From: Tony (aamercado31@xxxxxxxxx)
Date: Fri May 17 2002 - 14:05:53 GMT-3
HI
IP split horizon checking is disabled by default for
Frame Relay encapsulation on multipoint interface (ie
main physical interface and multipoint subinterface)
so that distance vector routing updates will not loop
in and out the same interface. An exception is EIGRP
for which split horizon must be explicitly disabled.
However, certain protocols such as AppleTalk,
transparent bridging, and IPX cannot be supported on
partially meshed networks because they require "split
horizon" so split-horizon cannot be disabled for
these protocols (ie IPX and AppleTalk). These other
protocols must use subinterfaces if dynamic routing is
desired.
Therefore the concept of subinterfaces was originally
created in order to better handle issues caused by
split-horizon NBMA networks (e.g. frame relay, ATM)
and distance-vector based routing protocols (e.g. IP
RIP, AppleTalk). Split-horizon dictates that a routing
update received on an interface cannot be
retransmitted out onto the same interface.
By dividing the partially-meshed frame relay network
into a number of virtual, point-to-point networks
using subinterfaces (NOT Multipoint Subinterface), the
split-horizon problem can be overcome.
Here's some links to refer to:
http://www.cisco.com/warp/public/125/26.html#26-B
http://www.cisco.com/warp/public/116/fr_faq.html#21
http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm
HTH
--- Phil <ciscostudent1@yahoo.com.br> wrote:
> Just to add a little bit, split-horizon is disabled
> by default in physical interfaces and is enabled by
> default in subinterfaces.
> Phil
>
> Vincent Tsang <cvst@wpcorp.com.au> escreveu:
> David,
>
> Split Horizon is turned off by default because in a
> partial mesh hub & spoke
> topology, spoke routers running distance vector
> routing protocol (EIGRP/RIP)
> will not be able to send routing update to other
> spoke routers. That is if
> you are not running point-to-point subinterfaces
>
> This is because routing updates enter and exits
> through the same physical
> interface, even though they are separate PVCs, Split
> Horizon stops the
> update for this reason along.
>
> I don't think Split Horizon gets disabled by default
> if you use
> point-to-point subinterfaces. Look at Caslow's
> explanation for more details.
>
> Hope this helps.
>
> Vincent Tsang
> ----- Original Message -----
> From: "David Lee Steele, Jr."
> To:
> Sent: Friday, May 17, 2002 5:30 PM
> Subject: Split Horizon/Frame Relay
>
>
> > I know that split horizon is disabled by default
> on Frame Relay, but
> > does anyone know why? Just a random choice or is
> there a technical
> > reason for doing so?
> >
> >
> >
> > David Lee Steele, Jr.
> >
> > [GroupStudy.com removed an attachment of type
> application/x-pkcs7-signature which had a name of
> smime.p7s]
> >
>
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