Re: EIGRP - Split Horizon

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_paper09186a0080094c
b7.shtml#splithorizon
>n
>
> "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
>>
>>
>> Blogs and organic
groups at http://www.ccie.net
>>
>>
Received on Sun Jan 09 2011 - 16:51:33 ART