission Unit (MTU) differs on the different types of

 

IEEE 802 networks. In the following there are comments on the MTU

 

for each type of IEEE 802 network. However, on any particular

 

network all hosts must use the same MTU. In the following, the terms

 

"maximum packet size" and "maximum transmission unit" are equivalent.

 

 

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