behavior of routers when MTU increased

[maximus_colt45]

Hi,

I am trying to design a system (transparent device) which takes a packet as input, adds an overhead of 4bytes, and sends it on line again. For example:

A packet of size 100 Bytes --> blackbox --> 104 Bytes out of the system

I have few queries here:

1- If the packet size input to the system is 1500, the system will make it 1504 hence making it invalid for the next router. In this case how will the routers reconfigure themselves?

2- What effect would this overhead have on the entire network?

3- Is this scheme even possible to implement in real scenario? Like is it possible to add an overhead of 4Bytes at maximum MTU limit?

4- Will the routers drop frames when MTU increases 1500B ?

Regards

Muhammad Hassan

[Jeff Van Houten]

Are you going to recalculate the checksums in the footers?

Sent from Cisco Technical Support iPad App

[maximus_colt45]

If you are talking about CRC at the end then yes, I will be recalculating it. Other checksums that are there in IPv4 header etc, i won't recalculate them. Actually the device will be encrypting the incoming stream at layer 2. So, it doesn't matter if I recalculate the internal checksums. But it does matter if I don't recalculate the CRC at the end of the packet because otherwise the packet will be dropped by the router flagged as incorrect CRC.

[adnane dakna]

if your blackBox work under TCP connection link  the best way to avoid MTU problems is the ajudst MSS TCP Packet ,

under ethenet interface connected to your LAN put the command "ip tcp ajudst mss 1460 ",

[adnane dakna]

when a packet size is more than 1500 octet and the DF bit of packet is not set "DF=0" , the packet is fragemented to other packets with size <1500 bytes.

[maximus_colt45]

Hi adnane,

My blackbox works under all kinds of traffic so MSS is irrelevant here. Kindly clarify one thing if you can, if I set MTU to 1460, will it be only on the transmitting end or my receive packet size will also be 1460? Because, if my blackbox is adding let's say 4bytes on every packet,

1460 (side A )--> blackbox --> 1464 (side B),

1464 (side B)--> blackbox --> 1460 (side A)

the receiving unit in this case will also be configured at 1460, right ? or it will still be at 1500MTU?

Like take the case when my blackbox is adding 4Bytes on every packet coming in from one side and removing 4Bytes the other way.

So if I set 1460 MTU on side A transmission, will my side a reception be 1460 too? or will it still be 1500?

also, will router on side B automatically change itself to 1460 or will it still be on 1500 MTU ?

Regards