[clug] A routing question

Kim Holburn kim.holburn at gmail.com
Wed Apr 18 15:08:37 GMT 2007

It really depends on what sort of routers you have and how much you  
control them.

The simplest way would probably be to remove the A to B link.  What  
do you need it for anyway?

Make all the machines on subnet A use the router that handles A to  
C.  Tell the A to C router that subnet B traffic goes to the router  
handing C to B.  On subnet B tell the router that to find subnet A go  
the the router handing B to C.

On 2007/Apr/18, at 3:54 PM, Christopher Zhang wrote:

> Hi list,
> Say if the connections from subnet A to B are throttled down, but  
> the connections from subnet A to C and from subnet B to C aren't.  
> The way the connections are throttled is by applying rules on the  
> default gateways of subnets A and B.
> Is there a way to speed up the connections from subnet A to B?
> The closest idea I can think of is to setup a gateway within subnet  
> A, let's call it D. Setup a host in subnet C, let's call it E, and  
> finally another gateway in subnet B, and call it F.
> The idea is to route all traffic from subnet A to C, then bounce it  
> off C to B. Since the connections from A to C and B to C are fast,  
> this effectively increases speed from A to B. So instead of using  
> the default gateways for subnet A and B, we can use our own new  
> gateway D, then somehow pipe all traffic to E, and then from E pipe  
> all traffic to our new gateway F in subnet B.
> The reason this increases the speed from subnet A to B is that the  
> connection is unthrottled from subnet A to C, and from subnet C to B.
> Eventually this is like a man in the middle setup, in subnet A,  
> tell all machines to use D as the default gateway. What D does is  
> to forward to the traffic to E, D still uses the real default  
> gateway for subnet A to do that however since this connection is to  
> host E in an unaffected subnet, the connection is fast. Then E  
> forwards whatever is forwarded to it to F, if we tell all computers  
> to use F in subnet B, the traffic will reach any host fin subnet B,  
> without any speed loss.
> It is easy to setup D as a gateway and route traffic through it,  
> but how can I tell D to route the traffic to E (in subnet C) and  
> from E route all traffic to F (in subnet B)? I cannot tell D to use  
> E as the default gateway since they are on different subnets. If I  
> use iptables to forward the traffic, the packet will lose the  
> original header which means the reverse won't come through.
> Maybe a tunnel needs to be setup, but I have no idea how to do  
> that, does anyone have better ideas?
> Thanks
> Chris
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Kim Holburn
IT Network & Security Consultant
Ph: +39 06 855 4294  M: +39 3494957443
mailto:kim at holburn.net  aim://kimholburn
skype://kholburn - PGP Public Key on request

Democracy imposed from without is the severest form of tyranny.
                           -- Lloyd Biggle, Jr. Analog, Apr 1961

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