[gazzzman]

hi there!
Sorry for the delay getting back to this
they have had me working like a slave

Originally Posted by Ubermik

Thats the point tho, they never DO leave the router in the first place because it IS a router so it doesnt even bother sending them to the modem because the gateway is incorrect

but if the gateway is inside the scope of the subnet mask it would forward it to the wan port regardless as it does not know the default gateway is wrong and therefore should be forwarded by the modem

The PC nor the modem looks at the routing information so even with an incorrect gateway packets flow as freely as guiness on st patricks day

if the modem is indeed just acting as a bridge and has no involvment at layer3
that would be the case

So yes IF the router would send the packets to the modem they WOULD be forwarded as they are from the PC, they would have an extra layer of information wrapped onto them by the router compared to the same packet from the PC, but to the modem thats irrelevant as it just sends whatever its sent to the telewest router and lets that sort it out, but the "IF" is the critical word there as they arent sent to the modem, they never get to the wan side of the router to begin with so the modem cant forward packets it hasnt been sent to begin with no matter how good a modem it might be

aha! I can now atlast (sorry for the delay here!) see where you are comming from
if the wrongly set gateway lies OUTSIDE the scope of the subnet mask on the wan interface it will be discarded by the router..
and if the gateway is INSIDE the scope of the subnet mask.. it will be forwarded to the modem regardless


"I tried an assortment of gateway addresses in the same subnet as the correct gateway address and got the same result with 2 routers and 1 pc the routers I used btw where and old cisco 4000 series and a draytek 3300"

But all other locations bar the actual gateway in that subnet will be a PC and WILL be obtainable without any routing being needed BECAUSE they are on the same subnet

something wrong here? surely the packets from the router or the pc will both contain source destination and next hop information?
if so then what you are saying is when the packets from the pc get to the modem
no routing occurs untill they get to TW UBRs? and then the next hop info is stripped and the destination address is examined.. so (assuming as I am) the routers gateway address is within the scope of the subnet mask.. the same should happen?

So they will reach someone elses PC and be discarded

so that should never happen because TW UBR is only looking at the source and destination addresses the packet should be forwarded to the correct destination

The difference with the ACTUAL fault is that the gateway address ISNT on the same network, so it DOES neet to be routed and despite having the wrong IP for the gateway is automatically sent to the actual gateway for that subnet merely because its not for a local destination on that subnet

and yes that is what I faild to realise I should have looked more closley at the earlier post that contained the network info

Thats why you werent getting any joy

I think the failure mechanism is slightly different with my modem I think it is actually doing some filtering at layer 3

Infact in theory the same might happen with any real world IP that isnt on the same subnet as the host, similar to a two status rule

1. If the final destination is on the same subnet send it to the destination
2. if its not on the same subnet send it to the gateway

For rule two tho the packets gateway information is irrelevant as the telewest router already knows its own gateway to begin with so it just peels off the incorect jacket applied on the PC and possibly reapplied by the modem and then replaces it with an acurate one

yes I can see that if your modem is acting the way you say

finally I must say it is nice to have a technical discussion without shouting and name calling and at a level higher than "someting I saw on TV"
again thanks!

PS I am hoping I may get my modem replaced soon as I have problems with it loosing connectivity (different post lol!)
so I will be looking at this again with great interest
Garry