Strange speed problems with ipv6 forwarding

[Frank Steiner]

Matt Rowley wrote

>> I got one step further. tracerout shows that route from inside (A)
>> outside (B) is A->F->B with F being my firewall.
>>
>> But route from B to A goes through the router. I've setup all hosts
>> in the subnet in front of the firewall to route their packets through
>> the router R that our data center configured for this subnet.
>>
>> Thus it's B->R->F->A. The same happens for ipv4, no ->R-> when
>> sending from A to B, but via R from B to A. While it's fast for
>> ipv4, it's slow for ipv6. So I added a route for the internal
>> subnet to the routing table of B so that the trace now shows
>> B->F->A. And then the copying between A and B is at full speed
>> of 112MB/s.
> 
> Hi Frank,
> 
> So, R, B, and F all have legs on a common network segment, right?  

Yes!

> And B points to R for default gateway?  

Right.

> Does B have routes in its table so that it knows to point to F in 
> order to reach A?  

That's what I tried to describe above. By default it doesn't and that's
when the traffic slow. When I add such a route, the traffic is fast.
I just don't why it's slow without that route for ipv6 only, while
ipv4 has no problems routing through R first. Well, maybe...

> If not, it is sending packets to R, who is probably returning ICMP 
> redirects to B. Perhaps B is dropping them?  

...sth. like that. As far as I understand, ipv6 tries to dynamically
 configure better routes while ipv4 doesn't.

> A tcpdump on R, B, and F might help show you what's going on.

I'm not really familiar with tcpdump, but I will figure out!

cu,
Frank


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