Re: Split Horizon

The term spit-horizon is the most overused term in all of Cisco
networking it seems. :-)

There is the traditional sense which refers to the dynamic routing
protocol loop prevention mechanism, and then there are indeed a bunch
of references from there.

Scott, I think the most famous is the one you mention here. The iBGP
Split Horizon Rule - an iBGP speaker will not advertise to another
iBGP peer an update that is learned via iBGP.

If this thread is indeed in reference to the Core Knowledge section,
it is so very important to read carefully and not knee-jerk a response
too quickly. More and more students are reporting to us that Cisco
tries to "trap" a student moving through the questions quickly and not
stopping to really think about the issues/fundamentals involving the
technology.

Warmest Regards,

Anthony J. Sequeira, CCIE #15626
Senior CCIE Instructor
http://www.InternetworkExpert.com

Internetwork Expert, Inc.
Toll Free: 877-224-8987
Outside US: 775-826-4344

Test your Core Knowledge today!
Q: IGMP snooping detects multicast routers by listening to what traffic?
A: Protocol Independent Multicast Messages
More Info: http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_25_see/configuration/guide/swigmp.html

On Apr 23, 2009, at 7:21 PM, Scott M Vermillion wrote:

> Yep Narbik, I was indeed referring to synchronization/full-mesh I
> said as much below. But in a couple of recent discussions on this
> very list people (including CCIEs) were referring to this as a BGP
> split horizon behavior. I have been traveling a lot lately and
> sometimes reading threads one or two full weeks after they've
> already played out, so I didn't bother to chime in. In fact, I was
> just reading one such thread this morning from maybe a week ago. I
> half wondered if R.B's question didn't stem from one of these other
> threads where the term "split horizon" was being applied to BGP and
> hence my post...
>
> Cheers sir,
>
> Scott
>
>
> On Apr 23, 2009, at 4:35 , Narbik Kocharians wrote:
>
>> I think you mean synchronization
>>
>> On Thu, Apr 23, 2009 at 2:43 PM, Scott M Vermillion <
>> scott_ccie_list_at_it-ag.com> wrote:
>>
>>> R.B,
>>>
>>> I would just clean that up a little and replace "packet" with
>>> "destination"
>>> or something along that line.
>>>
>>> People sometimes (recently) use this in iBGP discussions, which I
>>> believe
>>> to be a slightly improper application of the term. The full
>>> mesh/synchronization requirement has as much if not more to do
>>> with serving
>>> as an anti-blackholing mechanism vs. preventing loops from
>>> forming. And
>>> even to the extent that it does prevent loops, it does so slightly
>>> differently as contrasted to, say, RIP split-horizon, so this is
>>> not a term
>>> that I personally use in the context of BGP. Others do, though,
>>> and so this
>>> is probably one context worth making note of.
>>>
>>> The term has also been borrowed for split-horizon DNS and so
>>> forth. But it
>>> generally infers a behavior where otherwise flooded information is
>>> not
>>> reflected back towards its point of origin relative to any given
>>> point in a
>>> topology.
>>>
>>> What was the catalyst for your question?
>>>
>>> Regards,
>>>
>>> Scott
>>>
>>>
>>>
>>>
>>> On Apr 23, 2009, at 7:03 , Robin Betterley wrote:
>>>
>>> Hi GS,
>>>>
>>>> The basic principle is simple: Information about the routing for a
>>>> particular packet is never sent back in the direction from which it
>>>> was received.
>>>>
>>>> Is there any other known principle of split horizon?
>>>>
>>>> Cheers,
>>>> R.B
>>>>
>>>>
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>>>
>>>
>>>
>>>
>>
>>
>> --
>> Narbik Kocharians
>> CCSI#30832, CCIE# 12410 (R&S, SP, Security)
>> www.MicronicsTraining.com
>> www.Net-Workbooks.com
>> Sr. Technical Instructor
>>
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Received on Thu Apr 23 2009 - 22:53:44 ART