OK I finally read what you were talking about.
It says it creates a condition known as split-horizon. I believe the key
word here is "Condition". This is not the split-horizon that is used to
prevent loops in a Distance-vector algorithm. I have added comments below.
Let's disect these paragraphs:
This sample configuration routes TCP/IP over a partially-meshed frame relay
network using point-to-point subinterfaces. IP routes are dynamically
resolved using Open Shortest Path First (OSPF) as the routing protocol.
Special care is needed when configuring OSPF over partially-meshed frame
relay. ~~1~~ OSPF requires direct connections to each neighbor in order to
elect a Designated Router (DR) and form adjacencies with neighboring OSPF
routers. However, the physical topology of a partially-meshed frame relay
network does not provide the direct access that OSPF requires.
The solution to the adjacency problem is to use subinterfaces. Subinterfaces
logically divide a single, physical interface into separate, virtual
point-to-point interfaces. A single frame relay interface connecting to
multiple destinations can now be treated as a collection of point-to-point
network segments. The point-to-point connections allow routers to form OSPF
adjacencies without DR election. Although each subinterface segment requires
a unique IP subnet, unnumbered IP segments can be used to alleviate the
burden of designing the additional IP subnets.
In general, it is good practice to use subinterfaces for partially-meshed
frame relay networks. A frame relay network designed with subinterfaces
scales much easier to future expansion. Referring to the example,
subinterfaces allow routing updates to exchange between Boston and Chicago
through Atlanta. ~~2~~ Without subinterfaces, Boston is unable to receive
routing updates from Chicago and vice versa creating a condition known as
split-horizon.
1- this is what I feel they are referring to whe they say "a condition known
as split-horizon." If subinterfaces aren't used and physical interfaces are
on a partially mashed network OSPF does not provide the direct access that
OSPF requires.
2- This says "without subinterfaces" this condition occurs.
I feel you have taken this article out of context and have made an
assumption that this is saying that OSPF uses split-horizon which is an
added feature to Distance-vector protocol. I believe this "condition" they
call split-horizon has similar attributes to the distance-vector feature.
The big difference is that this is a condition, not a feature. In this
example split-horizon is not being used by OSPF to suppress updates from
going back out of the interface. This is a condition encountered when you
use physical interface configuration in a partially-meshed environment where
OSPF will not be able to operate properly. Boston will not be able to
receive updates from Chicago if physical interfaces are used because in this
environment direct access is not provided for DR/adjacencies to occur.
>>>Brian
>From: "Leigh Anne Chisholm" <[EMAIL PROTECTED]>
>Reply-To: "Leigh Anne Chisholm" <[EMAIL PROTECTED]>
>To: "Brian Lodwick" <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>,
> <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
>Subject: RE: FR + Poison Reverse...
>Date: Tue, 27 Feb 2001 11:03:21 -0700
>
>If you don't want to read all of my quotes, look for the portion contained
>therein enclosed by double asterisks (**).
>
>From Cisco's site:
>
>"Normally, routers that are connected to broadcast-type IP networks and
>that use distance-vector routing protocols employ the split
>horizon mechanism to reduce the possibility of routing loops. Split horizon
>blocks information about routes from being advertised by
>a router out any interface from which that information originated. This
>behavior usually optimizes communications among multiple
>routers, particularly when links are broken. However, with nonbroadcast
>networks, such as Frame Relay and SMDS, situations can arise
>for which this behavior is less than ideal. For these situations, you might
>want to disable split horizon. **This applies to IGRP
>and RIP.**" This excerpt can be found at:
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/cbook/ciproute.htm#xtocid16743169
>
>A few weeks ago, this same discussion was brought up. Someone posted the
>following link outlining "Hub And Spoke Frame Relay Sample
>Configuration - Dynamic (OSPF) IP Routing". Here's an excerpt from the
>link:
>
>"In general, it is good practice to use subinterfaces for partially-meshed
>frame relay networks. A frame relay network designed with
>subinterfaces scales much easier to future expansion. Referring to the
>example, subinterfaces allow routing updates to exchange
>between Boston and Chicago through Atlanta. **Without subinterfaces, Boston
>is unable to receive routing updates from Chicago and
>vice versa creating a condition known as split-horizon.**" This excerpt
>can be found at:
>
>http://www.cisco.com/warp/public/779/smbiz/service/configs/framerelay/fr_ip_ospf.htm
>
>Now this is in contrast to what I know of OSPF and in contrast to the first
>quote I've included. Quite frankly, I would think that
>the problems encountered in the Frame Relay example have more to do with
>the non-broadcast nature of Frame Relay than split-horizon
>so I think whomever wrote this may have misunderstood why subinterfaces
>enable routing updates--and I think perhaps this is why
>there's so much confusion over whether or not split-horizon plays any role
>in OSPF.
>
>Brian L:
>
>I think you've got it backwards as to when split-horizon operates on an
>interface. By default, all Cisco serial interfaces are
>multipoint unless specifically configured to be point-to-point. With
>multipoint, you'd want split-horizon enabled if you were using
>a broadcasting routing protocol. Here's two excerpts from Cisco's site:
>
>--Excerpt 1--
>
>"Note: For TCP/IP, Cisco routers can disable split-horizon limitations on
>all frame relay interfaces and multipoint subinterfaces
>and do this by default. However, split-horizon cannot be disabled for other
>protocols like IPX and AppleTalk. These other protocols
>must use subinterfaces if dynamic routing is desired."
>
>--Excerpt 2--
>
>"Cisco serial interfaces are multipoint interfaces by default unless
>specified as a point-to-point subinterface. Though less common
>than point-to-point subinterfaces, it is possible to divide the interface
>into separate virtual multipoint subinterfaces."
>
>"Multipoint interfaces/subinterfaces are still subject to the split-horizon
>limitations as discussed above. All nodes attached to a
>multipoint subinterface belong to the same network number. Typically,
>multipoint subinterfaces are used in conjunction with
>point-to-point interfaces in cases where an existing multipoint frame relay
>cloud is migrating to a subinterfaced point-to-point
>network design. A multipoint subinterface is used to keep remote sites on a
>single network number while slowly migrating remote
>sites to their own point-to-point subinterface network."
>
>"Figure 4 shows serial 0.1 as a multipoint subinterface connecting to three
>different locations. All devices on the multipoint
>subinterface belong to the same network number (100). Site E has migrated
>off of the multipoint network to its own point-to-point
>subinterface network (200). Eventually, all remote sites can be moved to
>their own point-to-point subinterface networks and the
>multipoint subinterface will not be necessary."
>
>Both of these excerpts can be found at:
>
>http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm
>
>
> -- Leigh Anne
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> > Brian Lodwick
> > Sent: February 27, 2001 10:21 AM
> > To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> > Subject: RE: FR + Poison Reverse...
> >
> >
> > OSPF uses split-horizon? I don't think this is correct. I hate to be
>quick
> > to reply but there is no reason for OSPF to use split horizon. OSPF
>sends
> > updates to all adjacent neighbors, and in NBMA , and Broadcast network
>types
> > the DR forwards the updates on.
> > Split-horizon is used to keep routing loops from happening for a
> > Distance-vector protocol not a link state protocol.
> > There is an issue split-horizon presents when using a distance-vector
> > protocol in a frame-relay hub spoke topology. As you can imagine (if you
> > know how a distance-vector protocol diseminates updates) the hub will
>need
> > to send the update back out of the interface it received it on so that
>the
> > other spokes will receive the update. This will not be allowed to happen
>if
> > split-horizon is enabled. The spokes will not be an issue.
> >
> > By default:
> > Physical interface frame-relay setup split-horizon will be disabled.
> > Subinterface frame-relay setup split-horizon will be enabled.
> >
> > Also keep in mind you cannot disable split-horizon on IPX RIP.
> >
> > Summation when you have a hub spoke frame-relay topology and you are
>using a
> > distance-vector routing protocol you need to disable split-horizon on
>the
> > hub. (which does leave you secceptible to the issue split-horizon was
> > designed to fix)
> >
> > Or use a link-state protocol.
> >
> > Or you could setup point-to-point subinterfaces on the hub for each
>spoke.
> >
> > >>>Brian
> >
> >
> > >From: "Maness, Drew" <[EMAIL PROTECTED]>
> > >Reply-To: "Maness, Drew" <[EMAIL PROTECTED]>
> > >To: "'Z'" <[EMAIL PROTECTED]>, Brian <[EMAIL PROTECTED]>,
> > >[EMAIL PROTECTED]
> > >Subject: RE: FR + Poison Reverse...
> > >Date: Tue, 27 Feb 2001 08:36:57 -0800
> > >
> > >OSPF does use Split horizon. I don't think OSPF uses PR because PR
>sets
> > >the
> > >route to infinity and I'm not sure what an ' infinity' cost would mean
>in
> > >OSPF.
> > >
> > >-----Original Message-----
> > >From: Z [mailto:[EMAIL PROTECTED]]
> > >Sent: Tuesday, February 27, 2001 12:47 AM
> > >To: Brian; [EMAIL PROTECTED]
> > >Subject: Re: FR + Poison Reverse...
> > >
> > >
> > >But I thought that I'd seen or heard of using split horizon with
> > >OSPF...maybe I'm thinking of Frame Relay...long day I guess...
> > >
> > >************************************************************
> > >This has been an Eyez Only streaming e-mail broadcast...We are
>watching.
> > >
> > >NetEyez ~ CCNP, CCDA
> > >
> > >----- Original Message -----
> > >From: "Brian" <[EMAIL PROTECTED]>
> > >To: "Z" <[EMAIL PROTECTED]>
> > >Cc: <[EMAIL PROTECTED]>
> > >Sent: Tuesday, February 27, 2001 1:16 AM
> > >Subject: Re: FR + Poison Reverse...
> > >
> > >
> > > >
> > > >
> > > > split horizon and PR are both associated with distance vector
>protocols,
> > > > OSPF is a link state protocol.
> > > >
> > > > Brian
> > > > On Mon, 26 Feb 2001, Z wrote:
> > > >
> > > > > Hey Group,
> > > > > I recently pondered something while at work and nobody could
>give
> > >me
> > >a
> > > > > strait answer. Actually I feel a little embarrassed asking this
>due to
> > >me
> > > > > being an NP and I feel like I should know this. Guess this type of
> > >thing
> > > > > doesn't come up much around me. Question is: Can poison reverse
>(PR)
> > >be
> > >used
> > > > > with OSPF? I know split horizon is used with it but I just cant
>see
> > >why/how
> > > > > poison reverse would. My definition of PR is that it sets the link
>to
> > >the
> > > > > max hop count and deems it unreachable, hence the term poison. I
>can't
> > >see
> > > > > how this would work with OSPF because it doesn't use a hop count.
> > >Maybe
> > >I'm
> > > > > confused about PR. Does it set the link to the highest metric, and
>not
> > >hop
> > > > > count? Maybe I'm just used to hearing about PR in discussions of
>RIP
> > >that
> > > > > I'm assuming it set the hop count to the highest and has nothing
>to do
> > >with
> > > > > metric. Any clarity would help, thanks all...
> > > > >
> > > > > ...sorry for the rambling...
> > > > >
> > > > > ************************************************************
> > > > > This has been an Eyez Only streaming e-mail broadcast...We are
> > >watching.
> > > > >
> > > > > NetEyez ~ CCNP, CCDA
> > > > >
> > > > >
> > > > >
> > > > > _________________________________
> > > > > FAQ, list archives, and subscription info:
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> > > > >
> > > >
> > > >
> > > > -----------------------------------------------
> > > > I'm buying / selling used CISCO gear!!
> > > > email me for a quote
> > > >
> > > > Brian Feeny,CCDP,CCNP+VAS Scarlett Parria
> > > > [EMAIL PROTECTED] [EMAIL PROTECTED]
> > > > 318-222-2638 x 109 318-222-2638 x 101
> > > >
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> > > > 1401 Oden St.
> > > > Suite 18
> > > > Shreveport, LA 71104
> > > > Fax 318-221-6612
> > > >
> > > >
> > >
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