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Posted by Barry Margolin on October 10, 2006, 9:22 pm
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> >Walter Roberson wrote:
>
> >>>I've got a question. If i got two router acting has default gateway
> >>>connected to a third router. How could I specified a default gateway by
> >>>ACL. Like, if I want all match with ACL 101 (who permit HTTP, SMTP)
> >>>router over Router A and the rest goes to router B.
>
> >> You want "policy based routing" (PBR). Probably you want to
> >> combine it with "ip nat".
>
> >Actually, there is no nat involved under this scenario.
>
> You want to select the outgoing routing by protocol, not by IP.
> You will presenting outgoing packets to router A and to router B that
> have the same source address (as other outgoing packets) {and possibly
> even the same destination address but different ports.)
>
> Now, when the remote systems respond to those packets, which path will
> the responses take back to you? Through router A or through router B?
> The remote systems don't know anything about your outgoing packet
> arrangements, so the remote systems are going to route back to you
> according to the IP only, without consideration of the port.
> And that implies that all the responses (for a given host) are going to
> come back through the same router, regardless of port.
He never said he wanted the return traffic to be routed specially, just
the outgoing traffic. Asymmetric routing isn't inherently bad, although
there certainly are times when you don't want it.
--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***
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