On 10/05/07 05:05, John Default wrote:
I was told that layer 3 switches are faster because "routing" there is
done by some ASIC hardware. Is there any advantage in having another
routing code in bridging when everything is done in software which is
same slow as normal routing? The only speed gain would be in keeping the
routing code very simple with limited functionality, but i think that
the trend is to put there more and more functionality which would end up
in having two same slow, same function code in two places.
Ah, there in lies the difference in what you are saying, which as a norm
is probably correct and something that I do not disagree with. I guess
I should say that my introduction to L3 switching is actually on Cisco
Catalyst 5000 / 5500 L2 switches where they depend on an external Cisco
L3 router to assist in the L3 switching. Rater that is to say that the
L2 switch and the L3 router communicate with each other to combined do
L3 switching. As I understand it, the L2 switch will send initial
packets to the L3 router along with some meta data. The L3 router will
route the packets and send them back to the L2 switch with updated meta
data. Then the L2 switch will have learned with the help of the L3
router that the packets can be altered on L2 to emulate L3 routing but
this time in hardware. Thus the L2 switch depends on the L3 router to
do the initial routing and then the L2 switch will subsequently step up
and L2 switch across L3 boundaries based on what it learned from the L3
router.
So, I guess I should say that I'm not wanting to (re)implement the
routing code in the kernel, it does quite fine for me thank you very
much. ;) I'm looking for a way to alter source / destination MAC
addresses of packets on L2 to emulate what happens in routing. I
believe that I could SNAT / DNAT the MAC addresses of the packet via
EBTables on L2 to achieve the effect of an L3 route. I would do this by
having the bridging code in the kernel learn from cached (?) results of
a previous L3 route.
In other words if the packet is in a NEW connection state, send it on up
to L3 routing. If the packet is in an ESTABLISHED state and we can pull
information form the systems ARP cache to know the destination MAC
address for the next subnet as well as pull the correct source MAC
address for the interface on the next subnet, then we could just SNAT /
DNAT the MAC addresses on L2 and send the packet back out on the
appropriate wire.
I'm wondering if this NATing of the source and destination MAC addresses
on L2 would be faster than passing the packet up to L3 routing. It is
my belief that L3 will do more sanity checks on packets than L2 will.
These sanity checks will take time to perform which could be avoided if
we could just NAT the source and destination MAC addresses on L2. Or at
least that's what I think. I could be very wrong about it.
(i was taugth that packets are routed on L3, frames are
switched(bridged) on L2. And L3 switch does L2 switching + L3 routing
but in hardware. routers are completely a software thing, switches
hardware thing, and bridge is switch in software.)
I can agree with that statement. However I'll spin what you said a
little bit and then I think you can see how I'm logically progressing on
down the line.
Switching is a L2 operation, no matter what that operation is. Routing
is a L3 operation, no matter what that operation is. Thus if we perform
some sort of L3 type operation on L2 then we are performing some sort of
switching operation. If that operation happens to be routing which is
normally a L3 operation, then we are doing a L3 like operation on L2,
thus L3 switching. So now that I have circularly argues that, how about
an example.
Let's say that we have two end point hosts on separate subnets with an
intermediary router.
+---------+ +-------------------+ +---------+
IP: | 4.0.0.9 +-----+ 4.0.0.1 : 5.0.0.1 +-----+ 5.0.0.9 |
MAC: | ..00:0f | | ..11:1e : ..22:2d | | ..33:3c |
+---------+ +-------------------+ +---------+
If I want to send an ICMP ping from 4.0.0.9 to 5.0.0.9 the ethernet
frames will be sent from ..00:0f to ..11:1e and from ..22:2d to ..33:3c.
Note that the routing code on the intermediary router will see that the
packet needs to be routed from one subnet to the other and will do so
just fine with out any problems at all. However this is a layer 3
operation.
What I'm wanting to do is educate L2 enough so that it can use cached
results from L3 to perform a similar operation on L2 in the future.
Thus when the frame from 4.0.0.9 with a MAC address of ..00:0f comes in
destined to 5.0.0.9 with the router's MAC address of ..11:1e I'm wanting
to alter the frame coming in to the switch such that the new destination
MAC address will be ..33:3c with a new source MAC address of ..22:2d
based on contents of the system's ARP cache with a little bit of help.
It is my belief that this L2 operation of SNATing and DNATing the MAC
addresses with out sending the data up to L3 will be faster than sending
the data up to L3 and doing its full processing. At least that is what
this entire discussion is based on. At the very least I believe I'm
going to do some controlled tests to see if this will even work with
manually entered static configurations.
If this does work, I think it would be possible to come up with a new
EBTables target that could alter the destination MAC address based on
the contents of the system's ARP cache (the system just spoke to the
target, thus the target MAC should be in the ARP cache, if not the ARP
code does a fine job at it's job and can get us the MAC address). The
only hiccup that I don't have an answer for at the moment is picking the
correct source MAC address. However looking at the contents of the
ARP cache we see that the interface is listed as well. So we could do a
simple translation from interface to source MAC address. Thus I believe
we have the basis of a rough crude logistical algorithm to L3 switch (a
n L3 operation on L2) traffic through a Linux system.
Please excuse me if i am missing your idea completely.
Please read and chew on what I've brain farted to the mailing list.
Poke holes in it and let's discuss this. If this truly will not work, I
have only wasted some bandwidth and bytes on drives, nothing else. All
the while we will have hopefully cleared a few cob webs from our
collective brains. ;) At least for a few minutes while I try to make a
fool of my self. :}
Grant. . . .
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