From: Jason Madsen (madsen.jason@gmail.com)
Date: Mon Nov 03 2008 - 12:12:44 ARST
group, sorry about this email chain. i didn't mean for it to take away from
what this site is intended. there actually was a reason other than
curiousity for me posting it initially.
I was doing speed drills where i'd randomly setup a few routers and do local
policy routing as fast as i could possibly type and then do it over again
with different IPs etc.
after doing a few I found that even though I "force" traffic out a
particular interface with local policy routing, traffic wasn't always
"sourced" from that interface and this issue cause me to have to "tell" the
connected router how to route traffic back. sometimes I had to and
sometimes I didn't. it completely depended on which interface the router
decided to source from.
my hopes for figuring out the logic behind this was so that if i encountered
a situation like this in a lab i could quickly identify it as an issue when
doing the initial read through of the lab. so i'd know to look for ospf
areas that need virtual links, x, y, z, and in certain cases which local
policy routing may need config's on a neighbor router in the RARE even that
it didn't know about it's neighbors addresses.
Jason
On Mon, Nov 3, 2008 at 6:49 AM, Jason Madsen <madsen.jason@gmail.com> wrote:
> Traffic actually did leave the local router and then of course it was
> unroutable at the neighbor. I enabled deb ip pack 100 with ACL 100 as a
> permit icmp any any and saw that it was doing this. It definitely didn't
> source with the lowest IP either. I initially had 10.10.10.10 and
> 20.20.20.20 on my two interfaces and in that case it always chose
> 20.20.20.20.
>
> With that result I thought maybe it used the highest IP, so I changed
> 20.20.20.20 to 19... and I also tried 30... and even 100...and got
> different results .
>
> As you mentioned it probably doesn't matter. I was just curious if there
> was an logic behind the selection process.
>
> Thanks,
> Jason
>
>
> On Mon, Nov 3, 2008 at 4:27 AM, Marko Milivojevic <markom@markom.info>wrote:
>
>> > I was trying to determine what method if any routers
>> > use to determine the source address used if/when the destination address
>> is
>> > not directly connected or in the routing table.
>>
>> The router "should not" try to generate traffic if there is no route
>> for the destination. As we all probably know, it does :-). However,
>> it's not really relevant, as this traffic is not leaving the router,
>> it's being dropped as unroutable. While I don't think it's really
>> relevant which interface it "uses" in this case, I believe it's using
>> the one with the lowest IP address.
>>
>> --
>> Marko
>> CCIE #18427 (SP)
>> My network blog: http://cisco.markom.info/
Blogs and organic groups at http://www.ccie.net
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