Re: EIGRP - Split Horizon

Glad to help Dave and I hope you had fun with that one - sounds like
you did! I used to hate (and thus avoid) EIGRP due to the simple fact
that it's proprietary. After going through the whole lab prep
experience (and being more or less forced to use it in a couple of
real-world designs), I now really like it for certain scenarios. But
it's one of those things you have to take to the lab to fully solidify
(at least I do, anyway). If you have a Dynamips setup, packet capture
can be pretty interesting with this protocol too...

Cheers sir,

Scott

On Jan 9, 2011, at 5:51 , Dave Serra wrote:

> Thank you Scott!
>
> Your posts have been Most helpful. I did take the time to "jack up"
> the metric on the R3 interface pointing back to R2. The results
> were exactly as you described. Because R3 now saw the best path to
> 1.1.1.1/32 through R4, it is no longer advertising that route back
> to R4. Interestingly, R3 and R2 now advertise the route to each
> other because neither router is using the other as 'Best'.
>
> Great posts my friend. Please keep up the good work.
>
> Regards,
>
> David Serra
>
>
> Make a small loan, Make a big difference - Kiva.org
>
>
> From: Scott M Vermillion <scott_ccie_list_at_it-ag.com>
> To: Cisco certification <ccielab_at_groupstudy.com>
> Sent: Sat, January 8, 2011 9:09:21 PM
> Subject: Re: EIGRP - Split Horizon
>
> Howdy All,
>
> I am signing off for the night but I've received a couple of
> unicasts pointing out what can be perceived as weaknesses in my
> below post, so I thought I would take one last stab to clear up my
> intention. The below quote (never advertise a route out of the
> interface through which you learned it) applies to a simple FR
> topology (depicted in the link) - which is different than the
> topology originally in question - and is thus a technically correct
> definition of Split Horizon. I wouldn't argue that. The problem I
> have with it is that it comes off as an >exclusive< definition of
> EIGRP Split Horizon. And one that's difficult to square - if
> interpreted literally - with a slightly more complex topology such
> as was depicted in the original post of this thread. I like the
> definition found in the Command Ref better: "The split-horizon rule
> prohibits a router from advertising a route through an interface
> that the router itself uses to reach the destination." Much less
> ambiguous, IMHO, and it allows for the behavior observed by the OP.
>
> Nighty night (MST) all ye GSers!
>
> Scott
>
>
> On Jan 8, 2011, at 5:19 , Scott M Vermillion wrote:
>
> > Sorry about that Jules! Hate to cause rather than to clear
> confusion.
> >
> > The below quote of my earlier post was relative to a more
> > "traditional" (and not entirely technically correct in this
> particular
> > context!) definition of Split Horizon that a lot of us probably grew
> > up with. For example, check out this old Cisco EIGRP link:
> >
> > http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#splithorizon
> >
> > "Never advertise a route out of the interface through which you
> > learned it."
> >
> > I think the OP was expecting that R3 and R4 would take a literal
> > interpretation of this and one would wind up advertising the R1 Lo0
> > route to the other and then the "learning router" (my term) wouldn't
> > be able to be advertised back in the other direction towards the
> > advertising router. That's not, in fact, what happens, as you point
> > out in your post. The rationale for my having said that in the
> first
> > place was to attempt to sum up what I believed to be the point of
> > confusion. And in doing so apparently generated even more! ;-)
> >
> > Apologies,
> >
> > Scott
> >
> >
> > On Jan 8, 2011, at 5:07 , jules NYA BAWEU wrote:
> >
> >> Scott you got me all confused with this Split Horizon thing -
> please
> >> feel
> >> free to weight in with more details if you could. Here is my take
> on
> >> this:
> >> What Dave is seeing is a normal behavior of Split Horizon. R3 and
> R4
> >> would
> >> advertise R1's Lo0 to each other as long as those routes pertain to
> >> their
> >> path that connect them to R2 - you will only see one route in your
> >> routing
> >> table unless you have a variance set up or they are exactly equal
> path
> >> routes, but you would see all those routes in your EIGRP topology
> >> table. Now
> >> with EIGRP, they would not be able to advertise the route back to
> >> their
> >> interface that they have picked as best path direction - even
> though
> >> that
> >> router was learned from a different interface - in this case,
> >> assuming that
> >> R3 picked R2 as its best path, R3 would not advertise the path via
> >> R4 to to
> >> R2.
> >>
> >> My issue in your prior is: "and then the learning router would not
> >> advertise
> >> R1's Lo0 back to the other neighbor"
> >>
> >> Please correct me if I'm wrong.
> >>
> >> Thx
> >>
> >>
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> >>
> >>
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Received on Sun Jan 09 2011 - 18:50:56 ART